#11371: Document lock ordering for hwcontext_vulkan.h
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Reporter: mstoeckl | Type:
| enhancement
Status: new | Priority: normal
Component: avutil | Version: git-
Keywords: | master
documentation, vulkan, deadlock | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
-------------------------------------+-------------------------------------
There are two types of locks exposed in libavutil/hwcontext_vulkan.h: a
lock for exclusive access to a Vulkan queue,
AVVulkanDeviceContext.lock_queue, and a lock for exclusive access to a
frame, AVVulkanFramesContext.lock_frame. As far as I can tell, the
documentation does not clearly say in which order such locks should be
taken, but I believe it should.
I currently have code (for `waypipe`'s video encoding) that locks an
AVVkFrame, builds a command buffer to fill its contents and updates the
AVVkFrame, locks a Vulkan queue, submits the command buffer, and then
promptly unlocks everything.
However, if ffmpeg ever has a thread operate in the reverse order (lock a
queue, and then locks a frame), then a deadlock could be possible if my
code locks frame A, ffmpeg locks queue B, and then my code tries to lock
queue B while ffmpeg tries to lock frame A for something.
Holding both queue and frame locks simultaneously appears necessary. The
AVVulkanFramesContext.lock_frame documentation, currently implies that at
command buffer submission time the frame lock should be held: "Users
SHOULD only ever lock just before command submission in order get accurate
frame properties, and unlock immediately after command submission without
waiting for it to finish." On the other hand, Vulkan's documentation marks
vkQueueSubmit as "externally synchronized", so it is unsafe to submit
command buffers without holding a queue lock.
--
Ticket URL: <https://trac.ffmpeg.org/ticket/11371>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker
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