Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

[Prateek Sood]

On 08/24/2017 06:22 PM, Peter Zijlstra wrote:
> On Thu, Aug 24, 2017 at 02:33:04PM +0200, Peter Zijlstra wrote:
>> On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote:
>>>
>>> WTH did you not Cc the people that commented on your patch last time?
>>>
>>> On Wed, Aug 23, 2017 at 04:58:55PM +0530, Prateek Sood wrote:
>>>> If a spinner is present, there is a chance that the load of
>>>> rwsem_has_spinner() in rwsem_wake() can be reordered with
>>>> respect to decrement of rwsem count in __up_write() leading
>>>> to wakeup being missed.
>>>
>>>>  spinning writer                  up_write caller
>>>>  ---------------                  -----------------------
>>>>  [S] osq_unlock()                 [L] osq
>>>>   spin_lock(wait_lock)
>>>>   sem->count=0xFFFFFFFF00000001
>>>>             +0xFFFFFFFF00000000
>>>>   count=sem->count
>>>>   MB
>>>>                                    sem->count=0xFFFFFFFE00000001
>>>>                                              -0xFFFFFFFF00000001
>>>>                                    RMB
>>>
>>> This doesn't make sense, it appears to order a STORE against something
>>> else.
>>>
>>>>                                    spin_trylock(wait_lock)
>>>>                                    return
>>>>  rwsem_try_write_lock(count)
>>>>  spin_unlock(wait_lock)
>>>>  schedule()
>>
>> Is this what you wanted to write?
> 
> And ideally there should be a comment near the atomic_long_add_return()
> in __rwsem_down_write_failed_common() to indicate we rely on the implied
> smp_mb() before it -- just in case someone goes and makes it
> atomic_long_add_return_relaxed().
> 
> And I suppose someone should look at the waiting branch of that thing
> too.. because I'm not sure what happens if waiting is true but count
> isn't big enough.
> 
> I bloody hate the rwsem code, that BIAS stuff forever confuses me. I
> have a start at rewriting the thing to put the owner in the lock word
> just like we now do for mutex, but never seem to get around to finishing
> it.
> 
>> ---
>>  kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
>>  1 file changed, 27 insertions(+)
>>
>> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
>> index 02f660666ab8..813b5d3654ce 100644
>> --- a/kernel/locking/rwsem-xadd.c
>> +++ b/kernel/locking/rwsem-xadd.c
>> @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
>>  	DEFINE_WAKE_Q(wake_q);
>>  
>>  	/*
>> +	 * __rwsem_down_write_failed_common(sem)
>> +	 *   rwsem_optimistic_spin(sem)
>> +	 *     osq_unlock(sem->osq)
>> +	 *   ...
>> +	 *   atomic_long_add_return(&sem->count)
>> +	 *
>> +	 *		- VS -
>> +	 *
>> +	 *			__up_write()
>> +	 *			  if (atomic_long_sub_return_release(&sem->count) < 0)
>> +	 *			    rwsem_wake(sem)
>> +	 *			      osq_is_locked(&sem->osq)
>> +	 *
>> +	 * And __up_write() must observe !osq_is_locked() when it observes the
>> +	 * atomic_long_add_return() in order to not miss a wakeup.
>> +	 *
>> +	 * This boils down to:
>> +	 *
>> +	 * [S.rel] X = 1		[RmW] r0 = (Y += 0)
>> +	 *	   MB			      RMB
>> +	 * [RmW]   Y += 1		[L]   r1 = X
>> +	 *
>> +	 * exists (r0=1 /\ r1=0)
>> +	 */
>> +	smp_rmb();
>> +
>> +	/*
>>  	 * If a spinner is present, it is not necessary to do the wakeup.
>>  	 * Try to do wakeup only if the trylock succeeds to minimize
>>  	 * spinlock contention which may introduce too much delay in the

Thanks Peter for your suggestion on comments.
I will resend the patch with updated comments

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