Messages in this thread Patch in this message | | | Date | Sun, 07 Oct 2012 23:58:47 +0100 | From | Ben Hutchings <> | Subject | [ 013/108] sched: Fix ancient race in do_exit() |
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3.2-stable review patch. If anyone has any objections, please let me know.
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From: Yasunori Goto <y-goto@jp.fujitsu.com>
commit b5740f4b2cb3503b436925eb2242bc3d75cd3dfe upstream.
try_to_wake_up() has a problem which may change status from TASK_DEAD to TASK_RUNNING in race condition with SMI or guest environment of virtual machine. As a result, exited task is scheduled() again and panic occurs.
Here is the sequence how it occurs:
----------------------------------+----------------------------- | CPU A | CPU B ----------------------------------+-----------------------------
TASK A calls exit()....
do_exit()
exit_mm() down_read(mm->mmap_sem);
rwsem_down_failed_common()
set TASK_UNINTERRUPTIBLE set waiter.task <= task A list_add to sem->wait_list : raw_spin_unlock_irq() (I/O interruption occured)
__rwsem_do_wake(mmap_sem)
list_del(&waiter->list); waiter->task = NULL wake_up_process(task A) try_to_wake_up() (task is still TASK_UNINTERRUPTIBLE) p->on_rq is still 1.)
ttwu_do_wakeup() (*A) : (I/O interruption handler finished)
if (!waiter.task) schedule() is not called due to waiter.task is NULL.
tsk->state = TASK_RUNNING
: check_preempt_curr(); : task->state = TASK_DEAD (*B) <--- set TASK_RUNNING (*C)
schedule() (exit task is running again) BUG_ON() is called! --------------------------------------------------------
The execution time between (*A) and (*B) is usually very short, because the interruption is disabled, and setting TASK_RUNNING at (*C) must be executed before setting TASK_DEAD.
HOWEVER, if SMI is interrupted between (*A) and (*B), (*C) is able to execute AFTER setting TASK_DEAD! Then, exited task is scheduled again, and BUG_ON() is called....
If the system works on guest system of virtual machine, the time between (*A) and (*B) may be also long due to scheduling of hypervisor, and same phenomenon can occur.
By this patch, do_exit() waits for releasing task->pi_lock which is used in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after waking up.
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com> Acked-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/20120117174031.3118.E1E9C6FF@jp.fujitsu.com Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Ben Hutchings <ben@decadent.org.uk> --- kernel/exit.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)
--- a/kernel/exit.c +++ b/kernel/exit.c @@ -1019,6 +1019,22 @@ NORET_TYPE void do_exit(long code) preempt_disable(); exit_rcu(); + + /* + * The setting of TASK_RUNNING by try_to_wake_up() may be delayed + * when the following two conditions become true. + * - There is race condition of mmap_sem (It is acquired by + * exit_mm()), and + * - SMI occurs before setting TASK_RUNINNG. + * (or hypervisor of virtual machine switches to other guest) + * As a result, we may become TASK_RUNNING after becoming TASK_DEAD + * + * To avoid it, we have to wait for releasing tsk->pi_lock which + * is held by try_to_wake_up() + */ + smp_mb(); + raw_spin_unlock_wait(&tsk->pi_lock); + /* causes final put_task_struct in finish_task_switch(). */ tsk->state = TASK_DEAD; schedule();
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