Re: + mm-oom_kill-wake-futex-waiters-before-annihilating-victim-shared-mutex.patch added to -mm tree

[Michal Hocko]

Btw. I do not think this patch should be in MM until we hear from Futex
maintainers and also learn more about the underlying issue.

Thanks.

On Tue 07-12-21 15:49:24, Andrew Morton wrote:
> From: Joel Savitz <jsavitz@xxxxxxxxxx>
> Subject: mm/oom_kill: wake futex waiters before annihilating victim shared mutex
> 
> In the case that two or more processes share a futex located within a
> shared mmaped region, such as a process that shares a lock between itself
> and a number of child processes, we have observed that when a process
> holding the lock is oom killed, at least one waiter is never alerted to
> this new development and simply continues to wait.
> 
> This is visible via pthreads by checking the __owner field of the
> pthread_mutex_t structure within a waiting process, perhaps with gdb.
> 
> We identify reproduction of this issue by checking a waiting process of a
> test program and viewing the contents of the pthread_mutex_t, taking note
> of the value in the owner field, and then checking dmesg to see if the
> owner has already been killed.
> 
> This issue can be tricky to reproduce, but with the modifications of this
> small patch, I have found it to be impossible to reproduce.  There may be
> additional considerations that I have not taken into account in this patch
> and I welcome any comments and criticism.
> 
> Link: https://lkml.kernel.org/r/20211207214902.772614-1-jsavitz@xxxxxxxxxx
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> Co-developed-by: Nico Pache <npache@xxxxxxxxxx>
> Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> 
>  mm/oom_kill.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/mm/oom_kill.c~mm-oom_kill-wake-futex-waiters-before-annihilating-victim-shared-mutex
> +++ a/mm/oom_kill.c
> @@ -44,6 +44,7 @@
>  #include <linux/kthread.h>
>  #include <linux/init.h>
>  #include <linux/mmu_notifier.h>
> +#include <linux/futex.h>
>  
>  #include <asm/tlb.h>
>  #include "internal.h"
> @@ -890,6 +891,7 @@ static void __oom_kill_process(struct ta
>  	 * in order to prevent the OOM victim from depleting the memory
>  	 * reserves from the user space under its control.
>  	 */
> +	futex_exit_release(victim);
>  	do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
>  	mark_oom_victim(victim);
>  	pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB, UID:%u pgtables:%lukB oom_score_adj:%hd\n",
> @@ -930,6 +932,7 @@ static void __oom_kill_process(struct ta
>  		 */
>  		if (unlikely(p->flags & PF_KTHREAD))
>  			continue;
> +		futex_exit_release(p);
>  		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_TGID);
>  	}
>  	rcu_read_unlock();
> _
> 
> Patches currently in -mm which might be from jsavitz@xxxxxxxxxx are
> 
> mm-oom_kill-wake-futex-waiters-before-annihilating-victim-shared-mutex.patch

-- 
Michal Hocko
SUSE Labs