Messages in this thread Patch in this message | | | Date | Tue, 17 Jan 2012 17:40:31 +0900 | From | Yasunori Goto <> | Subject | Re: [BUG] TASK_DEAD task is able to be woken up in special condition |
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> On Mon, 2012-01-16 at 20:51 +0900, Yasunori Goto wrote: > > > 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. (See: https://lkml.org/lkml/2011/12/21/523) > > As a result, exited task is scheduled() again and panic occurs. > > > > 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. > > > This Changelog isn't very good. Please spell out the problem instead of > referring to it so that people using git-blame and the like don't then > have to go look up some (possibly non-existent) web-resource.
Ahh, Ok.
I rewrote the description and comment in patch. Please check them.
----- 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>
--- kernel/exit.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)
Index: linux-3.2/kernel/exit.c =================================================================== --- linux-3.2.orig/kernel/exit.c +++ linux-3.2/kernel/exit.c @@ -1038,6 +1038,22 @@ 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(); -----
-- Yasunori Goto
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