Messages in this thread Patch in this message | | | Date | Thu, 18 Jul 2019 14:23:13 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty |
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On Thu, Jul 18, 2019 at 12:45:47PM +0100, Will Deacon wrote: > On Thu, Jul 18, 2019 at 12:58:12PM +0200, Peter Zijlstra wrote: > > On Thu, Jul 18, 2019 at 10:26:41AM +0100, Will Deacon wrote: > > > > > /* > > > * We need to ensure ACQUIRE semantics when reading sem->count so that > > > * we pair with the RELEASE store performed by an unlocking/downgrading > > > * writer. > > > * > > > * P0 (writer) P1 (reader) > > > * > > > * down_write(sem); > > > * <write shared data> > > > * downgrade_write(sem); > > > * -> fetch_add_release(&sem->count) > > > * > > > * down_read_slowpath(sem); > > > * -> atomic_read(&sem->count) > > > * <ctrl dep> > > > * smp_acquire__after_ctrl_dep() > > > * <read shared data> > > > */ > > > > So I'm thinking all this is excessive; the simple rule is: lock acquire > > should imply ACQUIRE, we all know why. > > Fair enough, I just thought this was worth highlighting because you can't > reply on the wait_lock to give you ACQUIRE ordering.
Right, not in this case, because sem->count is not fully serialized by it, whereas below the wait-queue is.
> > --- > > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c > > index 37524a47f002..9eb630904a17 100644 > > --- a/kernel/locking/rwsem.c > > +++ b/kernel/locking/rwsem.c > > @@ -1000,6 +1000,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) > > atomic_long_add(-RWSEM_READER_BIAS, &sem->count); > > adjustment = 0; > > if (rwsem_optimistic_spin(sem, false)) { > > + /* rwsem_optimistic_spin() implies ACQUIRE through rwsem_*trylock() */ > > I couldn't figure out if this was dependent on the return value or not,
I went with the fact that the only way to return true is if taken becomes true; and that only happens through rwsem_try_{read,write}_lock_unqueued(), and both imply ACQUIRE on success.
> and looking at osq_lock() I also couldn't see the ACQUIRE barrier when we're > spinning on node->locked. Hmm.
Yes, osq is not a full lock and does not imply these barriers. This came up somewhere, did we forget to write a comment on that? Lemme go look.
> > /* > > * Wake up other readers in the wait list if the front > > * waiter is a reader. > > @@ -1014,6 +1015,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) > > } > > return sem; > > } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) { > > + /* rwsem_reader_phase_trylock() implies ACQUIRE */ > > Can we add "on success" to the end of this, please?
Good point.
> > return sem; > > } > > > > @@ -1032,6 +1034,8 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) > > */ > > if (adjustment && !(atomic_long_read(&sem->count) & > > (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) { > > + /* Provide lock ACQUIRE */ > > + smp_acquire__after_ctrl_dep(); > > raw_spin_unlock_irq(&sem->wait_lock); > > rwsem_set_reader_owned(sem); > > lockevent_inc(rwsem_rlock_fast); > > @@ -1065,15 +1069,25 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) > > wake_up_q(&wake_q); > > > > /* wait to be given the lock */ > > - while (true) { > > + for (;;) { > > set_current_state(state); > > - if (!waiter.task) > > + if (!smp_load_acquire(&waiter.task)) { > > + /* > > + * Matches rwsem_mark_wake()'s smp_store_release() and ensures > > + * we're ordered against its sem->count operations. > > + */ > > break; > > + } > > Ack. Also, grepping for 'waiter.task' reveals a similar usage in > drivers/tty/tty_ldsem.c if you're feeling brave enough.
*sigh* of course, for every bug there needs to be a second copy somewhere.
I'll go look there too. Thanks!
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