On Fri 27-05-16 10:00:48, Arnd Bergmann wrote:
> The change to the oom_reaper to hold a mutex inside __oom_reap_task()
> accidentally started calling mmput_async() on the local
> mm before that variable got initialized, as reported by gcc
> in linux-next:
>
> mm/oom_kill.c: In function '__oom_reap_task':
> mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>
> This rearranges the code slightly back to the state before patch
> but leaves the lock in place. The error handling in the function
> still looks a bit confusing and could probably be improved
> but I could not come up with a solution that made me happy
> for now.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Fixes: mmotm ("oom_reaper: close race with exiting task")
Thanks for catching that Arnd?
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/oom_kill.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1685890d424e..255cb5f48019 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -447,7 +447,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
> struct task_struct *p;
> struct zap_details details = {.check_swap_entries = true,
> .ignore_dirty = true};
> - bool ret = true;
> + bool ret;
>
> /*
> * We have to make sure to not race with the victim exit path
> @@ -472,13 +472,16 @@ static bool __oom_reap_task(struct task_struct *tsk)
> * is no mm.
> */
> p = find_lock_task_mm(tsk);
> - if (!p)
> - goto unlock_oom;
> + if (!p) {
> + mutex_unlock(&oom_lock);
> + return true;
> + }
>
> mm = p->mm;
> if (!atomic_inc_not_zero(&mm->mm_users)) {
> task_unlock(p);
> - goto unlock_oom;
> + mutex_unlock(&oom_lock);
> + return true;
> }
>
> task_unlock(p);
> @@ -527,6 +530,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
> * to release its memory.
> */
> set_bit(MMF_OOM_REAPED, &mm->flags);
> + ret = true;
> unlock_oom:
> mutex_unlock(&oom_lock);
> /*
> --
> 2.7.0
>
--
Michal Hocko
SUSE Labs
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