[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>
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 7a51a55..b3f3edc 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -387,25 +387,19 @@
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.
- */
- 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);
+ /*
+ * 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.
+ */
+ list_del(&timer->it.cpu.entry);
+ put_task_struct(p);
}
- put_task_struct(p);
+ spin_unlock(&p->sighand->siglock);
return 0;
}