Re: [patch V3 01/18] posix-timers: Ensure that timer initialization is fully visible

[Frederic Weisbecker <frederic@kernel.org>]

Le Sat, Mar 08, 2025 at 05:48:10PM +0100, Thomas Gleixner a écrit :
> Frederic pointed out that the memory operations to initialize the timer are
> not guaranteed to be visible, when __lock_timer() observes timer::it_signal
> valid under timer::it_lock:
> 
>   T0                                      T1
>   ---------                               -----------
>   do_timer_create()
>       // A
>       new_timer->.... = ....
>       spin_lock(current->sighand)
>       // B
>       WRITE_ONCE(new_timer->it_signal, current->signal)
>       spin_unlock(current->sighand)
> 					sys_timer_*()
> 					   t =  __lock_timer()
> 						  spin_lock(&timr->it_lock)
> 						  // observes B
> 						  if (timr->it_signal == current->signal)
> 						    return timr;
> 			                   if (!t)
> 					       return;
> 					// Is not guaranteed to observe A
> 
> Protect the write of timer::it_signal, which makes the timer valid, with
> timer::it_lock as well. This guarantees that T1 must observe the
> initialization A completely, when it observes the valid signal pointer
> under timer::it_lock. sighand::siglock must still be taken to protect the
> signal::posix_timers list.
> 
> Reported-by: Frederic Weisbecker <frederic@kernel.org>
> Suggested-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>