Commit 95913d97 authored Sep 29, 2015 by Peter Zijlstra Committed by Ingo Molnar Oct 06, 2015

sched/core: Fix TASK_DEAD race in finish_task_switch()



So the problem this patch is trying to address is as follows:

        CPU0                            CPU1

        context_switch(A, B)
                                        ttwu(A)
                                          LOCK A->pi_lock
                                          A->on_cpu == 0
        finish_task_switch(A)
          prev_state = A->state  <-.
          WMB                      |
          A->on_cpu = 0;           |
          UNLOCK rq0->lock         |
                                   |    context_switch(C, A)
                                   `--  A->state = TASK_DEAD
          prev_state == TASK_DEAD
            put_task_struct(A)
                                        context_switch(A, C)
                                        finish_task_switch(A)
                                          A->state == TASK_DEAD
                                            put_task_struct(A)

The argument being that the WMB will allow the load of A->state on CPU0
to cross over and observe CPU1's store of A->state, which will then
result in a double-drop and use-after-free.

Now the comment states (and this was true once upon a long time ago)
that we need to observe A->state while holding rq->lock because that
will order us against the wakeup; however the wakeup will not in fact
acquire (that) rq->lock; it takes A->pi_lock these days.

We can obviously fix this by upgrading the WMB to an MB, but that is
expensive, so we'd rather avoid that.

The alternative this patch takes is: smp_store_release(&A->on_cpu, 0),
which avoids the MB on some archs, but not important ones like ARM.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org> # v3.1+
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: manfred@colorfullife.com
Cc: will.deacon@arm.com
Fixes: e4a52bcb ("sched: Remove rq->lock from the first half of ttwu()")
Link: http://lkml.kernel.org/r/20150929124509.GG3816@twins.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 049e6dde

kernel/sched/core.c

+5 −5

Original line number	Diff line number	Diff line
		@@ -2517,11 +2517,11 @@ static struct rq finish_task_switch(struct task_struct prev)
		* If a task dies, then it sets TASK_DEAD in tsk->state and calls
		* schedule one last time. The schedule call will never return, and
		* the scheduled task must drop that reference.
		* The test for TASK_DEAD must occur while the runqueue locks are
		* still held, otherwise prev could be scheduled on another cpu, die
		* there before we look at prev->state, and then the reference would
		* be dropped twice.
		* Manfred Spraul <manfred@colorfullife.com>
		*
		* We must observe prev->state before clearing prev->on_cpu (in
		* finish_lock_switch), otherwise a concurrent wakeup can get prev
		* running on another CPU and we could rave with its RUNNING -> DEAD
		* transition, resulting in a double drop.
		*/
		prev_state = prev->state;
		vtime_task_switch(prev);

kernel/sched/sched.h

+3 −2

Original line number	Diff line number	Diff line
		@@ -1078,9 +1078,10 @@ static inline void finish_lock_switch(struct rq rq, struct task_struct prev)
		* After ->on_cpu is cleared, the task can be moved to a different CPU.
		* We must ensure this doesn't happen until the switch is completely
		* finished.
		*
		* Pairs with the control dependency and rmb in try_to_wake_up().
		*/
		smp_wmb();
		prev->on_cpu = 0;
		smp_store_release(&prev->on_cpu, 0);
		#endif
		#ifdef CONFIG_DEBUG_SPINLOCK
		/* this is a valid case when another task releases the spinlock */

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