[vlc-devel] [PATCH 00/20] MR: Finishing vout life cycle work

[Thomas Guillem]

I backported my "merge both locks" commit into 3.0 to reproduce the deadlock.

Indeed, it protected a deadlock when the following happened:

The GUI (Qt here) hold vouts while a vout is created from an other thread.
cf. https://code.videolan.org/tguillem/vlc/snippets/955/raw for the backtrace

1/ A decoder thread request a vout, LOCK the input_resource mutex, and create a vout
2/ The GUI is on its mainloop and WAIT the input_resource mutex.
3/ From the decoder thread, The vout_Request() load a "vout window" plugin, that will emit and wait for the getVideoSlot() that need to be executed in the GUI thread

=> Deadlock

But this won't happen on VLC 4.0 (I tested it to made sure) since vout_Request() is called unlocked. Only the vout_Create() is locked, but this call only load the "vout window" plugin without enabling it.

I updated my vout-life/6 branch to fix some issues: https://code.videolan.org/tguillem/vlc/commits/vout-life/6

Mainly related to the "es_out: terminate free vout in more places" commit. The window was not disabled when I was disabled video tracks from the UI.

Also, this branch do revert a commit from Rémi: "resource: atomically return vout on failure (fixes #22284)" only to fix the issue in a diffrent way from the "resource: respect vout order" commit.

On Thu, May 9, 2019, at 12:01, Thomas Guillem wrote:
> 
> On Thu, May 9, 2019, at 11:52, Rémi Denis-Courmont wrote:
>> No. Plugins are supposed to be reentrant, and at least the Qt window provider was before you disabled it. Besides, the resource lock cannot prevent reentry into a vout window activation callback anyway.
> 
> The Qt window provider is protected by the resource lock since it's created from vout_Create() that is locked. Indeed this lock is not enough since there can be several input_resource.
> 
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