Commit e03d13e9 authored Oct 19, 2005 by Roland McGrath Committed by Linus Torvalds Oct 19, 2005

[PATCH] Fix cpu timers exit deadlock and races



Oleg Nesterov reported an SMP deadlock.  If there is a running timer
tracking a different process's CPU time clock when the process owning
the timer exits, we deadlock on tasklist_lock in posix_cpu_timer_del via
exit_itimers.

That code was using tasklist_lock to check for a race with __exit_signal
being called on the timer-target task and clearing its ->signal.
However, there is actually no such race.  __exit_signal will have called
posix_cpu_timers_exit and posix_cpu_timers_exit_group before it does
that.  Those will clear those k_itimer's association with the dying
task, so posix_cpu_timer_del will return early and never reach the code
in question.

In addition, posix_cpu_timer_del called from exit_itimers during execve
or directly from timer_delete in the process owning the timer can race
with an exiting timer-target task to cause a double put on timer-target
task struct.  Make sure we always access cpu_timers lists with sighand
lock held.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 3359b54c

kernel/posix-cpu-timers.c

+11 −17

Original line number	Diff line number	Diff line
		@@ -387,25 +387,19 @@ int posix_cpu_timer_del(struct k_itimer *timer)
		if (unlikely(p == NULL))
		return 0;

		spin_lock(&p->sighand->siglock);
		if (!list_empty(&timer->it.cpu.entry)) {
		read_lock(&tasklist_lock);
		if (unlikely(p->signal == NULL)) {
		/*
		* We raced with the reaping of the task.
		* The deletion should have cleared us off the list.
		* Take us off the task's timer list. We don't need to
		* take tasklist_lock and check for the task being reaped.
		* If it was reaped, it already called posix_cpu_timers_exit
		* and posix_cpu_timers_exit_group to clear all the timers
		* that pointed to it.
		*/
		BUG_ON(!list_empty(&timer->it.cpu.entry));
		} else {
		/*
		* Take us off the task's timer list.
		*/
		spin_lock(&p->sighand->siglock);
		list_del(&timer->it.cpu.entry);
		spin_unlock(&p->sighand->siglock);
		}
		read_unlock(&tasklist_lock);
		}
		put_task_struct(p);
		}
		spin_unlock(&p->sighand->siglock);

		return 0;
		}

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