vulkan: remove pointless mutex locks

This code was simply incorrect through and through. It did not
protect what actually has to be protected in a multi-threaded setup.
Perhaps it was used to silence threading errors?

Either way, remove it, and document the correct way to use execution
pools in a threaded environment.

libavutil/vulkan.c

@@ -251,7 +251,6 @@ void ff_vk_exec_pool_free(FFVulkanContext *s, FFVkExecPool *pool)
                 vk->WaitForFences(s->hwctx->act_dev, 1, &e->fence, VK_TRUE, UINT64_MAX);
             vk->DestroyFence(s->hwctx->act_dev, e->fence, s->hwctx->alloc);
         }
-        pthread_mutex_destroy(&e->lock);
 
         ff_vk_exec_discard_deps(s, e);
 
@@ -424,11 +423,6 @@ int ff_vk_exec_pool_init(FFVulkanContext *s, FFVkQueueFamilyCtx *qf,
             .flags = VK_FENCE_CREATE_SIGNALED_BIT,
         };
 
-        /* Mutex */
-        err = pthread_mutex_init(&e->lock, NULL);
-        if (err != 0)
-            return AVERROR(err);
-
         /* Fence */
         ret = vk->CreateFence(s->hwctx->act_dev, &fence_create, s->hwctx->alloc,
                               &e->fence);
@@ -498,10 +492,8 @@ FFVkExecContext *ff_vk_exec_get(FFVulkanContext *s, FFVkExecPool *pool)
 void ff_vk_exec_wait(FFVulkanContext *s, FFVkExecContext *e)
 {
     FFVulkanFunctions *vk = &s->vkfn;
-    pthread_mutex_lock(&e->lock);
     vk->WaitForFences(s->hwctx->act_dev, 1, &e->fence, VK_TRUE, UINT64_MAX);
     ff_vk_exec_discard_deps(s, e);
-    pthread_mutex_unlock(&e->lock);
 }
 
 int ff_vk_exec_start(FFVulkanContext *s, FFVkExecContext *e)
@@ -517,11 +509,7 @@ int ff_vk_exec_start(FFVulkanContext *s, FFVkExecContext *e)
 
     /* Wait for the fence to be signalled */
     vk->WaitForFences(s->hwctx->act_dev, 1, &e->fence, VK_TRUE, UINT64_MAX);
-
-    /* vkResetFences is defined as being host-synchronized */
-    pthread_mutex_lock(&e->lock);
     vk->ResetFences(s->hwctx->act_dev, 1, &e->fence);
-    pthread_mutex_unlock(&e->lock);
 
     /* Discard queue dependencies */
     ff_vk_exec_discard_deps(s, e);

libavutil/vulkan.h

@@ -400,6 +400,8 @@ int ff_vk_qf_init(FFVulkanContext *s, FFVkQueueFamilyCtx *qf,
 
 /**
  * Allocates/frees an execution pool.
+ * If used in a multi-threaded context, there must be at least as many contexts
+ * as there are threads.
  * ff_vk_exec_pool_init_desc() MUST be called if ff_vk_exec_descriptor_set_add()
  * has been called.
  */