Re: [PATCH] mm/oom_kill: wake futex waiters before annihilating victim shared mutex

[Andrew Morton]

(cc's added)

On Tue,  7 Dec 2021 16:49:02 -0500 Joel Savitz <jsavitz@xxxxxxxxxx> wrote:

> 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.

Well dang.  Is there any way of killing off that waiting process, or do
we have a resource leak here?

> 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.

> Co-developed-by: Nico Pache <npache@xxxxxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
> ---
>  mm/oom_kill.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1ddabefcfb5a..fa58bd10a0df 100644
> --- a/mm/oom_kill.c
> +++ b/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 task_struct *victim, const char *message)
>  	 * 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 task_struct *victim, const char *message)
>  		 */
>  		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();
> -- 
> 2.33.1