Re: [PATCH 1/6] proc, oom: drop bogus task_lock and mm check

From: Oleg Nesterov
Date: Mon May 30 2016 - 13:43:34 EST

On 05/30, Michal Hocko wrote:
> both oom_adj_write and oom_score_adj_write are using task_lock,
> check for task->mm and fail if it is NULL. This is not needed because
> the oom_score_adj is per signal struct so we do not need mm at all.
> The code has been introduced by 3d5992d2ac7d ("oom: add per-mm oom
> disable count") but we do not do per-mm oom disable since c9f01245b6a7
> ("oom: remove oom_disable_count").
> The task->mm check is even not correct because the current thread might
> have exited but the thread group might be still alive - e.g. thread
> group leader would lead that echo $VAL > /proc/pid/oom_score_adj would
> always fail with EINVAL while /proc/pid/task/$other_tid/oom_score_adj
> would succeed. This is unexpected at best.
> Remove the lock along with the check to fix the unexpected behavior
> and also because there is not real need for the lock in the first place.


and we should also remove lock_task_sighand(). as for oom_adj_read() and
oom_score_adj_read() we can just remove it right now; it was previously
needed to ensure the task->signal != NULL, today this is always true.