Re: [PATCH v6] mm, oom: Fix race when selecting process to kill

[Oleg Nesterov <oleg@redhat.com>]

On 11/13, Sameer Nanda wrote:
>
> The selection of the process to be killed happens in two spots:
> first in select_bad_process and then a further refinement by
> looking for child processes in oom_kill_process. Since this is
> a two step process, it is possible that the process selected by
> select_bad_process may get a SIGKILL just before oom_kill_process
> executes. If this were to happen, __unhash_process deletes this
> process from the thread_group list. This results in oom_kill_process
> getting stuck in an infinite loop when traversing the thread_group
> list of the selected process.
>
> Fix this race by adding a pid_alive check for the selected process
> with tasklist_lock held in oom_kill_process.

I am fine with this patch as well, but honestly I'd prefer the previous
v5. I won't argue though.

> +/*
> + * Careful: while_each_thread is not RCU safe. Callers should hold
> + * read_lock(tasklist_lock) across while_each_thread loops.
> + */

(tasklist_lock or siglock, in fact but this doesn't matter).

This is not that simple, even tasklist_lock can't help if the task is
already dead.

Oh. Yes, sorry. I promised to send the patches "soon" many times, but
still didn't find the time.

Perhaps I should try to start with the "make this all less buggy" changes,
the "complete" fix needs to change the callers as well.

Oleg.