Messages in this thread | | | Date | Wed, 10 Apr 2019 20:44:29 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH-tip v2 02/12] locking/rwsem: Implement lock handoff to prevent lock starvation |
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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.
> @@ -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).
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.
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.
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