Messages in this thread | | | Date | Thu, 16 Jun 2005 11:44:22 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Re: [BUG] Race condition with it_real_fn in kernel/itimer.c |
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Steven Rostedt wrote: > > So, timer_pending tests if timer->base is NULL, but here we see that > timer->base IS NULL before the function is called, and as I have said > earlier, the it_real_arm can be called on two CPUS simultaneously. So > here's another patch that should fix this race condition too. > > [...] > > + /* > + * Call del_timer_sync unconditionally, since we don't > + * know if it is running or not. We also need to unlock > + * the siglock so that the it_real_fn called by ksoftirqd > + * doesn't wait for us. > + */ > + spin_unlock(&tsk->sighand->siglock); > + del_timer_sync(&tsk->signal->real_timer); > + spin_lock(&tsk->sighand->siglock);
I don't think this is 100% correct. After del_timer_sync() returns another thread can come and call do_setitimer() and re-arm the timer (because with your patch we are dropping tsk->sighand->siglock here). So this patch does not garantees that the timer is not queued/running after del_timer_sync(), and the it_real_arm can be called on two CPUS simultaneously again.
There is a try_to_del_timer_sync() in the -mm tree which is suitable here:
again: spin_lock_irq(&tsk->sighand->siglock); if (try_to_del_timer_sync(&tsk->signal->real_timer) < 0) { spin_unlock_irq(&tsk->sighand->siglock); goto again; }
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