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 11:55:53AM +0100, Paolo Bonzini wrote:
> @@ -88,9 +88,9 @@ static void *worker_thread(void *opaque)
>  
>          do {
>              pool->idle_threads++;
> -            qemu_mutex_unlock(&pool->lock);
> +            qemu_lock_guard_unlock(&pool_guard);
>              ret = qemu_sem_timedwait(&pool->sem, 10000);
> -            qemu_mutex_lock(&pool->lock);
> +            qemu_lock_guard_lock(&pool_guard);

Or:

  QEMU_WITHOUT_LOCK_GUARD(pool_guard) {
      ret = qemu_sem_timedwait(&pool->sem, 10000);
  }

Seems funny but I like it. :)

Unfortunately it's tricky to come up with good semantics.  A 'return'
statement inside QEMU_WITHOUT_LOCK_GUARD() should leave the lock
unlocked.  But a 'break' statement may need to reacquire the lock...

> @@ -258,12 +254,12 @@ BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
>  
>      trace_thread_pool_submit(pool, req, arg);
>  
> -    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.

> @@ -330,7 +326,7 @@ void thread_pool_free(ThreadPool *pool)
>  
>      assert(QLIST_EMPTY(&pool->head));
>  
> -    qemu_mutex_lock(&pool->lock);
> +    QEMU_LOCK_GUARD(QemuMutex, pool_guard, &pool->lock);
>  
>      /* Stop new threads from spawning */
>      qemu_bh_delete(pool->new_thread_bh);
> @@ -344,7 +340,7 @@ void thread_pool_free(ThreadPool *pool)
>          qemu_cond_wait(&pool->worker_stopped, &pool->lock);
>      }
>  
> -    qemu_mutex_unlock(&pool->lock);
> +    qemu_lock_guard_unlock(&pool_guard);

Here too.  I don't see the advantage of replacing a single lock/unlock
with QEMU_LOCK_GUARD/unlock, if it cannot be made shorter/safer then
it's fine to use QemuMutex directly.

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