Re: [patch 01/20] posix-timers: Prevent RT livelock in itimer_delete()

[Frederic Weisbecker <frederic@kernel.org>]

Le Tue, Apr 25, 2023 at 08:48:56PM +0200, Thomas Gleixner a écrit :
> itimer_delete() has a retry loop when the timer is concurrently expired. On
> non-RT kernels this just spin-waits until the timer callback has
> completed. On RT kernels this is a potential livelock when the exiting task
> preempted the hrtimer soft interrupt.
> 
> This only affects hrtimer based timers as Posix CPU timers cannot be
> concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
> obviously impossible as the task cannot run task work and exit at the same
> time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
> because interrupts are disabled.

But the owner of the timer is not the same as the target of the timer, right?

Though I seem to remember that we forbid setting a timer to a target outside
the current process, in which case the owner and the target are the same at
this exit stage. But I can't remember what enforces that permission in pid_for_clock()...

Thanks.