Re: [PATCH v9] oom_kill.c: futex: Delay the OOM reaper to allow time for proper futex cleanup

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Apr 14 2022 at 10:40, Nico Pache wrote:
> The pthread struct is allocated on PRIVATE|ANONYMOUS memory [1] which can
> be targeted by the oom reaper. This mapping is used to store the futex
> robust list head; the kernel does not keep a copy of the robust list and
> instead references a userspace address to maintain the robustness during
> a process death. A race can occur between exit_mm and the oom reaper that
> allows the oom reaper to free the memory of the futex robust list before
> the exit path has handled the futex death:
>
>     CPU1                               CPU2
> ------------------------------------------------------------------------
>     page_fault
>     do_exit "signal"
>     wake_oom_reaper
>                                         oom_reaper
>                                         oom_reap_task_mm (invalidates mm)
>     exit_mm
>     exit_mm_release
>     futex_exit_release
>     futex_cleanup
>     exit_robust_list
>     get_user (EFAULT- can't access memory)
>
> If the get_user EFAULT's, the kernel will be unable to recover the
> waiters on the robust_list, leaving userspace mutexes hung indefinitely.
>
> Delay the OOM reaper, allowing more time for the exit path to perform
> the futex cleanup.
>
> Reproducer: https://gitlab.com/jsavitz/oom_futex_reproducer
>
> [1] https://elixir.bootlin.com/glibc/latest/source/nptl/allocatestack.c#L370

A link to the original discussion about this would be more useful than a
code reference which is stale tomorrow. The above explanation is good
enough to describe the problem.

>  
> +/*
> + * Give the OOM victim time to exit naturally before invoking the oom_reaping.
> + * The timers timeout is arbitrary... the longer it is, the longer the worst
> + * case scenario for the OOM can take. If it is too small, the oom_reaper can
> + * get in the way and release resources needed by the process exit path.
> + * e.g. The futex robust list can sit in Anon|Private memory that gets reaped
> + * before the exit path is able to wake the futex waiters.
> + */
> +#define OOM_REAPER_DELAY (2*HZ)
> +static void queue_oom_reaper(struct task_struct *tsk)

Bah. Did you run out of newlines? Glueing that define between the
comment and the function is unreadable.

Other than that.

Acked-by: Thomas Gleixner <tglx@linutronix.de>