Re: [Qemu-devel] [PATCH 5/5] thread-pool: convert to use lock guards

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Dec 08, 2017 at 02:02:32PM -0600, Eric Blake wrote:
> On 12/08/2017 12:12 PM, Paolo Bonzini wrote:
> > On 08/12/2017 16:13, Stefan Hajnoczi wrote:
> >>> -    qemu_mutex_lock(&pool->lock);
> >>> +    QEMU_LOCK_GUARD(QemuMutex, pool_guard, &pool->lock);
> >>>      if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
> >>>          spawn_thread(pool);
> >>>      }
> >>>      QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
> >>> -    qemu_mutex_unlock(&pool->lock);
> >>> +    qemu_lock_guard_unlock(&pool_guard);
> >> Why not QEMU_WITH_LOCK()?  Then you can get rid of the explicit unlock.
> > 
> > I agree that QEMU_WITH_LOCK_GUARD is better in this case. (IIRC I wrote
> > this patch before coming up with the is_taken trick!).
> > 
> > My main question for the series is what you think the balance should be
> > between a more widely applicable API and a simpler one.
> 
> If you require the user to provide the scope, this could be:
> 
> @@ -258,12 +254,12 @@ BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
> 
>      trace_thread_pool_submit(pool, req, arg);
> 
> -    qemu_mutex_lock(&pool->lock);
> -    if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
> -        spawn_thread(pool);
> -    QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
> +    {
> +        QEMU_LOCK_GUARD(QemuMutex, pool_guard, &pool->lock);
> +        if (pool->idle_threads == 0 && pool->cur_threads <
> pool->max_threads) {
> +            spawn_thread(pool);
> +        }
> +        QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
>      }
> -    qemu_mutex_unlock(&pool->lock);
>      qemu_sem_post(&pool->sem);
>      return &req->common;
>  }
> 
> In other words, I don't see what 'QEMU_WITH_LOCK_GUARD() {}' buys us
> over '{ QEMU_LOCK_GUARD() }'.

The QEMU_WITH_LOCK_GUARD() {} syntax is nice because it's similar to
if/while/for statements.

However, { QEMU_LOCK_GUARD() } doesn't hide a for statement in a macro
so the break statement works inside the scope.  Less chance of bugs.

I'd be okay without QEMU_WITH_LOCK_GUARD().

Stefan

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