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Date:   Wed, 10 Apr 2019 22:25:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On 04/10/2019 02:44 PM, Peter Zijlstra wrote:
> On Fri, Apr 05, 2019 at 03:21:05PM -0400, Waiman Long wrote:
>> Because of writer lock stealing, it is possible that a constant
>> stream of incoming writers will cause a waiting writer or reader to
>> wait indefinitely leading to lock starvation.
>>
>> The mutex code has a lock handoff mechanism to prevent lock starvation.
>> This patch implements a similar lock handoff mechanism to disable
>> lock stealing and force lock handoff to the first waiter in the queue
>> after at least a 5ms waiting period. The waiting period is used to
>> avoid discouraging lock stealing too much to affect performance.
> I would say the handoff it not at all similar to the mutex code. It is
> in fact radically different.
>

I mean they are similar in concept. Of course, the implementations are
quite different.

>> @@ -131,6 +138,15 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
>>  		adjustment = RWSEM_READER_BIAS;
>>  		oldcount = atomic_long_fetch_add(adjustment, &sem->count);
>>  		if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
>> +			/*
>> +			 * Initiate handoff to reader, if applicable.
>> +			 */
>> +			if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
>> +			    time_after(jiffies, waiter->timeout)) {
>> +				adjustment -= RWSEM_FLAG_HANDOFF;
>> +				lockevent_inc(rwsem_rlock_handoff);
>> +			}
>> +
>>  			atomic_long_sub(adjustment, &sem->count);
>>  			return;
>>  		}
> That confuses the heck out of me...
>
> The above seems to rely on __rwsem_mark_wake() to be fully serialized
> (and it is, by ->wait_lock, but that isn't spelled out anywhere) such
> that we don't get double increment of FLAG_HANDOFF.
>
> So there is NO __rwsem_mark_wake() vs __wesem_mark_wake() race like:
>
>   CPU0					CPU1
>
>   oldcount = atomic_long_fetch_add(adjustment, &sem->count)
>
> 					oldcount = atomic_long_fetch_add(adjustment, &sem->count)
>
>   if (!(oldcount & HANDOFF))
>     adjustment -= HANDOFF;
>
> 					if (!(oldcount & HANDOFF))
> 					  adjustment -= HANDOFF;
>   atomic_long_sub(adjustment)
> 					atomic_long_sub(adjustment)
>
>
> *whoops* double negative decrement of HANDOFF (aka double increment).

Yes, __rwsem_mark_wake() is always called with wait_lock held. I can add
a lockdep_assert() statement to clarify this point.

>
> However there is another site that fiddles with the HANDOFF bit, namely
> __rwsem_down_write_failed_common(), and that does:
>
> +                               atomic_long_or(RWSEM_FLAG_HANDOFF, &sem->count);
>
> _OUTSIDE_ of ->wait_lock, which would yield:
>
>   CPU0					CPU1
>
>   oldcount = atomic_long_fetch_add(adjustment, &sem->count)
>
> 					atomic_long_or(HANDOFF)
>
>   if (!(oldcount & HANDOFF))
>     adjustment -= HANDOFF;
>
>   atomic_long_sub(adjustment)
>
> *whoops*, incremented HANDOFF on HANDOFF.
>
>
> And there's not a comment in sight that would elucidate if this is
> possible or not.
>

A writer can only set the handoff bit if it is the first waiter in the
queue. If it is the first waiter, a racing __rwsem_mark_wake() will see
that the first waiter is a writer and so won't go into the reader path.
I know I something don't spell out all the conditions that may look
obvious to me but not to others. I will elaborate more in comments.

> Also:
>
> +                               atomic_long_or(RWSEM_FLAG_HANDOFF, &sem->count);
> +                               first++;
> +
> +                               /*
> +                                * Make sure the handoff bit is seen by
> +                                * others before proceeding.
> +                                */
> +                               smp_mb__after_atomic();
>
> That comment is utter nonsense. smp_mb() doesn't (and cannot) 'make
> visible'. There needs to be order between two memops on both sides.
>
I kind of add that for safety. I will take some time to rethink if it is
really necessary.

Cheers,
Longman


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