Re: [patch 07/44] posix-cpu-timers: Simplify sighand locking in run_posix_cpu_timers()

[Frederic Weisbecker]

On Wed, Aug 21, 2019 at 03:25:01PM +0200, Thomas Gleixner wrote:
> On Wed, 21 Aug 2019, Frederic Weisbecker wrote:
> 
> > On Mon, Aug 19, 2019 at 04:31:48PM +0200, Thomas Gleixner wrote:
> > > run_posix_cpu_timers() is called from the timer interrupt. The posix timer
> > > expiry always affects the current task which got interrupted.
> > > 
> > > sighand locking is only racy when done on a foreign task, which must use
> > > lock_task_sighand(). But in case of run_posix_cpu_timers() that's
> > > pointless.
> > > 
> > > sighand of a task can only be dropped or changed by the task itself. Drop
> > > happens in do_exit()
> > 
> > Well, that's only in case of autoreap. Otherwise this is dropped by the reaper.
> 
> Right, but in the reaper case the task cannot be on the CPU running and
> being interrupted by the tick. I might be missing something subtle though.

That looks possible. After exit_notify() and until the final schedule(), the exiting task
can execute concurrently with the reaper calling release_task().