Re: [PATCH] fs: process fput task_work with TWA_SIGNAL

[Al Viro]

On Tue, Jan 05, 2021 at 11:29:11AM -0700, Jens Axboe wrote:
> Song reported a boot regression in a kvm image with 5.11-rc, and bisected
> it down to the below patch. Debugging this issue, turns out that the boot
> stalled when a task is waiting on a pipe being released. As we no longer
> run task_work from get_signal() unless it's queued with TWA_SIGNAL, the
> task goes idle without running the task_work. This prevents ->release()
> from being called on the pipe, which another boot task is waiting on.
> 
> Use TWA_SIGNAL for the file fput work to ensure it's run before the task
> goes idle.
> 
> Fixes: 98b89b649fce ("signal: kill JOBCTL_TASK_WORK")
> Reported-by: Song Liu <songliubraving@xxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> 
> ---
> 
> The other alternative here is obviously to re-instate the:
> 
> if (unlikely(current->task_works))
> 	task_work_run();
> 
> in get_signal() that we had before this change. Might be safer in case
> there are other cases that need to ensure the work is run in a timely
> fashion, though I do think it's cleaner to long term to correctly mark
> task_work with the needed notification type. Comments welcome...

Interesting...  I think I've missed the discussion of that thing; could
you forward the relevant thread my way or give an archive link to it?