[Xen-devel] [PATCH 0/2] xen/locks: fix preempt disabling in lock handling

[Xen-devel] [PATCH 0/2] xen/locks: fix preempt disabling in lock handling

[Juergen Gross]

Xen's rwlocks don't disable preemption at all, while spinlocks are
doing it only after obtaining the lock.

While not really critical, it is wrong.

This series fixes that.

Juergen Gross (2):
  xen/rwlocks: call preempt_disable() when taking a rwlock
  xen/spinlocks: fix placement of preempt_[dis|en]able()

 xen/common/spinlock.c    |  9 ++++++---
 xen/include/xen/rwlock.h | 18 +++++++++++++++++-
 2 files changed, 23 insertions(+), 4 deletions(-)

-- 
2.16.4


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[Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Juergen Gross]

Similar to spinlocks preemption should be disabled while holding a
rwlock.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 xen/include/xen/rwlock.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
index 1c221dd0d9..4ee341a182 100644
--- a/xen/include/xen/rwlock.h
+++ b/xen/include/xen/rwlock.h
@@ -2,6 +2,7 @@
 #define __RWLOCK_H__
 
 #include <xen/percpu.h>
+#include <xen/preempt.h>
 #include <xen/smp.h>
 #include <xen/spinlock.h>
 
@@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
     cnts = atomic_read(&lock->cnts);
     if ( likely(_can_read_lock(cnts)) )
     {
+        preempt_disable();
         cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
         if ( likely(_can_read_lock(cnts)) )
             return 1;
         atomic_sub(_QR_BIAS, &lock->cnts);
+        preempt_enable();
     }
     return 0;
 }
@@ -73,6 +76,7 @@ static inline void _read_lock(rwlock_t *lock)
 {
     u32 cnts;
 
+    preempt_disable();
     cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
     if ( likely(_can_read_lock(cnts)) )
         return;
@@ -106,6 +110,7 @@ static inline void _read_unlock(rwlock_t *lock)
      * Atomically decrement the reader count
      */
     atomic_sub(_QR_BIAS, &lock->cnts);
+    preempt_enable();
 }
 
 static inline void _read_unlock_irq(rwlock_t *lock)
@@ -137,6 +142,7 @@ static inline unsigned int _write_lock_val(void)
 static inline void _write_lock(rwlock_t *lock)
 {
     /* Optimize for the unfair lock case where the fair flag is 0. */
+    preempt_disable();
     if ( atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) == 0 )
         return;
 
@@ -172,13 +178,21 @@ static inline int _write_trylock(rwlock_t *lock)
     if ( unlikely(cnts) )
         return 0;
 
-    return likely(atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) == 0);
+    preempt_disable();
+    if ( unlikely(atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) != 0) )
+    {
+        preempt_enable();
+        return 0;
+    }
+
+    return 1;
 }
 
 static inline void _write_unlock(rwlock_t *lock)
 {
     ASSERT(_is_write_locked_by_me(atomic_read(&lock->cnts)));
     atomic_and(~(_QW_CPUMASK | _QW_WMASK), &lock->cnts);
+    preempt_enable();
 }
 
 static inline void _write_unlock_irq(rwlock_t *lock)
@@ -274,6 +288,7 @@ static inline void _percpu_read_lock(percpu_rwlock_t **per_cpudata,
     }
 
     /* Indicate this cpu is reading. */
+    preempt_disable();
     this_cpu_ptr(per_cpudata) = percpu_rwlock;
     smp_mb();
     /* Check if a writer is waiting. */
@@ -308,6 +323,7 @@ static inline void _percpu_read_unlock(percpu_rwlock_t **per_cpudata,
         return;
     }
     this_cpu_ptr(per_cpudata) = NULL;
+    preempt_enable();
     smp_wmb();
 }
 
-- 
2.16.4


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[Xen-devel] [PATCH 2/2] xen/spinlocks: fix placement of preempt_[dis|en]able()

[Juergen Gross]

In case Xen ever gains preemption support the spinlock coding's
placement of preempt_disable() and preempt_enable() should be outside
of the locked section.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 xen/common/spinlock.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index 344981c54a..f05fb068cd 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -160,6 +160,7 @@ void inline _spin_lock_cb(spinlock_t *lock, void (*cb)(void *), void *data)
     LOCK_PROFILE_VAR;
 
     check_lock(&lock->debug);
+    preempt_disable();
     tickets.head_tail = arch_fetch_and_add(&lock->tickets.head_tail,
                                            tickets.head_tail);
     while ( tickets.tail != observe_head(&lock->tickets) )
@@ -171,7 +172,6 @@ void inline _spin_lock_cb(spinlock_t *lock, void (*cb)(void *), void *data)
     }
     got_lock(&lock->debug);
     LOCK_PROFILE_GOT;
-    preempt_disable();
     arch_lock_acquire_barrier();
 }
 
@@ -199,10 +199,10 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
 void _spin_unlock(spinlock_t *lock)
 {
     arch_lock_release_barrier();
-    preempt_enable();
     LOCK_PROFILE_REL;
     rel_lock(&lock->debug);
     add_sized(&lock->tickets.head, 1);
+    preempt_enable();
     arch_lock_signal();
 }
 
@@ -242,15 +242,18 @@ int _spin_trylock(spinlock_t *lock)
         return 0;
     new = old;
     new.tail++;
+    preempt_disable();
     if ( cmpxchg(&lock->tickets.head_tail,
                  old.head_tail, new.head_tail) != old.head_tail )
+    {
+        preempt_enable();
         return 0;
+    }
     got_lock(&lock->debug);
 #ifdef CONFIG_DEBUG_LOCK_PROFILE
     if (lock->profile)
         lock->profile->time_locked = NOW();
 #endif
-    preempt_disable();
     /*
      * cmpxchg() is a full barrier so no need for an
      * arch_lock_acquire_barrier().
-- 
2.16.4


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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Jan Beulich]

On 13.03.2020 09:05, Juergen Gross wrote:
> Similar to spinlocks preemption should be disabled while holding a
> rwlock.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>

Just one note/question:

> @@ -308,6 +323,7 @@ static inline void _percpu_read_unlock(percpu_rwlock_t **per_cpudata,
>          return;
>      }
>      this_cpu_ptr(per_cpudata) = NULL;
> +    preempt_enable();
>      smp_wmb();
>  }

It would seem more logical to me to insert this after the smp_wmb().
Thoughts? I'll be happy to give my R-b once we've settled on this.

Jan

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Re: [Xen-devel] [PATCH 2/2] xen/spinlocks: fix placement of preempt_[dis|en]able()

[Jan Beulich]

On 13.03.2020 09:05, Juergen Gross wrote:
> @@ -199,10 +199,10 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
>  void _spin_unlock(spinlock_t *lock)
>  {
>      arch_lock_release_barrier();
> -    preempt_enable();
>      LOCK_PROFILE_REL;
>      rel_lock(&lock->debug);
>      add_sized(&lock->tickets.head, 1);
> +    preempt_enable();
>      arch_lock_signal();
>  }

arch_lock_signal() is a barrier on Arm, hence just like for patch 1
I wonder whether the insertion wouldn't better come after it.

Jan

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Re: [Xen-devel] [PATCH 2/2] xen/spinlocks: fix placement of preempt_[dis|en]able()

[Jürgen Groß]

On 13.03.20 09:55, Jan Beulich wrote:
> On 13.03.2020 09:05, Juergen Gross wrote:
>> @@ -199,10 +199,10 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
>>   void _spin_unlock(spinlock_t *lock)
>>   {
>>       arch_lock_release_barrier();
>> -    preempt_enable();
>>       LOCK_PROFILE_REL;
>>       rel_lock(&lock->debug);
>>       add_sized(&lock->tickets.head, 1);
>> +    preempt_enable();
>>       arch_lock_signal();
>>   }
> 
> arch_lock_signal() is a barrier on Arm, hence just like for patch 1
> I wonder whether the insertion wouldn't better come after it.

Either way is fine for me. It should be noted that preemption is only
relevant on the local cpu. So this is about trading lock state
visibility against preemption disabled time, and I agree the visible
time of the lock held should be minimized at higher priority than the
preemption disabled time.

I'll modify my patches accordingly, adding a note in this regard to
the commit message.


Juergen

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Re: [Xen-devel] [PATCH 2/2] xen/spinlocks: fix placement of preempt_[dis|en]able()

[Julien Grall]

Hi,

On 13/03/2020 09:00, Jürgen Groß wrote:
> On 13.03.20 09:55, Jan Beulich wrote:
>> On 13.03.2020 09:05, Juergen Gross wrote:
>>> @@ -199,10 +199,10 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
>>>   void _spin_unlock(spinlock_t *lock)
>>>   {
>>>       arch_lock_release_barrier();
>>> -    preempt_enable();
>>>       LOCK_PROFILE_REL;
>>>       rel_lock(&lock->debug);
>>>       add_sized(&lock->tickets.head, 1);
>>> +    preempt_enable();
>>>       arch_lock_signal();
>>>   }
>>
>> arch_lock_signal() is a barrier on Arm, hence just like for patch 1
>> I wonder whether the insertion wouldn't better come after it.

The important barrier in spin_unlock() is arch_lock_release_barrier().

The one in arch_lock_signal() is just to ensure that waking up the other 
CPUs will not happen before the unlock is seen. The barrier would not 
have been necessary if the we didn't use 'sev'.

> 
> Either way is fine for me. It should be noted that preemption is only
> relevant on the local cpu. So this is about trading lock state
> visibility against preemption disabled time, and I agree the visible
> time of the lock held should be minimized at higher priority than the
> preemption disabled time.

I don't think the rationale is about "performance" here. The rationale 
is you don't know the implementation of arch_lock_signal(). If you get 
preempted by a thread trying to acquire the same lock, then it may not 
do the right thing.

Linux will also re-enable preemption only after the unlock has been 
completed. So it would be best to follow the same pattern.

Cheers,

-- 
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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Julien Grall]

Hi Juergen,

On 13/03/2020 08:05, Juergen Gross wrote:
> Similar to spinlocks preemption should be disabled while holding a
> rwlock.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
> index 1c221dd0d9..4ee341a182 100644
> --- a/xen/include/xen/rwlock.h
> +++ b/xen/include/xen/rwlock.h
> @@ -2,6 +2,7 @@
>   #define __RWLOCK_H__
>   
>   #include <xen/percpu.h>
> +#include <xen/preempt.h>
>   #include <xen/smp.h>
>   #include <xen/spinlock.h>
>   
> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>       cnts = atomic_read(&lock->cnts);
>       if ( likely(_can_read_lock(cnts)) )
>       {

If you get preempted here, then it means the check below is likely going 
to fail. So I think it would be best to disable preemption before, to 
give more chance to succeed.

> +        preempt_disable();
>           cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
>           if ( likely(_can_read_lock(cnts)) )
>               return 1;
>           atomic_sub(_QR_BIAS, &lock->cnts);
> +        preempt_enable();
>       }
>       return 0;
>   }
> @@ -73,6 +76,7 @@ static inline void _read_lock(rwlock_t *lock)
>   {
>       u32 cnts;
>   
> +    preempt_disable();
>       cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
>       if ( likely(_can_read_lock(cnts)) )
>           return;
> @@ -106,6 +110,7 @@ static inline void _read_unlock(rwlock_t *lock)
>        * Atomically decrement the reader count
>        */
>       atomic_sub(_QR_BIAS, &lock->cnts);
> +    preempt_enable();
>   }
>   
>   static inline void _read_unlock_irq(rwlock_t *lock)
> @@ -137,6 +142,7 @@ static inline unsigned int _write_lock_val(void)
>   static inline void _write_lock(rwlock_t *lock)
>   {
>       /* Optimize for the unfair lock case where the fair flag is 0. */
> +    preempt_disable();
>       if ( atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) == 0 )
>           return;
>   
> @@ -172,13 +178,21 @@ static inline int _write_trylock(rwlock_t *lock)
>       if ( unlikely(cnts) )
>           return 0;
>   
> -    return likely(atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) == 0);
> +    preempt_disable();

Similar remark as the read_trylock().

> +    if ( unlikely(atomic_cmpxchg(&lock->cnts, 0, _write_lock_val()) != 0) )
> +    {
> +        preempt_enable();
> +        return 0;
> +    }
> +
> +    return 1;
>   }
>   
>   static inline void _write_unlock(rwlock_t *lock)
>   {
>       ASSERT(_is_write_locked_by_me(atomic_read(&lock->cnts)));
>       atomic_and(~(_QW_CPUMASK | _QW_WMASK), &lock->cnts);
> +    preempt_enable();
>   }
>   
>   static inline void _write_unlock_irq(rwlock_t *lock)
> @@ -274,6 +288,7 @@ static inline void _percpu_read_lock(percpu_rwlock_t **per_cpudata,
>       }
>   
>       /* Indicate this cpu is reading. */
> +    preempt_disable();
>       this_cpu_ptr(per_cpudata) = percpu_rwlock;
>       smp_mb();
>       /* Check if a writer is waiting. */
> @@ -308,6 +323,7 @@ static inline void _percpu_read_unlock(percpu_rwlock_t **per_cpudata,
>           return;
>       }
>       this_cpu_ptr(per_cpudata) = NULL;
> +    preempt_enable();
>       smp_wmb();
>   }
>   
> 

Cheers,

-- 
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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Julien Grall]

On 13/03/2020 08:48, Jan Beulich wrote:
> On 13.03.2020 09:05, Juergen Gross wrote:
>> Similar to spinlocks preemption should be disabled while holding a
>> rwlock.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> Just one note/question:
> 
>> @@ -308,6 +323,7 @@ static inline void _percpu_read_unlock(percpu_rwlock_t **per_cpudata,
>>           return;
>>       }
>>       this_cpu_ptr(per_cpudata) = NULL;
>> +    preempt_enable();
>>       smp_wmb();
>>   }
> 
> It would seem more logical to me to insert this after the smp_wmb().

+1

> Thoughts? I'll be happy to give my R-b once we've settled on this.
> 
> Jan
> 

-- 
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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Jürgen Groß]

On 13.03.20 11:02, Julien Grall wrote:
> Hi Juergen,
> 
> On 13/03/2020 08:05, Juergen Gross wrote:
>> Similar to spinlocks preemption should be disabled while holding a
>> rwlock.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>> index 1c221dd0d9..4ee341a182 100644
>> --- a/xen/include/xen/rwlock.h
>> +++ b/xen/include/xen/rwlock.h
>> @@ -2,6 +2,7 @@
>>   #define __RWLOCK_H__
>>   #include <xen/percpu.h>
>> +#include <xen/preempt.h>
>>   #include <xen/smp.h>
>>   #include <xen/spinlock.h>
>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>       cnts = atomic_read(&lock->cnts);
>>       if ( likely(_can_read_lock(cnts)) )
>>       {
> 
> If you get preempted here, then it means the check below is likely going 
> to fail. So I think it would be best to disable preemption before, to 
> give more chance to succeed.

As preemption probability at this very point should be much lower than
that of held locks I think that is optimizing the wrong path. I'm not
opposed doing the modification you are requesting, but would like to
hear a second opinion on that topic, especially as I'd need to add
another preempt_enable() call when following your advice.


Juergen

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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Jan Beulich]

On 13.03.2020 11:15, Jürgen Groß wrote:
> On 13.03.20 11:02, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 13/03/2020 08:05, Juergen Gross wrote:
>>> Similar to spinlocks preemption should be disabled while holding a
>>> rwlock.
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>>> index 1c221dd0d9..4ee341a182 100644
>>> --- a/xen/include/xen/rwlock.h
>>> +++ b/xen/include/xen/rwlock.h
>>> @@ -2,6 +2,7 @@
>>>   #define __RWLOCK_H__
>>>   #include <xen/percpu.h>
>>> +#include <xen/preempt.h>
>>>   #include <xen/smp.h>
>>>   #include <xen/spinlock.h>
>>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>>       cnts = atomic_read(&lock->cnts);
>>>       if ( likely(_can_read_lock(cnts)) )
>>>       {
>>
>> If you get preempted here, then it means the check below is likely going 
>> to fail. So I think it would be best to disable preemption before, to 
>> give more chance to succeed.
> 
> As preemption probability at this very point should be much lower than
> that of held locks I think that is optimizing the wrong path. I'm not
> opposed doing the modification you are requesting, but would like to
> hear a second opinion on that topic, especially as I'd need to add
> another preempt_enable() call when following your advice.

I can see arguments for both placements, and hence I'm fine either
way.

Jan

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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Julien Grall]

Hi Juergen,

On 13/03/2020 10:15, Jürgen Groß wrote:
> On 13.03.20 11:02, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 13/03/2020 08:05, Juergen Gross wrote:
>>> Similar to spinlocks preemption should be disabled while holding a
>>> rwlock.
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>>> index 1c221dd0d9..4ee341a182 100644
>>> --- a/xen/include/xen/rwlock.h
>>> +++ b/xen/include/xen/rwlock.h
>>> @@ -2,6 +2,7 @@
>>>   #define __RWLOCK_H__
>>>   #include <xen/percpu.h>
>>> +#include <xen/preempt.h>
>>>   #include <xen/smp.h>
>>>   #include <xen/spinlock.h>
>>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>>       cnts = atomic_read(&lock->cnts);
>>>       if ( likely(_can_read_lock(cnts)) )
>>>       {
>>
>> If you get preempted here, then it means the check below is likely 
>> going to fail. So I think it would be best to disable preemption 
>> before, to give more chance to succeed.
> 
> As preemption probability at this very point should be much lower than
> that of held locks I think that is optimizing the wrong path.

Why so? Lock contention should be fairly limited or you already have a 
problem on your system. So preemption is more likely.

> I'm not
> opposed doing the modification you are requesting, but would like to
> hear a second opinion on that topic, especially as I'd need to add
> another preempt_enable() call when following your advice.

I don't really see the problem with adding a new preemption_enable() 
call. But the code can also be reworked to have only one call...

Cheers,


-- 
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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Jürgen Groß]

On 13.03.20 11:40, Julien Grall wrote:
> Hi Juergen,
> 
> On 13/03/2020 10:15, Jürgen Groß wrote:
>> On 13.03.20 11:02, Julien Grall wrote:
>>> Hi Juergen,
>>>
>>> On 13/03/2020 08:05, Juergen Gross wrote:
>>>> Similar to spinlocks preemption should be disabled while holding a
>>>> rwlock.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>> ---
>>>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>>>> index 1c221dd0d9..4ee341a182 100644
>>>> --- a/xen/include/xen/rwlock.h
>>>> +++ b/xen/include/xen/rwlock.h
>>>> @@ -2,6 +2,7 @@
>>>>   #define __RWLOCK_H__
>>>>   #include <xen/percpu.h>
>>>> +#include <xen/preempt.h>
>>>>   #include <xen/smp.h>
>>>>   #include <xen/spinlock.h>
>>>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>>>       cnts = atomic_read(&lock->cnts);
>>>>       if ( likely(_can_read_lock(cnts)) )
>>>>       {
>>>
>>> If you get preempted here, then it means the check below is likely 
>>> going to fail. So I think it would be best to disable preemption 
>>> before, to give more chance to succeed.
>>
>> As preemption probability at this very point should be much lower than
>> that of held locks I think that is optimizing the wrong path.
> 
> Why so? Lock contention should be fairly limited or you already have a 
> problem on your system. So preemption is more likely.

Today probability of preemption is 0.

Even with preemption added the probability to be preempted in a sequence
of about a dozen instructions is _very_ low.

> 
>> I'm not
>> opposed doing the modification you are requesting, but would like to
>> hear a second opinion on that topic, especially as I'd need to add
>> another preempt_enable() call when following your advice.
> 
> I don't really see the problem with adding a new preemption_enable() 
> call. But the code can also be reworked to have only one call...

Its the dynamical path I'm speaking of. Accessing a local cpu variable
is not that cheap, and in case we add preemption in the future
preempt_enable() will become even more expensive.


Juergen

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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Julien Grall]

Hi Juergen,

On 13/03/2020 11:23, Jürgen Groß wrote:
> On 13.03.20 11:40, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 13/03/2020 10:15, Jürgen Groß wrote:
>>> On 13.03.20 11:02, Julien Grall wrote:
>>>> Hi Juergen,
>>>>
>>>> On 13/03/2020 08:05, Juergen Gross wrote:
>>>>> Similar to spinlocks preemption should be disabled while holding a
>>>>> rwlock.
>>>>>
>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>> ---
>>>>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>>>>> index 1c221dd0d9..4ee341a182 100644
>>>>> --- a/xen/include/xen/rwlock.h
>>>>> +++ b/xen/include/xen/rwlock.h
>>>>> @@ -2,6 +2,7 @@
>>>>>   #define __RWLOCK_H__
>>>>>   #include <xen/percpu.h>
>>>>> +#include <xen/preempt.h>
>>>>>   #include <xen/smp.h>
>>>>>   #include <xen/spinlock.h>
>>>>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>>>>       cnts = atomic_read(&lock->cnts);
>>>>>       if ( likely(_can_read_lock(cnts)) )
>>>>>       {
>>>>
>>>> If you get preempted here, then it means the check below is likely 
>>>> going to fail. So I think it would be best to disable preemption 
>>>> before, to give more chance to succeed.
>>>
>>> As preemption probability at this very point should be much lower than
>>> that of held locks I think that is optimizing the wrong path.
>>
>> Why so? Lock contention should be fairly limited or you already have a 
>> problem on your system. So preemption is more likely.
> 
> Today probability of preemption is 0.

I am aware of that...

> 
> Even with preemption added the probability to be preempted in a sequence
> of about a dozen instructions is _very_ low.

... but I am not convinced of the low probability here.

> 
>>
>>> I'm not
>>> opposed doing the modification you are requesting, but would like to
>>> hear a second opinion on that topic, especially as I'd need to add
>>> another preempt_enable() call when following your advice.
>>
>> I don't really see the problem with adding a new preemption_enable() 
>> call. But the code can also be reworked to have only one call...
> 
> Its the dynamical path I'm speaking of. Accessing a local cpu variable
> is not that cheap, and in case we add preemption in the future
> preempt_enable() will become even more expensive.

Do you realize that the lock is most likely be uncontented? And if it 
were, the caller would likely not spin in a tight loop, otherwise it 
would have used read_lock().

So until you proved me otherwise (with numbers), this is 
micro-optimization that is not going to be seen in a workload.

Cheers,

-- 
Julien Grall

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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

[Jürgen Groß]

On 13.03.20 12:39, Julien Grall wrote:
> Hi Juergen,
> 
> On 13/03/2020 11:23, Jürgen Groß wrote:
>> On 13.03.20 11:40, Julien Grall wrote:
>>> Hi Juergen,
>>>
>>> On 13/03/2020 10:15, Jürgen Groß wrote:
>>>> On 13.03.20 11:02, Julien Grall wrote:
>>>>> Hi Juergen,
>>>>>
>>>>> On 13/03/2020 08:05, Juergen Gross wrote:
>>>>>> Similar to spinlocks preemption should be disabled while holding a
>>>>>> rwlock.
>>>>>>
>>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>>> ---
>>>>>>   xen/include/xen/rwlock.h | 18 +++++++++++++++++-
>>>>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
>>>>>> index 1c221dd0d9..4ee341a182 100644
>>>>>> --- a/xen/include/xen/rwlock.h
>>>>>> +++ b/xen/include/xen/rwlock.h
>>>>>> @@ -2,6 +2,7 @@
>>>>>>   #define __RWLOCK_H__
>>>>>>   #include <xen/percpu.h>
>>>>>> +#include <xen/preempt.h>
>>>>>>   #include <xen/smp.h>
>>>>>>   #include <xen/spinlock.h>
>>>>>> @@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
>>>>>>       cnts = atomic_read(&lock->cnts);
>>>>>>       if ( likely(_can_read_lock(cnts)) )
>>>>>>       {
>>>>>
>>>>> If you get preempted here, then it means the check below is likely 
>>>>> going to fail. So I think it would be best to disable preemption 
>>>>> before, to give more chance to succeed.
>>>>
>>>> As preemption probability at this very point should be much lower than
>>>> that of held locks I think that is optimizing the wrong path.
>>>
>>> Why so? Lock contention should be fairly limited or you already have 
>>> a problem on your system. So preemption is more likely.
>>
>> Today probability of preemption is 0.
> 
> I am aware of that...
> 
>>
>> Even with preemption added the probability to be preempted in a sequence
>> of about a dozen instructions is _very_ low.
> 
> ... but I am not convinced of the low probability here.
> 
>>
>>>
>>>> I'm not
>>>> opposed doing the modification you are requesting, but would like to
>>>> hear a second opinion on that topic, especially as I'd need to add
>>>> another preempt_enable() call when following your advice.
>>>
>>> I don't really see the problem with adding a new preemption_enable() 
>>> call. But the code can also be reworked to have only one call...
>>
>> Its the dynamical path I'm speaking of. Accessing a local cpu variable
>> is not that cheap, and in case we add preemption in the future
>> preempt_enable() will become even more expensive.
> 
> Do you realize that the lock is most likely be uncontented? And if it 
> were, the caller would likely not spin in a tight loop, otherwise it 
> would have used read_lock().
> 
> So until you proved me otherwise (with numbers), this is 
> micro-optimization that is not going to be seen in a workload.

Fine, in case you feeling so strong about that, I'll change it.


Juergen

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