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Re: [Qemu-block] [Qemu-devel] [PATCH 6/6] coroutine-lock: make CoRwlock
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH 6/6] coroutine-lock: make CoRwlock thread-safe and fair |
Date: |
Wed, 15 Feb 2017 19:37:11 +0800 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Wed, 02/15 12:20, Paolo Bonzini wrote:
>
>
> On 15/02/2017 10:23, Fam Zheng wrote:
> > On Mon, 02/13 19:12, Paolo Bonzini wrote:
> >> This adds a CoMutex around the existing CoQueue. Because the write-side
> >
> > s/CoQueue/CoRwlock/
>
> No, I meant that CoRwlock has a CoQueue, and after this patch it is
> wrapped by a CoMutex too.
OK.
>
>
> >> @@ -375,16 +384,20 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
> >> qemu_co_queue_next(&lock->queue);
> >> }
> >> }
> >> - self->locks_held--;
> >> + qemu_co_mutex_unlock(&lock->mutex);
> >> }
> >>
> >> void qemu_co_rwlock_wrlock(CoRwlock *lock)
> >> {
> >> - Coroutine *self = qemu_coroutine_self();
> >> -
> >> - while (lock->writer || lock->reader) {
> >> - qemu_co_queue_wait(&lock->queue, NULL);
> >> + qemu_co_mutex_lock(&lock->mutex);
> >> + lock->pending_writer++;
> >> + while (lock->reader) {
> >> + qemu_co_queue_wait(&lock->queue, &lock->mutex);
> >> }
> >> - lock->writer = true;
> >> - self->locks_held++;
> >> + lock->pending_writer--;
> >> +
> >> + /* The rest of the write-side critical section is run with
> >> + * the mutex taken, so that lock->reader remains zero.
> >> + * There is no need to update self->locks_held.
> >> + */
> >
> > But is it still better to update self->locks_held anyway for the
> > 'assert(!co->locks_held)' in qemu_coroutine_enter? Or is the same thing
> > checked
> > elsewhere?
>
> self->locks_held is already incremented by the qemu_co_mutex_lock call
> at the beginning of qemu_co_rwlock_wrlock. It is then decremented in
> qemu_co_rwlock_unlock.
>
> For the read side, rdlock _unlocks_ lock->mutex before returning, so
> self->locks_held must be incremented by rdlock and decremented by unlock.
Makes sense.
Fam
- [Qemu-block] [PATCH 1/6] coroutine-lock: make CoMutex thread-safe, (continued)
- [Qemu-block] [PATCH 1/6] coroutine-lock: make CoMutex thread-safe, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 2/6] coroutine-lock: add limited spinning to CoMutex, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 3/6] test-aio-multithread: add performance comparison with thread-based mutexes, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 4/6] coroutine-lock: place CoMutex before CoQueue in header, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 5/6] coroutine-lock: add mutex argument to CoQueue APIs, Paolo Bonzini, 2017/02/13
- [Qemu-block] [PATCH 6/6] coroutine-lock: make CoRwlock thread-safe and fair, Paolo Bonzini, 2017/02/13
- Re: [Qemu-block] [PATCH 0/6] Make CoMutex/CoQueue/CoRwlock thread-safe, Fam Zheng, 2017/02/15
- Re: [Qemu-block] [PATCH 0/6] Make CoMutex/CoQueue/CoRwlock thread-safe, Stefan Hajnoczi, 2017/02/16