Messages in this thread Patch in this message | | | Date | Tue, 28 Mar 2023 16:02:59 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 8/8] locking/rwsem: Restore old write lock slow path behavior |
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On Mon, Mar 27, 2023 at 04:24:13PM -0400, Waiman Long wrote:
> kernel/locking/rwsem.c | 34 +++++++++++++++++++++++++++++++--- > 1 file changed, 31 insertions(+), 3 deletions(-) > > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c > index 7bd26e64827a..cf9dc1e250e0 100644 > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -426,6 +426,7 @@ rwsem_waiter_wake(struct rwsem_waiter *waiter, struct wake_q_head *wake_q) > static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > struct rwsem_waiter *waiter) > { > + bool first = (rwsem_first_waiter(sem) == waiter); > long count, new; > > lockdep_assert_held(&sem->wait_lock); > @@ -434,6 +435,9 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > do { > new = count; > > + if (!first && (count & (RWSEM_FLAG_HANDOFF | RWSEM_LOCK_MASK))) > + return false; > + > if (count & RWSEM_LOCK_MASK) { > /* > * A waiter (first or not) can set the handoff bit
I couldn't immediately make sense of the above, and I think there's case where not-first would still want to set handoff you're missing.
After a few detours, I ended up with the below; does that make sense or did I just make a bigger mess? (entirely possible due to insufficient sleep etc..).
--- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -426,12 +426,27 @@ rwsem_waiter_wake(struct rwsem_waiter *w static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, struct rwsem_waiter *waiter) { + bool first = (rwsem_first_waiter(sem) == waiter); long count, new; lockdep_assert_held(&sem->wait_lock); count = atomic_long_read(&sem->count); do { + /* + * first handoff + * + * 0 0 | take + * 0 1 | not take + * 1 0 | take + * 1 1 | take + * + */ + bool handoff = count & RWSEM_FLAG_HANDOFF; + + if (!first && handoff) + return false; + new = count; if (count & RWSEM_LOCK_MASK) { @@ -440,7 +455,7 @@ static inline bool rwsem_try_write_lock( * if it is an RT task or wait in the wait queue * for too long. */ - if ((count & RWSEM_FLAG_HANDOFF) || + if (handoff || (!rt_task(waiter->task) && !time_after(jiffies, waiter->timeout))) return false; @@ -501,11 +516,19 @@ static void rwsem_writer_wake(struct rw_ */ list_del(&waiter->list); atomic_long_set(&sem->owner, (long)waiter->task); - - } else if (!rwsem_try_write_lock(sem, waiter)) + rwsem_waiter_wake(waiter, wake_q); return; + } - rwsem_waiter_wake(waiter, wake_q); + /* + * Mark writer at the front of the queue for wakeup. + * + * Until the task is actually awoken later by the caller, other writers + * are able to steal it. Readers, on the other hand, will block as they + * will notice the queued writer. + */ + wake_q_add(wake_q, waiter->task); + lockevent_inc(rwsem_wake_writer); } static void rwsem_reader_wake(struct rw_semaphore *sem, @@ -1038,6 +1061,25 @@ rwsem_waiter_wait(struct rw_semaphore *s /* Matches rwsem_waiter_wake()'s smp_store_release(). */ break; } + if (!reader) { + /* + * If rwsem_writer_wake() did not take the lock, we must + * rwsem_try_write_lock() here. + */ + raw_spin_lock_irq(&sem->wait_lock); + if (waiter->task && rwsem_try_write_lock(sem, waiter)) { + waiter->task = NULL; + raw_spin_unlock_irq(&sem->wait_lock); + break; + } + raw_spin_unlock_irq(&sem->wait_lock); + + if (waiter->handoff_set) + rwsem_spin_on_owner(sem); + + if (!smp_load_acquire(&waiter->task)) + break; + } if (signal_pending_state(state, current)) { raw_spin_lock_irq(&sem->wait_lock); if (waiter->task)
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