[PATCH] Fix data race in mark_rt_mutex_waiters

[PATCH] Fix data race in mark_rt_mutex_waiters

[Hernan Ponce de Leon]

From: Hernan Ponce de Leon <hernanl.leon@huawei.com>

Following the defition of data race in
tools/memory-model/linux-kernel.cat the dartagnan tool
https://github.com/hernanponcedeleon/Dat3M
reported a race between mark_rt_mutex_waiters and rt_mutex_cmpxchg_release.

Commit 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
later removed in commit d0aa7a70bf03 ("futex_requeue_pi optimization")
and reverted in commit bd197234b0a6
("Revert "futex_requeue_pi optimization"")

The original commit introduced the data race.

Cc: stable@vger.kernel.org # v2.6.18.x
Fixes: 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
Signed-off-by: Hernan Ponce de Leon <hernanl.leon@huawei.com>
---
 kernel/locking/rtmutex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 010cf4e6d0b8..7ed9472edd48 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
 	unsigned long owner, *p = (unsigned long *) &lock->owner;
 
 	do {
-		owner = *p;
+		owner = READ_ONCE(*p);
 	} while (cmpxchg_relaxed(p, owner,
 				 owner | RT_MUTEX_HAS_WAITERS) != owner);
 
-- 
2.25.1

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Arjan van de Ven]

On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
> From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> 

>   kernel/locking/rtmutex.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 010cf4e6d0b8..7ed9472edd48 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
>   	unsigned long owner, *p = (unsigned long *) &lock->owner;
>   
>   	do {
> -		owner = *p;
> +		owner = READ_ONCE(*p);
>   	} while (cmpxchg_relaxed(p, owner,


I don't see how this makes any difference at all.
*p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Paul E. McKenney]

On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote:
> On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
> > From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> > 
> 
> >   kernel/locking/rtmutex.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > index 010cf4e6d0b8..7ed9472edd48 100644
> > --- a/kernel/locking/rtmutex.c
> > +++ b/kernel/locking/rtmutex.c
> > @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
> >   	unsigned long owner, *p = (unsigned long *) &lock->owner;
> >   	do {
> > -		owner = *p;
> > +		owner = READ_ONCE(*p);
> >   	} while (cmpxchg_relaxed(p, owner,
> 
> 
> I don't see how this makes any difference at all.
> *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics

Doing so does suppress a KCSAN warning.  You could also use data_race()
if it turns out that the volatile semantics would prevent a valuable
compiler optimization.

							Thanx, Paul

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Hernan Ponce de Leon]

On 1/20/2023 4:54 PM, Paul E. McKenney wrote:
> On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote:
>> On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
>>> From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
>>>
>>
>>>    kernel/locking/rtmutex.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>>> index 010cf4e6d0b8..7ed9472edd48 100644
>>> --- a/kernel/locking/rtmutex.c
>>> +++ b/kernel/locking/rtmutex.c
>>> @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
>>>    	unsigned long owner, *p = (unsigned long *) &lock->owner;
>>>    	do {
>>> -		owner = *p;
>>> +		owner = READ_ONCE(*p);
>>>    	} while (cmpxchg_relaxed(p, owner,
>>
>>
>> I don't see how this makes any difference at all.
>> *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics
> 
> Doing so does suppress a KCSAN warning.  You could also use data_race()
> if it turns out that the volatile semantics would prevent a valuable
> compiler optimization.

I think the import question is "is this a harmful data race (and needs 
to be fixed as proposed by the patch) or a harmless one (and we should 
use data_race() to silence tools)?".

In https://lkml.org/lkml/2023/1/22/160 I describe how this data race can 
affect important ordering guarantees for the rest of the code. For this 
reason I consider it a harmful one. If this is not the case, I would 
appreciate some feedback or pointer to resources about what races care 
to avoid spamming the mailing list in the future.

Hernan

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Paul E. McKenney]

On Sun, Jan 22, 2023 at 04:24:21PM +0100, Hernan Ponce de Leon wrote:
> On 1/20/2023 4:54 PM, Paul E. McKenney wrote:
> > On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote:
> > > On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
> > > > From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> > > > 
> > > 
> > > >    kernel/locking/rtmutex.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > > > index 010cf4e6d0b8..7ed9472edd48 100644
> > > > --- a/kernel/locking/rtmutex.c
> > > > +++ b/kernel/locking/rtmutex.c
> > > > @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
> > > >    	unsigned long owner, *p = (unsigned long *) &lock->owner;
> > > >    	do {
> > > > -		owner = *p;
> > > > +		owner = READ_ONCE(*p);
> > > >    	} while (cmpxchg_relaxed(p, owner,
> > > 
> > > 
> > > I don't see how this makes any difference at all.
> > > *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics
> > 
> > Doing so does suppress a KCSAN warning.  You could also use data_race()
> > if it turns out that the volatile semantics would prevent a valuable
> > compiler optimization.
> 
> I think the import question is "is this a harmful data race (and needs to be
> fixed as proposed by the patch) or a harmless one (and we should use
> data_race() to silence tools)?".
> 
> In https://lkml.org/lkml/2023/1/22/160 I describe how this data race can
> affect important ordering guarantees for the rest of the code. For this
> reason I consider it a harmful one. If this is not the case, I would
> appreciate some feedback or pointer to resources about what races care to
> avoid spamming the mailing list in the future.

In the case, the value read is passed into cmpxchg_relaxed(), which
checks the value against memory.  In this case, as Arjan noted, the only
compiler-and-silicon difference between data_race() and READ_ONCE()
is that use of data_race() might allow the compiler to do things like
tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
failure.  In contrast, LKMM (by design) throws up its hands when it sees
a data race.  Something about not being eager to track the idiosyncrasies
of many compiler versions.

My approach in my own code is to use *_ONCE() unless it causes a visible
performance regression or if it confuses KCSAN.  An example of the latter
can be debug code, in which case use of data_race() avoids suppressing
KCSAN warnings (and also false positives, depending).

Except that your other email seems to also be arguing that additional
ordering is required.  So is https://lkml.org/lkml/2023/1/20/702 really
sufficient just by itself, or is additional ordering required?

							Thanx, Paul

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Alan Stern]

On Mon, Jan 23, 2023 at 08:40:14AM -0800, Paul E. McKenney wrote:
> In the case, the value read is passed into cmpxchg_relaxed(), which
> checks the value against memory.  In this case, as Arjan noted, the only
> compiler-and-silicon difference between data_race() and READ_ONCE()
> is that use of data_race() might allow the compiler to do things like
> tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
> failure.

Is it possible in theory for a torn load to cause a spurious 
cmpxchg_relaxed() success?  Or would that not matter here?

Alan

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Paul E. McKenney]

On Mon, Jan 23, 2023 at 12:34:37PM -0500, Alan Stern wrote:
> On Mon, Jan 23, 2023 at 08:40:14AM -0800, Paul E. McKenney wrote:
> > In the case, the value read is passed into cmpxchg_relaxed(), which
> > checks the value against memory.  In this case, as Arjan noted, the only
> > compiler-and-silicon difference between data_race() and READ_ONCE()
> > is that use of data_race() might allow the compiler to do things like
> > tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
> > failure.
> 
> Is it possible in theory for a torn load to cause a spurious 
> cmpxchg_relaxed() success?  Or would that not matter here?

In this case, the new value is the old value with an additional bit set.
There is no check for that bit being clear, so I am having a hard time
seeing a difference.

Then again, much might depend on the ordering that Hernan is
referring to.

And Peter Zijlstra's suggestion of set_bit() is quite attractive,
give or take the casting issues called out by David Laight.

							Thanx, Paul

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Jonas Oberhauser]

On 1/23/2023 6:34 PM, Alan Stern wrote:
> On Mon, Jan 23, 2023 at 08:40:14AM -0800, Paul E. McKenney wrote:
>> In the case, the value read is passed into cmpxchg_relaxed(), which
>> checks the value against memory.  In this case, as Arjan noted, the only
>> compiler-and-silicon difference between data_race() and READ_ONCE()
>> is that use of data_race() might allow the compiler to do things like
>> tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
>> failure.
> Is it possible in theory for a torn load to cause a spurious
> cmpxchg_relaxed() success?  Or would that not matter here?

Note that in this example there are no memory accesses between the read 
and the CAS.
So if the cmpxchg succeeds, what you non-atomically read must be exactly 
the value that is read by the cmpxchg, and you could pretend that the 
torn read happened at the same time as the cmpxchg.

This "pretend" part requires that there are no other events in the 
middle, otherwise you could be violating some ordering constraint 
between those events and the torn reads. Otherwise you might get some 
issues. E.g., you might read a sequence count of 259 from reading the 
lower half when the count is 3 and the upper half when the count is 256, 
and then do the CAS when the sequence count is 259, so if you had two 
peeks at sequence-count-protected data between that read and the CAS you 
might see different states despite the CAS succeeding.

have fun, jonas

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Hernan Ponce de Leon]

On 1/23/2023 5:40 PM, Paul E. McKenney wrote:
> On Sun, Jan 22, 2023 at 04:24:21PM +0100, Hernan Ponce de Leon wrote:
>> On 1/20/2023 4:54 PM, Paul E. McKenney wrote:
>>> On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote:
>>>> On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
>>>>> From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
>>>>>
>>>>
>>>>>     kernel/locking/rtmutex.c | 2 +-
>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>>>>> index 010cf4e6d0b8..7ed9472edd48 100644
>>>>> --- a/kernel/locking/rtmutex.c
>>>>> +++ b/kernel/locking/rtmutex.c
>>>>> @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
>>>>>     	unsigned long owner, *p = (unsigned long *) &lock->owner;
>>>>>     	do {
>>>>> -		owner = *p;
>>>>> +		owner = READ_ONCE(*p);
>>>>>     	} while (cmpxchg_relaxed(p, owner,
>>>>
>>>>
>>>> I don't see how this makes any difference at all.
>>>> *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics
>>>
>>> Doing so does suppress a KCSAN warning.  You could also use data_race()
>>> if it turns out that the volatile semantics would prevent a valuable
>>> compiler optimization.
>>
>> I think the import question is "is this a harmful data race (and needs to be
>> fixed as proposed by the patch) or a harmless one (and we should use
>> data_race() to silence tools)?".
>>
>> In https://lkml.org/lkml/2023/1/22/160 I describe how this data race can
>> affect important ordering guarantees for the rest of the code. For this
>> reason I consider it a harmful one. If this is not the case, I would
>> appreciate some feedback or pointer to resources about what races care to
>> avoid spamming the mailing list in the future.
> 
> In the case, the value read is passed into cmpxchg_relaxed(), which
> checks the value against memory.  In this case, as Arjan noted, the only
> compiler-and-silicon difference between data_race() and READ_ONCE()
> is that use of data_race() might allow the compiler to do things like
> tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
> failure.  In contrast, LKMM (by design) throws up its hands when it sees
> a data race.  Something about not being eager to track the idiosyncrasies
> of many compiler versions.
> 
> My approach in my own code is to use *_ONCE() unless it causes a visible
> performance regression or if it confuses KCSAN.  An example of the latter
> can be debug code, in which case use of data_race() avoids suppressing
> KCSAN warnings (and also false positives, depending).

I understand that *_ONCE() might avoid some compiler optimization and 
reduce performance in the general case. However, if I understand your 
first paragraph correctly, in this particular case data_race() could 
allow the CAS to fail more often, resulting in more spinning iterations 
and degraded performance. Am I right?

> 
> Except that your other email seems to also be arguing that additional
> ordering is required.  So is https://lkml.org/lkml/2023/1/20/702 really
> sufficient just by itself, or is additional ordering required?

I do not claim that we need to mark the read to add the ordering that is 
needed for correctness (mutual exclusion). What I claim in this patch is 
that there is a data race, and since it can affect ordering constrains 
in subtle ways, I consider it harmful and thus I want to fix it.

What I explain in the other email is that if we fix the data race, 
either the fence or the acquire store might be relaxed (because marking 
the read gives us some extra ordering guarantees). If the race is not 
fixed, both the fence and the acquire are needed according to LKMM. The 
situation is different wrt hardware models. In that case the tool cannot 
find any violation even if we don't fix the race and we relax the store 
/ remove the fence.

Hernan

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Waiman Long]

On 1/24/23 09:57, Hernan Ponce de Leon wrote:
>> In the case, the value read is passed into cmpxchg_relaxed(), which
>> checks the value against memory.  In this case, as Arjan noted, the only
>> compiler-and-silicon difference between data_race() and READ_ONCE()
>> is that use of data_race() might allow the compiler to do things like
>> tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
>> failure.  In contrast, LKMM (by design) throws up its hands when it sees
>> a data race.  Something about not being eager to track the 
>> idiosyncrasies
>> of many compiler versions.
>>
>> My approach in my own code is to use *_ONCE() unless it causes a visible
>> performance regression or if it confuses KCSAN.  An example of the 
>> latter
>> can be debug code, in which case use of data_race() avoids suppressing
>> KCSAN warnings (and also false positives, depending).
>
> I understand that *_ONCE() might avoid some compiler optimization and 
> reduce performance in the general case. However, if I understand your 
> first paragraph correctly, in this particular case data_race() could 
> allow the CAS to fail more often, resulting in more spinning 
> iterations and degraded performance. Am I right?
>
>>
>> Except that your other email seems to also be arguing that additional
>> ordering is required.  So is https://lkml.org/lkml/2023/1/20/702 really
>> sufficient just by itself, or is additional ordering required?
>
> I do not claim that we need to mark the read to add the ordering that 
> is needed for correctness (mutual exclusion). What I claim in this 
> patch is that there is a data race, and since it can affect ordering 
> constrains in subtle ways, I consider it harmful and thus I want to 
> fix it.
>
> What I explain in the other email is that if we fix the data race, 
> either the fence or the acquire store might be relaxed (because 
> marking the read gives us some extra ordering guarantees). If the race 
> is not fixed, both the fence and the acquire are needed according to 
> LKMM. The situation is different wrt hardware models. In that case the 
> tool cannot find any violation even if we don't fix the race and we 
> relax the store / remove the fence.

I would suggest to do it as suggested by PeterZ. Instead of set_bit(), 
however, it is probably better to use atomic_long_or() like

atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner)

The mutex code stores the lock owner as atomic_long_t. So it is natural 
to treat &lock->owner as atomic_long_t here too.

Cheers,
Longman

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Peter Zijlstra]

On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:

> I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
> however, it is probably better to use atomic_long_or() like
> 
> atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner)

That function doesn't exist, atomic_long_or() is implicitly relaxed for
not returning a value.

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Waiman Long]

On 1/24/23 10:52, Peter Zijlstra wrote:
> On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:
>
>> I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
>> however, it is probably better to use atomic_long_or() like
>>
>> atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner)
> That function doesn't exist, atomic_long_or() is implicitly relaxed for
> not returning a value.
>
You are right. atomic_long_or() doesn't have variants like some others.

Cheers,
Longman

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Hernan Ponce de Leon]

On 1/24/2023 5:04 PM, Waiman Long wrote:
> 
> On 1/24/23 10:52, Peter Zijlstra wrote:
>> On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:
>>
>>> I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
>>> however, it is probably better to use atomic_long_or() like
>>>
>>> atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t 
>>> *)&lock->owner)
>> That function doesn't exist, atomic_long_or() is implicitly relaxed for
>> not returning a value.
>>
> You are right. atomic_long_or() doesn't have variants like some others.
> 
> Cheers,
> Longman
> 

When you say "replace the whole of that function", do you mean "barrier 
included"? I argue in the other email that I think this should not 
affect correctness (at least not obviously), but removing the barrier is 
doing more than just fixing the data race as this patch suggests.

Hernan

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Peter Zijlstra]

On Thu, Jan 26, 2023 at 10:42:07AM +0100, Hernan Ponce de Leon wrote:
> On 1/24/2023 5:04 PM, Waiman Long wrote:
> > 
> > On 1/24/23 10:52, Peter Zijlstra wrote:
> > > On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:
> > > 
> > > > I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
> > > > however, it is probably better to use atomic_long_or() like
> > > > 
> > > > atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t
> > > > *)&lock->owner)
> > > That function doesn't exist, atomic_long_or() is implicitly relaxed for
> > > not returning a value.
> > > 
> > You are right. atomic_long_or() doesn't have variants like some others.
> > 
> > Cheers,
> > Longman
> > 
> 
> When you say "replace the whole of that function", do you mean "barrier
> included"? I argue in the other email that I think this should not affect
> correctness (at least not obviously), but removing the barrier is doing more
> than just fixing the data race as this patch suggests.

Well, set_bit() implies smp_mb(), atomic_long_or() does not and would
need to retain the barrier.

That said, the comments are all batshit. The top comment states relaxed
ordering is suffient since holding lock, the comment with the barrier
utterly fails to explain what it's ordering against.

So all that would need to be updated as well.

That said, looking at 1c0908d8e441 I'm not at all sure we need that
barrier. Even in the try_to_take_rt_mutex(.waiter=NULL) case, where we
skip over the task->pi_lock region, rt_mutex_set_owner(.acquire=true)
will be ACQUIRE.

And in case of rt_mutex_owner(), we fail the trylock (return with 0) and
a failed trylock does not imply any ordering.

So why are we having this barrier?

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Peter Zijlstra]

On Thu, Jan 26, 2023 at 01:20:07PM +0100, Peter Zijlstra wrote:
> Well, set_bit() implies smp_mb(), atomic_long_or() does not and would
> need to retain the barrier.

set_bit() does not, must've had a brain-fart or so.

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Hernan Ponce de Leon]

On 1/26/2023 1:20 PM, Peter Zijlstra wrote:
> On Thu, Jan 26, 2023 at 10:42:07AM +0100, Hernan Ponce de Leon wrote:
>> On 1/24/2023 5:04 PM, Waiman Long wrote:
>>>
>>> On 1/24/23 10:52, Peter Zijlstra wrote:
>>>> On Tue, Jan 24, 2023 at 10:42:24AM -0500, Waiman Long wrote:
>>>>
>>>>> I would suggest to do it as suggested by PeterZ. Instead of set_bit(),
>>>>> however, it is probably better to use atomic_long_or() like
>>>>>
>>>>> atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t
>>>>> *)&lock->owner)
>>>> That function doesn't exist, atomic_long_or() is implicitly relaxed for
>>>> not returning a value.
>>>>
>>> You are right. atomic_long_or() doesn't have variants like some others.
>>>
>>> Cheers,
>>> Longman
>>>
>>
>> When you say "replace the whole of that function", do you mean "barrier
>> included"? I argue in the other email that I think this should not affect
>> correctness (at least not obviously), but removing the barrier is doing more
>> than just fixing the data race as this patch suggests.
> 
> Well, set_bit() implies smp_mb(), atomic_long_or() does not and would
> need to retain the barrier.
> 
> That said, the comments are all batshit. The top comment states relaxed
> ordering is suffient since holding lock, the comment with the barrier
> utterly fails to explain what it's ordering against.

I think the top comment became obsolete after 1c0908d8e441 and this just 
went unnoticed. I agree the comment with the barrier does not say much 
and getting some more detailed information was one of the goals of my 
other email.

> 
> So all that would need to be updated as well.
> 
> That said, looking at 1c0908d8e441 I'm not at all sure we need that
> barrier. Even in the try_to_take_rt_mutex(.waiter=NULL) case, where we
> skip over the task->pi_lock region, rt_mutex_set_owner(.acquire=true)
> will be ACQUIRE.

This sentence states in a clear way the idea I was trying to express in 
my other email about why the barrier is not necessary. I think the same 
argument holds if we keep the barrier and relax the store in 
rt_mutex_set_owner as suggested by Boqun (see patch below).

> 
> And in case of rt_mutex_owner(), we fail the trylock (return with 0) and
> a failed trylock does not imply any ordering.
> 
> So why are we having this barrier?

I run again the verification with the following patch (I am aware the 
comments still need to be updated, this was just to be able to run the 
tool) and the tool still finds no violation.

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 010cf4e6d0b8..c62e409906a2 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -107,7 +107,7 @@ rt_mutex_set_owner(struct rt_mutex_base *lock, 
struct task_struct *owner)
  	 * lock->wait_lock is held but explicit acquire semantics are needed
  	 * for a new lock owner so WRITE_ONCE is insufficient.
  	 */
-	xchg_acquire(&lock->owner, rt_mutex_owner_encode(lock, owner));
+	WRITE_ONCE(lock->owner, rt_mutex_owner_encode(lock, owner));
  }

  static __always_inline void rt_mutex_clear_owner(struct rt_mutex_base 
*lock)
@@ -232,12 +232,7 @@ static __always_inline bool 
rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
   */
  static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base 
*lock)
  {
-	unsigned long owner, *p = (unsigned long *) &lock->owner;
-
-	do {
-		owner = *p;
-	} while (cmpxchg_relaxed(p, owner,
-				 owner | RT_MUTEX_HAS_WAITERS) != owner);
+	atomic_long_or(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner);

  	/*
  	 * The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE
-- 

RE: [PATCH] Fix data race in mark_rt_mutex_waiters

[David Laight]

From: Hernan Ponce de Leon
> Sent: 26 January 2023 21:07
...
>   static __always_inline void rt_mutex_clear_owner(struct rt_mutex_base
> *lock)
> @@ -232,12 +232,7 @@ static __always_inline bool
> rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
>    */
>   static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base
> *lock)
>   {
> -	unsigned long owner, *p = (unsigned long *) &lock->owner;
> -
> -	do {
> -		owner = *p;
> -	} while (cmpxchg_relaxed(p, owner,
> -				 owner | RT_MUTEX_HAS_WAITERS) != owner);
> +	atomic_long_or(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner);

These *(int_type *)&foo accesses (quite often just plain wrong)
made me look up the definitions.

All one big accident waiting to happen...
RT_MUTEX_HAS_WAITERS is defined in a different header to the structure.
The explanatory comment is in a 3rd file.

It would all be safer if lock->owner were atomic_long_t with a comment
that it was the waiting task_struct | RT_MUTEX_HAS_WAITERS.

Given the actual definition is rt_mutex_base_is_locked() even correct?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Waiman Long]

On 1/26/23 17:10, David Laight wrote:
> From: Hernan Ponce de Leon
>> Sent: 26 January 2023 21:07
> ...
>>    static __always_inline void rt_mutex_clear_owner(struct rt_mutex_base
>> *lock)
>> @@ -232,12 +232,7 @@ static __always_inline bool
>> rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
>>     */
>>    static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base
>> *lock)
>>    {
>> -	unsigned long owner, *p = (unsigned long *) &lock->owner;
>> -
>> -	do {
>> -		owner = *p;
>> -	} while (cmpxchg_relaxed(p, owner,
>> -				 owner | RT_MUTEX_HAS_WAITERS) != owner);
>> +	atomic_long_or(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner);
> These *(int_type *)&foo accesses (quite often just plain wrong)
> made me look up the definitions.
>
> All one big accident waiting to happen...
> RT_MUTEX_HAS_WAITERS is defined in a different header to the structure.
> The explanatory comment is in a 3rd file.
>
> It would all be safer if lock->owner were atomic_long_t with a comment
> that it was the waiting task_struct | RT_MUTEX_HAS_WAITERS.
>
> Given the actual definition is rt_mutex_base_is_locked() even correct?

It is arguable if it should be considered locked if a waiter is waiting 
but the lock is at an unlock state at the moment. Mutex has a narrower 
definition of locked while others have a broader one.

Cheers,
Longman

lock_torture results for different patches:

[Antonio Paolillo]

Dear all,

I want to provide some support to Hernan regarding performance claims.

I used lock_torture to evaluate the different proposed patches on two
different server machines:
- a Huawei TaiShan 200 (Model 2280) rack server that has 128 GB of RAM
  and 2x Kunpeng 920-4826 processors, a HiSilicon chip with 48 ARMv8.2
  64-bit cores totaling 96 cores (no SMT) [1, 2],
  denoted as taishan200-96c;
- a GIGABYTE R182-Z91-00 rack server that has 128 GB of RAM and 2x
  EPYC 7352 processors, an AMD chip with 24 x86_64 cores, totaling 48
  cores (96 CPUs when counting hyperthreading) [3, 4],
  denoted as gigabyte-96c.

I ran the evaluation on a Ubuntu 22.04 distro, with custom kernels based
on v6.2-rc6 (6d796c50f84ca79f1722bb131799e5a5710c4700).
The different kernels are combination of patches:
- (0) Stock kernel;
- (1) With relaxed set owner barrier (as discussed in [5] and questioned
  by Peter, the barrier seems not to be needed);
- (2) With READ_ONCE(), as originally proposed in this thread;
- (3) With atomic_long_or() as proposed by Peter;
- (4) With relaxed set owner barrier and READ_ONCE();
- (5) With relaxed set owner barrier and atomic_long_or().

I ran lock_torture several times, exploring the following parameter
space:
- torture_type="rtmutex_lock",
- nwriters_stress=[1, 2, 3, 4, 8, 16, 32, 64, 95],
- stat_interval=4,
- stutter=0,
- shuffle_interval=0.
For each value of "nwriters_stress", I ran the configuration 5 times.

By feeding the lock_torture kthread pids to "taskset -p", I overruled
the scheduling such that the distribution of kthreads to CPUs is fixed.
I also disabled "irq balance" and "numa balance" daemons, fixed the
frequency to 1.5GHz using the "userspace" cpufreq governor and isolated
all the cores used (using isolcpus=1-95 at boot-time) to avoid any
source of interference.

As a warm-up phase, I ignored the first reported results and only
considered the latest 60 seconds of execution (after all kthreads
migrated to their final CPU).
The reported throughput is computed by dividing the reported number of
operations by the duration of the measurement for each dot (60 seconds),
so higher is better.

Here follows the results on taishan200-96c (the 'rel' column is the mean
relative to the mean of the stock kernel, in percent, and each mean is
the average over 5 independent runs):

Kernel:             k0-stock-6.2.0-rc6       k1-rmacq             k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor            
Statistic (kops/s):               mean   std     mean   std   rel        mean   std   rel       mean   std   rel              mean   std   rel             mean   std   rel
nwriters_stress:                                                                                                                                                           
1                               899.91 24.95   880.10 29.62   -2%      871.57 44.27   -3%     888.65 37.90   -1%            898.63 29.82   -0%           889.83 25.64   -1%
2                               359.30 25.92   416.83 32.77  +16%      360.65 28.32   +0%     404.79 42.64  +13%            380.65 21.29   +6%           404.37 23.27  +13%
3                               314.97 24.32   308.41  9.68   -2%      315.00  9.97   +0%     313.86 13.47   -0%            313.47  4.01   -0%           322.77 20.82   +2%
4                               328.02 15.09   330.65 29.33   +1%      314.83 24.28   -4%     305.71 12.72   -7%            322.95 10.39   -2%           343.32 13.73   +5%
8                               292.16 22.03   288.85 10.50   -1%      288.28 18.84   -1%     285.42 24.58   -2%            310.23 26.08   +6%           285.67 20.03   -2%
16                              297.03 26.89   281.89 29.22   -5%      265.19 33.73  -11%     279.02 22.43   -6%            284.40 36.21   -4%           285.21 36.33   -4%
32                              187.36 28.59   175.71 19.77   -6%      186.44 48.15   -0%     206.59 14.11  +10%            174.08 24.30   -7%           185.80 45.12   -1%
64                              148.13 48.65   172.48 34.29  +16%      154.59 47.05   +4%     164.22 29.81  +11%            142.13 47.40   -4%           136.39 29.95   -8%
95                              174.35 57.89   148.59 38.03  -15%      156.85 43.64  -10%     132.92 32.35  -24%            126.44 28.24  -27%           146.82 60.04  -16%

And the results on gigabyte-96c:

Kernel:             k0-stock-6.2.0-rc6       k1-rmacq               k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor            
Statistic (kops/s):               mean   std     mean    std    rel        mean   std   rel       mean   std   rel              mean   std   rel             mean    std  rel
nwriters_stress:                                                                                                                                                             
1                               713.72 25.68   707.32  17.73    -1%      718.81 12.63   +1%     712.80 13.57   -0%            709.17 14.10   -1%           730.33   9.14  +2%
2                               376.25  8.19   400.09  16.24    +6%      396.71 26.09   +5%     412.61 17.80  +10%            396.48  7.02   +5%           409.90  14.61  +9%
3                               415.07 16.83   410.19  19.82    -1%      423.39  9.68   +2%     417.28 10.23   +1%            424.94 17.48   +2%           422.92  11.75  +2%
4                               286.77 26.63   285.13   6.80    -1%      297.33 23.62   +4%     296.49 16.60   +3%            303.99 30.38   +6%           296.93   9.90  +4%
8                               296.56 20.45   308.97  12.53    +4%      305.49 19.91   +3%     294.24 17.24   -1%            294.71 24.03   -1%           294.09  25.20  -1%
16                              257.34 33.94   266.03  29.60    +3%      270.72 35.22   +5%     252.28 50.16   -2%            263.83 45.84   +3%           247.42  41.01  -4%
32                              278.78 51.45   215.35  68.40   -23%      259.77 87.44   -7%     217.26 79.67  -22%            201.23 70.46  -28%           282.47 116.65  +1%
64                               75.82 64.87   194.52 137.19  +157%       35.57 12.14  -53%      74.24 72.04   -2%             71.29 45.55   -6%            77.93  43.57  +3%
95                               60.37 68.13   198.38 116.93  +229%       43.12 17.60  -29%      58.80 36.47   -3%             57.78 63.00   -4%            61.33  71.18  +2%

We can safely conclude that the patches do not significatively affect
the throughput of the lock_torture benchmark for rtmutex_lock.
The values for nwriters_stress>=64 can safely be ignored as they are too
spread.

Please notice that I pushed a landing page [6] with results in HTML that
may be more convenient to browse together with interactive charts.

Cheers,

Antonio

[1] https://e.huawei.com/uk/products/servers/taishan-server/taishan-2280-v2
[2] https://en.wikichip.org/wiki/hisilicon/kunpeng/920-4826
[3] https://www.gigabyte.com/Rack-Server/R182-Z91-rev-100
[4] https://www.amd.com/en/products/cpu/amd-epyc-7352
[5] https://lkml.org/lkml/2023/1/22/160
[6] https://antonio.paolillo.be/public/rtlocks-locktorture-patches.html

Re: lock_torture results for different patches:

[Hernan Ponce de Leon]

Thanks Antonio for the performance results. Taking both correctness and 
performance into consideration, this is my current understanding of the 
situation.

Removing the acquire barrier does not improve performance and while it 
might be unnecessary, it seems better to play it safe and keep it. That 
being said, a comment about this should be added to the code (Paul 
suggested something like this at some point).

Neither of the proposed fixes to the data race significantly affect 
performance. Using READ_ONCE shows better performance in the AMD 
machine, but using atomic_long_or shows the opposite in the HiSilicon 
one. However, the difference is more prominent using READ_ONCE.

I am not sure I got David's comment. Are you proposing to change the 
definition of rt_mutex_base to

struct rt_mutex_base {
	raw_spinlock_t		wait_lock;
	struct rb_root_cached   waiters;
	/* This the waiting task_struct | RT_MUTEX_HAS_WAITERS. */
	atomic_long_t 		*owner;
};

I don't fully understand the consequences of this. At minimum this would 
require changes in several methods such as the one below, right?

static __always_inline bool rt_mutex_cmpxchg_release(struct 
rt_mutex_base *lock,
						     atomic_long_t  *old,
						     atomic_long_t  *new)

Is there anything else that I am missing?

Hernan

On 3/1/2023 5:32 PM, Antonio Paolillo wrote:
> Dear all,
> 
> I want to provide some support to Hernan regarding performance claims.
> 
> I used lock_torture to evaluate the different proposed patches on two
> different server machines:
> - a Huawei TaiShan 200 (Model 2280) rack server that has 128 GB of RAM
>    and 2x Kunpeng 920-4826 processors, a HiSilicon chip with 48 ARMv8.2
>    64-bit cores totaling 96 cores (no SMT) [1, 2],
>    denoted as taishan200-96c;
> - a GIGABYTE R182-Z91-00 rack server that has 128 GB of RAM and 2x
>    EPYC 7352 processors, an AMD chip with 24 x86_64 cores, totaling 48
>    cores (96 CPUs when counting hyperthreading) [3, 4],
>    denoted as gigabyte-96c.
> 
> I ran the evaluation on a Ubuntu 22.04 distro, with custom kernels based
> on v6.2-rc6 (6d796c50f84ca79f1722bb131799e5a5710c4700).
> The different kernels are combination of patches:
> - (0) Stock kernel;
> - (1) With relaxed set owner barrier (as discussed in [5] and questioned
>    by Peter, the barrier seems not to be needed);
> - (2) With READ_ONCE(), as originally proposed in this thread;
> - (3) With atomic_long_or() as proposed by Peter;
> - (4) With relaxed set owner barrier and READ_ONCE();
> - (5) With relaxed set owner barrier and atomic_long_or().
> 
> I ran lock_torture several times, exploring the following parameter
> space:
> - torture_type="rtmutex_lock",
> - nwriters_stress=[1, 2, 3, 4, 8, 16, 32, 64, 95],
> - stat_interval=4,
> - stutter=0,
> - shuffle_interval=0.
> For each value of "nwriters_stress", I ran the configuration 5 times.
> 
> By feeding the lock_torture kthread pids to "taskset -p", I overruled
> the scheduling such that the distribution of kthreads to CPUs is fixed.
> I also disabled "irq balance" and "numa balance" daemons, fixed the
> frequency to 1.5GHz using the "userspace" cpufreq governor and isolated
> all the cores used (using isolcpus=1-95 at boot-time) to avoid any
> source of interference.
> 
> As a warm-up phase, I ignored the first reported results and only
> considered the latest 60 seconds of execution (after all kthreads
> migrated to their final CPU).
> The reported throughput is computed by dividing the reported number of
> operations by the duration of the measurement for each dot (60 seconds),
> so higher is better.
> 
> Here follows the results on taishan200-96c (the 'rel' column is the mean
> relative to the mean of the stock kernel, in percent, and each mean is
> the average over 5 independent runs):
> 
> Kernel:             k0-stock-6.2.0-rc6       k1-rmacq             k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor
> Statistic (kops/s):               mean   std     mean   std   rel        mean   std   rel       mean   std   rel              mean   std   rel             mean   std   rel
> nwriters_stress:
> 1                               899.91 24.95   880.10 29.62   -2%      871.57 44.27   -3%     888.65 37.90   -1%            898.63 29.82   -0%           889.83 25.64   -1%
> 2                               359.30 25.92   416.83 32.77  +16%      360.65 28.32   +0%     404.79 42.64  +13%            380.65 21.29   +6%           404.37 23.27  +13%
> 3                               314.97 24.32   308.41  9.68   -2%      315.00  9.97   +0%     313.86 13.47   -0%            313.47  4.01   -0%           322.77 20.82   +2%
> 4                               328.02 15.09   330.65 29.33   +1%      314.83 24.28   -4%     305.71 12.72   -7%            322.95 10.39   -2%           343.32 13.73   +5%
> 8                               292.16 22.03   288.85 10.50   -1%      288.28 18.84   -1%     285.42 24.58   -2%            310.23 26.08   +6%           285.67 20.03   -2%
> 16                              297.03 26.89   281.89 29.22   -5%      265.19 33.73  -11%     279.02 22.43   -6%            284.40 36.21   -4%           285.21 36.33   -4%
> 32                              187.36 28.59   175.71 19.77   -6%      186.44 48.15   -0%     206.59 14.11  +10%            174.08 24.30   -7%           185.80 45.12   -1%
> 64                              148.13 48.65   172.48 34.29  +16%      154.59 47.05   +4%     164.22 29.81  +11%            142.13 47.40   -4%           136.39 29.95   -8%
> 95                              174.35 57.89   148.59 38.03  -15%      156.85 43.64  -10%     132.92 32.35  -24%            126.44 28.24  -27%           146.82 60.04  -16%
> 
> And the results on gigabyte-96c:
> 
> Kernel:             k0-stock-6.2.0-rc6       k1-rmacq               k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor
> Statistic (kops/s):               mean   std     mean    std    rel        mean   std   rel       mean   std   rel              mean   std   rel             mean    std  rel
> nwriters_stress:
> 1                               713.72 25.68   707.32  17.73    -1%      718.81 12.63   +1%     712.80 13.57   -0%            709.17 14.10   -1%           730.33   9.14  +2%
> 2                               376.25  8.19   400.09  16.24    +6%      396.71 26.09   +5%     412.61 17.80  +10%            396.48  7.02   +5%           409.90  14.61  +9%
> 3                               415.07 16.83   410.19  19.82    -1%      423.39  9.68   +2%     417.28 10.23   +1%            424.94 17.48   +2%           422.92  11.75  +2%
> 4                               286.77 26.63   285.13   6.80    -1%      297.33 23.62   +4%     296.49 16.60   +3%            303.99 30.38   +6%           296.93   9.90  +4%
> 8                               296.56 20.45   308.97  12.53    +4%      305.49 19.91   +3%     294.24 17.24   -1%            294.71 24.03   -1%           294.09  25.20  -1%
> 16                              257.34 33.94   266.03  29.60    +3%      270.72 35.22   +5%     252.28 50.16   -2%            263.83 45.84   +3%           247.42  41.01  -4%
> 32                              278.78 51.45   215.35  68.40   -23%      259.77 87.44   -7%     217.26 79.67  -22%            201.23 70.46  -28%           282.47 116.65  +1%
> 64                               75.82 64.87   194.52 137.19  +157%       35.57 12.14  -53%      74.24 72.04   -2%             71.29 45.55   -6%            77.93  43.57  +3%
> 95                               60.37 68.13   198.38 116.93  +229%       43.12 17.60  -29%      58.80 36.47   -3%             57.78 63.00   -4%            61.33  71.18  +2%
> 
> We can safely conclude that the patches do not significatively affect
> the throughput of the lock_torture benchmark for rtmutex_lock.
> The values for nwriters_stress>=64 can safely be ignored as they are too
> spread.
> 
> Please notice that I pushed a landing page [6] with results in HTML that
> may be more convenient to browse together with interactive charts.
> 
> Cheers,
> 
> Antonio
> 
> [1] https://e.huawei.com/uk/products/servers/taishan-server/taishan-2280-v2
> [2] https://en.wikichip.org/wiki/hisilicon/kunpeng/920-4826
> [3] https://www.gigabyte.com/Rack-Server/R182-Z91-rev-100
> [4] https://www.amd.com/en/products/cpu/amd-epyc-7352
> [5] https://lkml.org/lkml/2023/1/22/160
> [6] https://antonio.paolillo.be/public/rtlocks-locktorture-patches.html

Re: lock_torture results for different patches:

[Paul E. McKenney]

On Wed, Mar 01, 2023 at 05:32:14PM +0100, Antonio Paolillo wrote:
> Dear all,
> 
> I want to provide some support to Hernan regarding performance claims.
> 
> I used lock_torture to evaluate the different proposed patches on two
> different server machines:
> - a Huawei TaiShan 200 (Model 2280) rack server that has 128 GB of RAM
>   and 2x Kunpeng 920-4826 processors, a HiSilicon chip with 48 ARMv8.2
>   64-bit cores totaling 96 cores (no SMT) [1, 2],
>   denoted as taishan200-96c;
> - a GIGABYTE R182-Z91-00 rack server that has 128 GB of RAM and 2x
>   EPYC 7352 processors, an AMD chip with 24 x86_64 cores, totaling 48
>   cores (96 CPUs when counting hyperthreading) [3, 4],
>   denoted as gigabyte-96c.
> 
> I ran the evaluation on a Ubuntu 22.04 distro, with custom kernels based
> on v6.2-rc6 (6d796c50f84ca79f1722bb131799e5a5710c4700).
> The different kernels are combination of patches:
> - (0) Stock kernel;
> - (1) With relaxed set owner barrier (as discussed in [5] and questioned
>   by Peter, the barrier seems not to be needed);
> - (2) With READ_ONCE(), as originally proposed in this thread;
> - (3) With atomic_long_or() as proposed by Peter;
> - (4) With relaxed set owner barrier and READ_ONCE();
> - (5) With relaxed set owner barrier and atomic_long_or().
> 
> I ran lock_torture several times, exploring the following parameter
> space:
> - torture_type="rtmutex_lock",
> - nwriters_stress=[1, 2, 3, 4, 8, 16, 32, 64, 95],
> - stat_interval=4,
> - stutter=0,
> - shuffle_interval=0.
> For each value of "nwriters_stress", I ran the configuration 5 times.
> 
> By feeding the lock_torture kthread pids to "taskset -p", I overruled
> the scheduling such that the distribution of kthreads to CPUs is fixed.
> I also disabled "irq balance" and "numa balance" daemons, fixed the
> frequency to 1.5GHz using the "userspace" cpufreq governor and isolated
> all the cores used (using isolcpus=1-95 at boot-time) to avoid any
> source of interference.
> 
> As a warm-up phase, I ignored the first reported results and only
> considered the latest 60 seconds of execution (after all kthreads
> migrated to their final CPU).
> The reported throughput is computed by dividing the reported number of
> operations by the duration of the measurement for each dot (60 seconds),
> so higher is better.
> 
> Here follows the results on taishan200-96c (the 'rel' column is the mean
> relative to the mean of the stock kernel, in percent, and each mean is
> the average over 5 independent runs):
> 
> Kernel:             k0-stock-6.2.0-rc6       k1-rmacq             k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor            
> Statistic (kops/s):               mean   std     mean   std   rel        mean   std   rel       mean   std   rel              mean   std   rel             mean   std   rel
> nwriters_stress:                                                                                                                                                           
> 1                               899.91 24.95   880.10 29.62   -2%      871.57 44.27   -3%     888.65 37.90   -1%            898.63 29.82   -0%           889.83 25.64   -1%
> 2                               359.30 25.92   416.83 32.77  +16%      360.65 28.32   +0%     404.79 42.64  +13%            380.65 21.29   +6%           404.37 23.27  +13%
> 3                               314.97 24.32   308.41  9.68   -2%      315.00  9.97   +0%     313.86 13.47   -0%            313.47  4.01   -0%           322.77 20.82   +2%
> 4                               328.02 15.09   330.65 29.33   +1%      314.83 24.28   -4%     305.71 12.72   -7%            322.95 10.39   -2%           343.32 13.73   +5%
> 8                               292.16 22.03   288.85 10.50   -1%      288.28 18.84   -1%     285.42 24.58   -2%            310.23 26.08   +6%           285.67 20.03   -2%
> 16                              297.03 26.89   281.89 29.22   -5%      265.19 33.73  -11%     279.02 22.43   -6%            284.40 36.21   -4%           285.21 36.33   -4%
> 32                              187.36 28.59   175.71 19.77   -6%      186.44 48.15   -0%     206.59 14.11  +10%            174.08 24.30   -7%           185.80 45.12   -1%
> 64                              148.13 48.65   172.48 34.29  +16%      154.59 47.05   +4%     164.22 29.81  +11%            142.13 47.40   -4%           136.39 29.95   -8%
> 95                              174.35 57.89   148.59 38.03  -15%      156.85 43.64  -10%     132.92 32.35  -24%            126.44 28.24  -27%           146.82 60.04  -16%
> 
> And the results on gigabyte-96c:
> 
> Kernel:             k0-stock-6.2.0-rc6       k1-rmacq               k2-readonce             k3-alongor             k4-rmacq+readonce             k5-rmacq+alongor            
> Statistic (kops/s):               mean   std     mean    std    rel        mean   std   rel       mean   std   rel              mean   std   rel             mean    std  rel
> nwriters_stress:                                                                                                                                                             
> 1                               713.72 25.68   707.32  17.73    -1%      718.81 12.63   +1%     712.80 13.57   -0%            709.17 14.10   -1%           730.33   9.14  +2%
> 2                               376.25  8.19   400.09  16.24    +6%      396.71 26.09   +5%     412.61 17.80  +10%            396.48  7.02   +5%           409.90  14.61  +9%
> 3                               415.07 16.83   410.19  19.82    -1%      423.39  9.68   +2%     417.28 10.23   +1%            424.94 17.48   +2%           422.92  11.75  +2%
> 4                               286.77 26.63   285.13   6.80    -1%      297.33 23.62   +4%     296.49 16.60   +3%            303.99 30.38   +6%           296.93   9.90  +4%
> 8                               296.56 20.45   308.97  12.53    +4%      305.49 19.91   +3%     294.24 17.24   -1%            294.71 24.03   -1%           294.09  25.20  -1%
> 16                              257.34 33.94   266.03  29.60    +3%      270.72 35.22   +5%     252.28 50.16   -2%            263.83 45.84   +3%           247.42  41.01  -4%
> 32                              278.78 51.45   215.35  68.40   -23%      259.77 87.44   -7%     217.26 79.67  -22%            201.23 70.46  -28%           282.47 116.65  +1%
> 64                               75.82 64.87   194.52 137.19  +157%       35.57 12.14  -53%      74.24 72.04   -2%             71.29 45.55   -6%            77.93  43.57  +3%
> 95                               60.37 68.13   198.38 116.93  +229%       43.12 17.60  -29%      58.80 36.47   -3%             57.78 63.00   -4%            61.33  71.18  +2%
> 
> We can safely conclude that the patches do not significatively affect
> the throughput of the lock_torture benchmark for rtmutex_lock.
> The values for nwriters_stress>=64 can safely be ignored as they are too
> spread.

Just so you know, locktorture is intended to be a stress test rather
than a performance benchmark.  Hugo Guiroux's dissertation gives a
much better locking performance methodology:

https://hugoguiroux.github.io/assets/these.pdf

							Thanx, Paul

> Please notice that I pushed a landing page [6] with results in HTML that
> may be more convenient to browse together with interactive charts.
> 
> Cheers,
> 
> Antonio
> 
> [1] https://e.huawei.com/uk/products/servers/taishan-server/taishan-2280-v2
> [2] https://en.wikichip.org/wiki/hisilicon/kunpeng/920-4826
> [3] https://www.gigabyte.com/Rack-Server/R182-Z91-rev-100
> [4] https://www.amd.com/en/products/cpu/amd-epyc-7352
> [5] https://lkml.org/lkml/2023/1/22/160
> [6] https://antonio.paolillo.be/public/rtlocks-locktorture-patches.html
> 

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Paul E. McKenney]

On Tue, Jan 24, 2023 at 03:57:55PM +0100, Hernan Ponce de Leon wrote:
> On 1/23/2023 5:40 PM, Paul E. McKenney wrote:
> > On Sun, Jan 22, 2023 at 04:24:21PM +0100, Hernan Ponce de Leon wrote:
> > > On 1/20/2023 4:54 PM, Paul E. McKenney wrote:
> > > > On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote:
> > > > > On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote:
> > > > > > From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> > > > > > 
> > > > > 
> > > > > >     kernel/locking/rtmutex.c | 2 +-
> > > > > >     1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > > > > > index 010cf4e6d0b8..7ed9472edd48 100644
> > > > > > --- a/kernel/locking/rtmutex.c
> > > > > > +++ b/kernel/locking/rtmutex.c
> > > > > > @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
> > > > > >     	unsigned long owner, *p = (unsigned long *) &lock->owner;
> > > > > >     	do {
> > > > > > -		owner = *p;
> > > > > > +		owner = READ_ONCE(*p);
> > > > > >     	} while (cmpxchg_relaxed(p, owner,
> > > > > 
> > > > > 
> > > > > I don't see how this makes any difference at all.
> > > > > *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics
> > > > 
> > > > Doing so does suppress a KCSAN warning.  You could also use data_race()
> > > > if it turns out that the volatile semantics would prevent a valuable
> > > > compiler optimization.
> > > 
> > > I think the import question is "is this a harmful data race (and needs to be
> > > fixed as proposed by the patch) or a harmless one (and we should use
> > > data_race() to silence tools)?".
> > > 
> > > In https://lkml.org/lkml/2023/1/22/160 I describe how this data race can
> > > affect important ordering guarantees for the rest of the code. For this
> > > reason I consider it a harmful one. If this is not the case, I would
> > > appreciate some feedback or pointer to resources about what races care to
> > > avoid spamming the mailing list in the future.
> > 
> > In the case, the value read is passed into cmpxchg_relaxed(), which
> > checks the value against memory.  In this case, as Arjan noted, the only
> > compiler-and-silicon difference between data_race() and READ_ONCE()
> > is that use of data_race() might allow the compiler to do things like
> > tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
> > failure.  In contrast, LKMM (by design) throws up its hands when it sees
> > a data race.  Something about not being eager to track the idiosyncrasies
> > of many compiler versions.
> > 
> > My approach in my own code is to use *_ONCE() unless it causes a visible
> > performance regression or if it confuses KCSAN.  An example of the latter
> > can be debug code, in which case use of data_race() avoids suppressing
> > KCSAN warnings (and also false positives, depending).
> 
> I understand that *_ONCE() might avoid some compiler optimization and reduce
> performance in the general case. However, if I understand your first
> paragraph correctly, in this particular case data_race() could allow the CAS
> to fail more often, resulting in more spinning iterations and degraded
> performance. Am I right?

In theory, yes.  The overall effect on performance will depend on the
hardware, the compiler, the compiler version, the flags passed to that
compiler, and who knows what all else.

> > Except that your other email seems to also be arguing that additional
> > ordering is required.  So is https://lkml.org/lkml/2023/1/20/702 really
> > sufficient just by itself, or is additional ordering required?
> 
> I do not claim that we need to mark the read to add the ordering that is
> needed for correctness (mutual exclusion). What I claim in this patch is
> that there is a data race, and since it can affect ordering constrains in
> subtle ways, I consider it harmful and thus I want to fix it.
> 
> What I explain in the other email is that if we fix the data race, either
> the fence or the acquire store might be relaxed (because marking the read
> gives us some extra ordering guarantees). If the race is not fixed, both the
> fence and the acquire are needed according to LKMM. The situation is
> different wrt hardware models. In that case the tool cannot find any
> violation even if we don't fix the race and we relax the store / remove the
> fence.

Plus there might be other options, as Waiman and Peter are discussing.

							Thanx, Paul

Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Peter Zijlstra]

On Fri, Jan 20, 2023 at 02:55:25PM +0100, Hernan Ponce de Leon wrote:
> From: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> 
> Following the defition of data race in
> tools/memory-model/linux-kernel.cat the dartagnan tool
> https://github.com/hernanponcedeleon/Dat3M
> reported a race between mark_rt_mutex_waiters and rt_mutex_cmpxchg_release.
> 
> Commit 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
> later removed in commit d0aa7a70bf03 ("futex_requeue_pi optimization")
> and reverted in commit bd197234b0a6
> ("Revert "futex_requeue_pi optimization"")
> 
> The original commit introduced the data race.
> 
> Cc: stable@vger.kernel.org # v2.6.18.x
> Fixes: 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
> Signed-off-by: Hernan Ponce de Leon <hernanl.leon@huawei.com>
> ---
>  kernel/locking/rtmutex.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 010cf4e6d0b8..7ed9472edd48 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
>  	unsigned long owner, *p = (unsigned long *) &lock->owner;
>  
>  	do {
> -		owner = *p;
> +		owner = READ_ONCE(*p);
>  	} while (cmpxchg_relaxed(p, owner,
>  				 owner | RT_MUTEX_HAS_WAITERS) != owner);
>  

Can't we replace the whole of that function with:

	set_bit(0, (unsigned long *)&lock->owner);

?

RE: [PATCH] Fix data race in mark_rt_mutex_waiters

[David Laight]

From: Peter Zijlstra
> Sent: 20 January 2023 16:23
...
> >  	do {
> > -		owner = *p;
> > +		owner = READ_ONCE(*p);
> >  	} while (cmpxchg_relaxed(p, owner,
> >  				 owner | RT_MUTEX_HAS_WAITERS) != owner);
> >
> 
> Can't we replace the whole of that function with:
> 
> 	set_bit(0, (unsigned long *)&lock->owner);
> 
> ?

If you need the cast then probably not...

There really ought to be a compile-time test (somehow)
that set_bit() is only used on large bit arrays.

OTOH atomic_or32/64() and atomic_and32/64() might use
usable in many places.

On x86 I doubt it makes much difference whether you use
'bis' or 'lock or'.

	David

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