* [PATCH] rcutorture: remove unneeded check
@ 2020-10-09 19:47 trix
2020-10-09 20:18 ` Paul E. McKenney
0 siblings, 1 reply; 9+ messages in thread
From: trix @ 2020-10-09 19:47 UTC (permalink / raw)
To: dave, paulmck, josh, rostedt, mathieu.desnoyers, jiangshanlai,
joel, natechancellor, ndesaulniers
Cc: linux-kernel, rcu, clang-built-linux, Tom Rix
From: Tom Rix <trix@redhat.com>
clang static analysis reports this problem:
rcutorture.c:1999:2: warning: Called function pointer
is null (null dereference)
cur_ops->sync(); /* Later readers see above write. */
^~~~~~~~~~~~~~~
This is a false positive triggered by an earlier, later ignored
NULL check of sync() op. By inspection of the rcu_torture_ops,
the sync() op is never uninitialized. So this earlier check is
not needed.
Signed-off-by: Tom Rix <trix@redhat.com>
---
kernel/rcu/rcutorture.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7963c8..6efc03a1d623 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
unsigned long stopat;
static DEFINE_TORTURE_RANDOM(trs);
- if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+ if (cur_ops->call && cur_ops->cb_barrier) {
init_rcu_head_on_stack(&fcs.rh);
selfpropcb = true;
}
--
2.18.1
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-09 19:47 [PATCH] rcutorture: remove unneeded check trix
@ 2020-10-09 20:18 ` Paul E. McKenney
2020-10-09 21:18 ` Tom Rix
0 siblings, 1 reply; 9+ messages in thread
From: Paul E. McKenney @ 2020-10-09 20:18 UTC (permalink / raw)
To: trix
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> From: Tom Rix <trix@redhat.com>
>
> clang static analysis reports this problem:
>
> rcutorture.c:1999:2: warning: Called function pointer
> is null (null dereference)
> cur_ops->sync(); /* Later readers see above write. */
> ^~~~~~~~~~~~~~~
>
> This is a false positive triggered by an earlier, later ignored
> NULL check of sync() op. By inspection of the rcu_torture_ops,
> the sync() op is never uninitialized. So this earlier check is
> not needed.
You lost me on this one. This check is at the very beginning of
rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
where exactly is this check?
In any case, the check is needed because all three functions are invoked
if there is a self-propagating RCU callback that ensures that there is
always an RCU grace period outstanding.
Ah. Is clang doing local analysis and assuming that because there was
a NULL check earlier, then the pointer might be NULL later? That does
not seem to me to be a sound check.
So please let me know exactly what is causing clang to emit this
diagnostic. It might or might not be worth fixing this, but either way
I need to understand the situation so as to be able to understand the
set of feasible fixes.
Thanx, Paul
> Signed-off-by: Tom Rix <trix@redhat.com>
> ---
> kernel/rcu/rcutorture.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7963c8..6efc03a1d623 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
> unsigned long stopat;
> static DEFINE_TORTURE_RANDOM(trs);
>
> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> + if (cur_ops->call && cur_ops->cb_barrier) {
> init_rcu_head_on_stack(&fcs.rh);
> selfpropcb = true;
> }
> --
> 2.18.1
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-09 20:18 ` Paul E. McKenney
@ 2020-10-09 21:18 ` Tom Rix
2020-10-09 23:50 ` Paul E. McKenney
0 siblings, 1 reply; 9+ messages in thread
From: Tom Rix @ 2020-10-09 21:18 UTC (permalink / raw)
To: paulmck
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>> From: Tom Rix <trix@redhat.com>
>>
>> clang static analysis reports this problem:
>>
>> rcutorture.c:1999:2: warning: Called function pointer
>> is null (null dereference)
>> cur_ops->sync(); /* Later readers see above write. */
>> ^~~~~~~~~~~~~~~
>>
>> This is a false positive triggered by an earlier, later ignored
>> NULL check of sync() op. By inspection of the rcu_torture_ops,
>> the sync() op is never uninitialized. So this earlier check is
>> not needed.
> You lost me on this one. This check is at the very beginning of
> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
> where exactly is this check?
>
> In any case, the check is needed because all three functions are invoked
> if there is a self-propagating RCU callback that ensures that there is
> always an RCU grace period outstanding.
>
> Ah. Is clang doing local analysis and assuming that because there was
> a NULL check earlier, then the pointer might be NULL later? That does
> not seem to me to be a sound check.
>
> So please let me know exactly what is causing clang to emit this
> diagnostic. It might or might not be worth fixing this, but either way
> I need to understand the situation so as to be able to understand the
> set of feasible fixes.
>
> Thanx, Paul
In rcu_prog_nr() there is check for for sync.
if ( ... cur_op->sync ...
do something
This flags in clang's static analyzer as 'could be null'
later in the function, in a reachable block it is used
cur_ops->sync()
I agree this is not a good check that's why i said is was a false positive.
However when looking closer at how cur_ops is set, it is never uninitialized.
So the check is not needed.
This is not a fix, the code works fine. It is a small optimization.
Tom
>
>> Signed-off-by: Tom Rix <trix@redhat.com>
>> ---
>> kernel/rcu/rcutorture.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>> index beba9e7963c8..6efc03a1d623 100644
>> --- a/kernel/rcu/rcutorture.c
>> +++ b/kernel/rcu/rcutorture.c
>> @@ -1989,7 +1989,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>> unsigned long stopat;
>> static DEFINE_TORTURE_RANDOM(trs);
>>
>> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
>> + if (cur_ops->call && cur_ops->cb_barrier) {
>> init_rcu_head_on_stack(&fcs.rh);
>> selfpropcb = true;
>> }
>> --
>> 2.18.1
>>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-09 21:18 ` Tom Rix
@ 2020-10-09 23:50 ` Paul E. McKenney
2020-10-10 0:24 ` Tom Rix
0 siblings, 1 reply; 9+ messages in thread
From: Paul E. McKenney @ 2020-10-09 23:50 UTC (permalink / raw)
To: Tom Rix
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>
> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> > On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> >> From: Tom Rix <trix@redhat.com>
> >>
> >> clang static analysis reports this problem:
> >>
> >> rcutorture.c:1999:2: warning: Called function pointer
> >> is null (null dereference)
> >> cur_ops->sync(); /* Later readers see above write. */
> >> ^~~~~~~~~~~~~~~
> >>
> >> This is a false positive triggered by an earlier, later ignored
> >> NULL check of sync() op. By inspection of the rcu_torture_ops,
> >> the sync() op is never uninitialized. So this earlier check is
> >> not needed.
> > You lost me on this one. This check is at the very beginning of
> > rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
> > earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
> > where exactly is this check?
> >
> > In any case, the check is needed because all three functions are invoked
> > if there is a self-propagating RCU callback that ensures that there is
> > always an RCU grace period outstanding.
> >
> > Ah. Is clang doing local analysis and assuming that because there was
> > a NULL check earlier, then the pointer might be NULL later? That does
> > not seem to me to be a sound check.
> >
> > So please let me know exactly what is causing clang to emit this
> > diagnostic. It might or might not be worth fixing this, but either way
> > I need to understand the situation so as to be able to understand the
> > set of feasible fixes.
> >
> > Thanx, Paul
>
> In rcu_prog_nr() there is check for for sync.
>
> if ( ... cur_op->sync ...
>
> do something
>
> This flags in clang's static analyzer as 'could be null'
>
> later in the function, in a reachable block it is used
>
> cur_ops->sync()
>
> I agree this is not a good check that's why i said is was a false positive.
>
> However when looking closer at how cur_ops is set, it is never uninitialized.
>
> So the check is not needed.
OK, got it, and thank you for the explanation.
> This is not a fix, the code works fine. It is a small optimization.
Well, there really is a bug. Yes, right now all ->sync pointers are
non-NULL, but they have not been in the past and might not be in the
future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
not be called or it should return immediately without doing anything.
My current thought is something like the (untested) patch below, of
course with your Reported-by.
Thoughts?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7..44749be 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
unsigned long stopat;
static DEFINE_TORTURE_RANDOM(trs);
- if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+ if (!cur_ops->sync)
+ return; // Cannot do need_resched() forward progress testing without ->sync.
+ if (cur_ops->call && cur_ops->cb_barrier) {
init_rcu_head_on_stack(&fcs.rh);
selfpropcb = true;
}
@@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
if (!fwd_progress)
return 0; /* Not requested, so don't do it. */
- if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
- cur_ops == &rcu_busted_ops) {
+ if ((!cur_ops->sync && !cur_ops->call) ||
+ !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
return 0;
}
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-09 23:50 ` Paul E. McKenney
@ 2020-10-10 0:24 ` Tom Rix
2020-10-10 2:57 ` Paul E. McKenney
0 siblings, 1 reply; 9+ messages in thread
From: Tom Rix @ 2020-10-10 0:24 UTC (permalink / raw)
To: paulmck
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>> From: Tom Rix <trix@redhat.com>
>>>>
>>>> clang static analysis reports this problem:
>>>>
>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>> is null (null dereference)
>>>> cur_ops->sync(); /* Later readers see above write. */
>>>> ^~~~~~~~~~~~~~~
>>>>
>>>> This is a false positive triggered by an earlier, later ignored
>>>> NULL check of sync() op. By inspection of the rcu_torture_ops,
>>>> the sync() op is never uninitialized. So this earlier check is
>>>> not needed.
>>> You lost me on this one. This check is at the very beginning of
>>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
>>> where exactly is this check?
>>>
>>> In any case, the check is needed because all three functions are invoked
>>> if there is a self-propagating RCU callback that ensures that there is
>>> always an RCU grace period outstanding.
>>>
>>> Ah. Is clang doing local analysis and assuming that because there was
>>> a NULL check earlier, then the pointer might be NULL later? That does
>>> not seem to me to be a sound check.
>>>
>>> So please let me know exactly what is causing clang to emit this
>>> diagnostic. It might or might not be worth fixing this, but either way
>>> I need to understand the situation so as to be able to understand the
>>> set of feasible fixes.
>>>
>>> Thanx, Paul
>> In rcu_prog_nr() there is check for for sync.
>>
>> if ( ... cur_op->sync ...
>>
>> do something
>>
>> This flags in clang's static analyzer as 'could be null'
>>
>> later in the function, in a reachable block it is used
>>
>> cur_ops->sync()
>>
>> I agree this is not a good check that's why i said is was a false positive.
>>
>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>
>> So the check is not needed.
> OK, got it, and thank you for the explanation.
>
>> This is not a fix, the code works fine. It is a small optimization.
> Well, there really is a bug. Yes, right now all ->sync pointers are
> non-NULL, but they have not been in the past and might not be in the
> future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> not be called or it should return immediately without doing anything.
>
> My current thought is something like the (untested) patch below, of
> course with your Reported-by.
>
> Thoughts?
Yes that would be fine.
In in review these other cases need to be been take care of.
Thanks,
Reported-by: Tom Rix <trix@redhat.com>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
> unsigned long stopat;
> static DEFINE_TORTURE_RANDOM(trs);
>
> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> + if (!cur_ops->sync)
> + return; // Cannot do need_resched() forward progress testing without ->sync.
> + if (cur_ops->call && cur_ops->cb_barrier) {
> init_rcu_head_on_stack(&fcs.rh);
> selfpropcb = true;
> }
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>
> if (!fwd_progress)
> return 0; /* Not requested, so don't do it. */
> - if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> - cur_ops == &rcu_busted_ops) {
> + if ((!cur_ops->sync && !cur_ops->call) ||
> + !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
> VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
> return 0;
> }
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-10 0:24 ` Tom Rix
@ 2020-10-10 2:57 ` Paul E. McKenney
2020-10-10 4:47 ` Tom Rix
2020-10-10 17:57 ` Paul E. McKenney
0 siblings, 2 replies; 9+ messages in thread
From: Paul E. McKenney @ 2020-10-10 2:57 UTC (permalink / raw)
To: Tom Rix
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
>
> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> > On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
> >> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> >>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> >>>> From: Tom Rix <trix@redhat.com>
> >>>>
> >>>> clang static analysis reports this problem:
> >>>>
> >>>> rcutorture.c:1999:2: warning: Called function pointer
> >>>> is null (null dereference)
> >>>> cur_ops->sync(); /* Later readers see above write. */
> >>>> ^~~~~~~~~~~~~~~
> >>>>
> >>>> This is a false positive triggered by an earlier, later ignored
> >>>> NULL check of sync() op. By inspection of the rcu_torture_ops,
> >>>> the sync() op is never uninitialized. So this earlier check is
> >>>> not needed.
> >>> You lost me on this one. This check is at the very beginning of
> >>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
> >>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
> >>> where exactly is this check?
> >>>
> >>> In any case, the check is needed because all three functions are invoked
> >>> if there is a self-propagating RCU callback that ensures that there is
> >>> always an RCU grace period outstanding.
> >>>
> >>> Ah. Is clang doing local analysis and assuming that because there was
> >>> a NULL check earlier, then the pointer might be NULL later? That does
> >>> not seem to me to be a sound check.
> >>>
> >>> So please let me know exactly what is causing clang to emit this
> >>> diagnostic. It might or might not be worth fixing this, but either way
> >>> I need to understand the situation so as to be able to understand the
> >>> set of feasible fixes.
> >>>
> >>> Thanx, Paul
> >> In rcu_prog_nr() there is check for for sync.
> >>
> >> if ( ... cur_op->sync ...
> >>
> >> do something
> >>
> >> This flags in clang's static analyzer as 'could be null'
> >>
> >> later in the function, in a reachable block it is used
> >>
> >> cur_ops->sync()
> >>
> >> I agree this is not a good check that's why i said is was a false positive.
> >>
> >> However when looking closer at how cur_ops is set, it is never uninitialized.
> >>
> >> So the check is not needed.
> > OK, got it, and thank you for the explanation.
> >
> >> This is not a fix, the code works fine. It is a small optimization.
> > Well, there really is a bug. Yes, right now all ->sync pointers are
> > non-NULL, but they have not been in the past and might not be in the
> > future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> > not be called or it should return immediately without doing anything.
> >
> > My current thought is something like the (untested) patch below, of
> > course with your Reported-by.
> >
> > Thoughts?
>
> Yes that would be fine.
>
> In in review these other cases need to be been take care of.
I am having a difficult time interpreting this sentence, but will
optimistically assume that it means that you are good with this approach.
If my optimism is unwarranted, please let me know so I can fix whatever
might be broken.
> Reported-by: Tom Rix <trix@redhat.com>
How does the commit below look?
Thanx, Paul
------------------------------------------------------------------------
commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Fri Oct 9 19:51:55 2020 -0700
rcutorture: Don't do need_resched() testing if ->sync is NULL
If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
attempt to call through it. This commit therefore flags cases where
neither need_resched() nor call_rcu() forward-progress testing
can be performed due to NULL function pointers, and also causes
rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
is NULL.
Reported-by: Tom Rix <trix@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index beba9e7..44749be 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
unsigned long stopat;
static DEFINE_TORTURE_RANDOM(trs);
- if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+ if (!cur_ops->sync)
+ return; // Cannot do need_resched() forward progress testing without ->sync.
+ if (cur_ops->call && cur_ops->cb_barrier) {
init_rcu_head_on_stack(&fcs.rh);
selfpropcb = true;
}
@@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
if (!fwd_progress)
return 0; /* Not requested, so don't do it. */
- if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
- cur_ops == &rcu_busted_ops) {
+ if ((!cur_ops->sync && !cur_ops->call) ||
+ !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
return 0;
}
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-10 2:57 ` Paul E. McKenney
@ 2020-10-10 4:47 ` Tom Rix
2020-10-10 17:57 ` Paul E. McKenney
1 sibling, 0 replies; 9+ messages in thread
From: Tom Rix @ 2020-10-10 4:47 UTC (permalink / raw)
To: paulmck
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On 10/9/20 7:57 PM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
>> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
>>> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>>>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>>>> From: Tom Rix <trix@redhat.com>
>>>>>>
>>>>>> clang static analysis reports this problem:
>>>>>>
>>>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>>>> is null (null dereference)
>>>>>> cur_ops->sync(); /* Later readers see above write. */
>>>>>> ^~~~~~~~~~~~~~~
>>>>>>
>>>>>> This is a false positive triggered by an earlier, later ignored
>>>>>> NULL check of sync() op. By inspection of the rcu_torture_ops,
>>>>>> the sync() op is never uninitialized. So this earlier check is
>>>>>> not needed.
>>>>> You lost me on this one. This check is at the very beginning of
>>>>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
>>>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
>>>>> where exactly is this check?
>>>>>
>>>>> In any case, the check is needed because all three functions are invoked
>>>>> if there is a self-propagating RCU callback that ensures that there is
>>>>> always an RCU grace period outstanding.
>>>>>
>>>>> Ah. Is clang doing local analysis and assuming that because there was
>>>>> a NULL check earlier, then the pointer might be NULL later? That does
>>>>> not seem to me to be a sound check.
>>>>>
>>>>> So please let me know exactly what is causing clang to emit this
>>>>> diagnostic. It might or might not be worth fixing this, but either way
>>>>> I need to understand the situation so as to be able to understand the
>>>>> set of feasible fixes.
>>>>>
>>>>> Thanx, Paul
>>>> In rcu_prog_nr() there is check for for sync.
>>>>
>>>> if ( ... cur_op->sync ...
>>>>
>>>>   do something
>>>>
>>>> This flags in clang's static analyzer as 'could be null'
>>>>
>>>> later in the function, in a reachable block it is used
>>>>
>>>> cur_ops->sync()
>>>>
>>>> I agree this is not a good check that's why i said is was a false positive.
>>>>
>>>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>>>
>>>> So the check is not needed.
>>> OK, got it, and thank you for the explanation.
>>>
>>>> This is not a fix, the code works fine. It is a small optimization.
>>> Well, there really is a bug. Yes, right now all ->sync pointers are
>>> non-NULL, but they have not been in the past and might not be in the
>>> future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
>>> not be called or it should return immediately without doing anything.
>>>
>>> My current thought is something like the (untested) patch below, of
>>> course with your Reported-by.
>>>
>>> Thoughts?
>> Yes that would be fine.
>>
>> In in review these other cases need to be been take care of.
> I am having a difficult time interpreting this sentence, but will
> optimistically assume that it means that you are good with this approach.
> If my optimism is unwarranted, please let me know so I can fix whatever
> might be broken.
>
>> Reported-by: Tom Rix <trix@redhat.com>
> How does the commit below look?
Looks fine.
Thanks
Tom
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date: Fri Oct 9 19:51:55 2020 -0700
>
> rcutorture: Don't do need_resched() testing if ->sync is NULL
>
> If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
> attempt to call through it. This commit therefore flags cases where
> neither need_resched() nor call_rcu() forward-progress testing
> can be performed due to NULL function pointers, and also causes
> rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
> is NULL.
>
> Reported-by: Tom Rix <trix@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
> unsigned long stopat;
> static DEFINE_TORTURE_RANDOM(trs);
>
> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> + if (!cur_ops->sync)
> + return; // Cannot do need_resched() forward progress testing without ->sync.
> + if (cur_ops->call && cur_ops->cb_barrier) {
> init_rcu_head_on_stack(&fcs.rh);
> selfpropcb = true;
> }
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>
> if (!fwd_progress)
> return 0; /* Not requested, so don't do it. */
> - if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> - cur_ops == &rcu_busted_ops) {
> + if ((!cur_ops->sync && !cur_ops->call) ||
> + !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
> VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
> return 0;
> }
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-10 2:57 ` Paul E. McKenney
2020-10-10 4:47 ` Tom Rix
@ 2020-10-10 17:57 ` Paul E. McKenney
2020-10-10 18:07 ` Tom Rix
1 sibling, 1 reply; 9+ messages in thread
From: Paul E. McKenney @ 2020-10-10 17:57 UTC (permalink / raw)
To: Tom Rix
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On Fri, Oct 09, 2020 at 07:57:43PM -0700, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
> >
> > On 10/9/20 4:50 PM, Paul E. McKenney wrote:
> > > On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
> > >> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
> > >>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
> > >>>> From: Tom Rix <trix@redhat.com>
> > >>>>
> > >>>> clang static analysis reports this problem:
> > >>>>
> > >>>> rcutorture.c:1999:2: warning: Called function pointer
> > >>>> is null (null dereference)
> > >>>> cur_ops->sync(); /* Later readers see above write. */
> > >>>> ^~~~~~~~~~~~~~~
> > >>>>
> > >>>> This is a false positive triggered by an earlier, later ignored
> > >>>> NULL check of sync() op. By inspection of the rcu_torture_ops,
> > >>>> the sync() op is never uninitialized. So this earlier check is
> > >>>> not needed.
> > >>> You lost me on this one. This check is at the very beginning of
> > >>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
> > >>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
> > >>> where exactly is this check?
> > >>>
> > >>> In any case, the check is needed because all three functions are invoked
> > >>> if there is a self-propagating RCU callback that ensures that there is
> > >>> always an RCU grace period outstanding.
> > >>>
> > >>> Ah. Is clang doing local analysis and assuming that because there was
> > >>> a NULL check earlier, then the pointer might be NULL later? That does
> > >>> not seem to me to be a sound check.
In this case, the diagnostic was clearly pointing out a latent bug, so
my bad. So more of a code-review comment than a diagnostic. Therefore,
if clang really wants to be in the code-review space, I suggest that it
more clearly explain its code-review feedback. ;-)
Thanx, Paul
> > >>> So please let me know exactly what is causing clang to emit this
> > >>> diagnostic. It might or might not be worth fixing this, but either way
> > >>> I need to understand the situation so as to be able to understand the
> > >>> set of feasible fixes.
> > >>>
> > >>> Thanx, Paul
> > >> In rcu_prog_nr() there is check for for sync.
> > >>
> > >> if ( ... cur_op->sync ...
> > >>
> > >> do something
> > >>
> > >> This flags in clang's static analyzer as 'could be null'
> > >>
> > >> later in the function, in a reachable block it is used
> > >>
> > >> cur_ops->sync()
> > >>
> > >> I agree this is not a good check that's why i said is was a false positive.
> > >>
> > >> However when looking closer at how cur_ops is set, it is never uninitialized.
> > >>
> > >> So the check is not needed.
> > > OK, got it, and thank you for the explanation.
> > >
> > >> This is not a fix, the code works fine. It is a small optimization.
> > > Well, there really is a bug. Yes, right now all ->sync pointers are
> > > non-NULL, but they have not been in the past and might not be in the
> > > future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
> > > not be called or it should return immediately without doing anything.
> > >
> > > My current thought is something like the (untested) patch below, of
> > > course with your Reported-by.
> > >
> > > Thoughts?
> >
> > Yes that would be fine.
> >
> > In in review these other cases need to be been take care of.
>
> I am having a difficult time interpreting this sentence, but will
> optimistically assume that it means that you are good with this approach.
> If my optimism is unwarranted, please let me know so I can fix whatever
> might be broken.
>
> > Reported-by: Tom Rix <trix@redhat.com>
>
> How does the commit below look?
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date: Fri Oct 9 19:51:55 2020 -0700
>
> rcutorture: Don't do need_resched() testing if ->sync is NULL
>
> If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
> attempt to call through it. This commit therefore flags cases where
> neither need_resched() nor call_rcu() forward-progress testing
> can be performed due to NULL function pointers, and also causes
> rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
> is NULL.
>
> Reported-by: Tom Rix <trix@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index beba9e7..44749be 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
> unsigned long stopat;
> static DEFINE_TORTURE_RANDOM(trs);
>
> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
> + if (!cur_ops->sync)
> + return; // Cannot do need_resched() forward progress testing without ->sync.
> + if (cur_ops->call && cur_ops->cb_barrier) {
> init_rcu_head_on_stack(&fcs.rh);
> selfpropcb = true;
> }
> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>
> if (!fwd_progress)
> return 0; /* Not requested, so don't do it. */
> - if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
> - cur_ops == &rcu_busted_ops) {
> + if ((!cur_ops->sync && !cur_ops->call) ||
> + !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
> VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
> return 0;
> }
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH] rcutorture: remove unneeded check
2020-10-10 17:57 ` Paul E. McKenney
@ 2020-10-10 18:07 ` Tom Rix
0 siblings, 0 replies; 9+ messages in thread
From: Tom Rix @ 2020-10-10 18:07 UTC (permalink / raw)
To: paulmck
Cc: dave, josh, rostedt, mathieu.desnoyers, jiangshanlai, joel,
natechancellor, ndesaulniers, linux-kernel, rcu,
clang-built-linux
On 10/10/20 10:57 AM, Paul E. McKenney wrote:
> On Fri, Oct 09, 2020 at 07:57:43PM -0700, Paul E. McKenney wrote:
>> On Fri, Oct 09, 2020 at 05:24:37PM -0700, Tom Rix wrote:
>>> On 10/9/20 4:50 PM, Paul E. McKenney wrote:
>>>> On Fri, Oct 09, 2020 at 02:18:41PM -0700, Tom Rix wrote:
>>>>> On 10/9/20 1:18 PM, Paul E. McKenney wrote:
>>>>>> On Fri, Oct 09, 2020 at 12:47:36PM -0700, trix@redhat.com wrote:
>>>>>>> From: Tom Rix <trix@redhat.com>
>>>>>>>
>>>>>>> clang static analysis reports this problem:
>>>>>>>
>>>>>>> rcutorture.c:1999:2: warning: Called function pointer
>>>>>>> is null (null dereference)
>>>>>>> cur_ops->sync(); /* Later readers see above write. */
>>>>>>> ^~~~~~~~~~~~~~~
>>>>>>>
>>>>>>> This is a false positive triggered by an earlier, later ignored
>>>>>>> NULL check of sync() op. By inspection of the rcu_torture_ops,
>>>>>>> the sync() op is never uninitialized. So this earlier check is
>>>>>>> not needed.
>>>>>> You lost me on this one. This check is at the very beginning of
>>>>>> rcu_torture_fwd_prog_nr(). Or are you saying that clang is seeing an
>>>>>> earlier check in one of rcu_torture_fwd_prog_nr()'s callers? If so,
>>>>>> where exactly is this check?
>>>>>>
>>>>>> In any case, the check is needed because all three functions are invoked
>>>>>> if there is a self-propagating RCU callback that ensures that there is
>>>>>> always an RCU grace period outstanding.
>>>>>>
>>>>>> Ah. Is clang doing local analysis and assuming that because there was
>>>>>> a NULL check earlier, then the pointer might be NULL later? That does
>>>>>> not seem to me to be a sound check.
> In this case, the diagnostic was clearly pointing out a latent bug, so
> my bad. So more of a code-review comment than a diagnostic. Therefore,
> if clang really wants to be in the code-review space, I suggest that it
> more clearly explain its code-review feedback. ;-)
Ok.
I have another a change in other area queued up.
I'll improve it's and future comments.
Tom
> Thanx, Paul
>
>>>>>> So please let me know exactly what is causing clang to emit this
>>>>>> diagnostic. It might or might not be worth fixing this, but either way
>>>>>> I need to understand the situation so as to be able to understand the
>>>>>> set of feasible fixes.
>>>>>>
>>>>>> Thanx, Paul
>>>>> In rcu_prog_nr() there is check for for sync.
>>>>>
>>>>> if ( ... cur_op->sync ...
>>>>>
>>>>> do something
>>>>>
>>>>> This flags in clang's static analyzer as 'could be null'
>>>>>
>>>>> later in the function, in a reachable block it is used
>>>>>
>>>>> cur_ops->sync()
>>>>>
>>>>> I agree this is not a good check that's why i said is was a false positive.
>>>>>
>>>>> However when looking closer at how cur_ops is set, it is never uninitialized.
>>>>>
>>>>> So the check is not needed.
>>>> OK, got it, and thank you for the explanation.
>>>>
>>>>> This is not a fix, the code works fine. It is a small optimization.
>>>> Well, there really is a bug. Yes, right now all ->sync pointers are
>>>> non-NULL, but they have not been in the past and might not be in the
>>>> future. So if ->sync is NULL, rcu_torture_fwd_prog_nr() either should
>>>> not be called or it should return immediately without doing anything.
>>>>
>>>> My current thought is something like the (untested) patch below, of
>>>> course with your Reported-by.
>>>>
>>>> Thoughts?
>>> Yes that would be fine.
>>>
>>> In in review these other cases need to be been take care of.
>> I am having a difficult time interpreting this sentence, but will
>> optimistically assume that it means that you are good with this approach.
>> If my optimism is unwarranted, please let me know so I can fix whatever
>> might be broken.
>>
>>> Reported-by: Tom Rix <trix@redhat.com>
>> How does the commit below look?
>>
>> Thanx, Paul
>>
>> ------------------------------------------------------------------------
>>
>> commit 75c79a5dd72c1bb59f6bd6c5ec36f3a6516795cd
>> Author: Paul E. McKenney <paulmck@kernel.org>
>> Date: Fri Oct 9 19:51:55 2020 -0700
>>
>> rcutorture: Don't do need_resched() testing if ->sync is NULL
>>
>> If cur_ops->sync is NULL, rcu_torture_fwd_prog_nr() will nevertheless
>> attempt to call through it. This commit therefore flags cases where
>> neither need_resched() nor call_rcu() forward-progress testing
>> can be performed due to NULL function pointers, and also causes
>> rcu_torture_fwd_prog_nr() to take an early exit if cur_ops->sync()
>> is NULL.
>>
>> Reported-by: Tom Rix <trix@redhat.com>
>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>
>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>> index beba9e7..44749be 100644
>> --- a/kernel/rcu/rcutorture.c
>> +++ b/kernel/rcu/rcutorture.c
>> @@ -1989,7 +1989,9 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>> unsigned long stopat;
>> static DEFINE_TORTURE_RANDOM(trs);
>>
>> - if (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
>> + if (!cur_ops->sync)
>> + return; // Cannot do need_resched() forward progress testing without ->sync.
>> + if (cur_ops->call && cur_ops->cb_barrier) {
>> init_rcu_head_on_stack(&fcs.rh);
>> selfpropcb = true;
>> }
>> @@ -2215,8 +2217,8 @@ static int __init rcu_torture_fwd_prog_init(void)
>>
>> if (!fwd_progress)
>> return 0; /* Not requested, so don't do it. */
>> - if (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0 ||
>> - cur_ops == &rcu_busted_ops) {
>> + if ((!cur_ops->sync && !cur_ops->call) ||
>> + !cur_ops->stall_dur || cur_ops->stall_dur() <= 0 || cur_ops == &rcu_busted_ops) {
>> VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
>> return 0;
>> }
^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2020-10-10 22:51 UTC | newest]
Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-10-09 19:47 [PATCH] rcutorture: remove unneeded check trix
2020-10-09 20:18 ` Paul E. McKenney
2020-10-09 21:18 ` Tom Rix
2020-10-09 23:50 ` Paul E. McKenney
2020-10-10 0:24 ` Tom Rix
2020-10-10 2:57 ` Paul E. McKenney
2020-10-10 4:47 ` Tom Rix
2020-10-10 17:57 ` Paul E. McKenney
2020-10-10 18:07 ` Tom Rix
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