On Tue 07-12-21 15:47:59, Andrew Morton wrote:
> (cc's added)
Extend CC to have all futex maintainers on board.
> 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.
Why does OOM killer need a special handling. All the oom killer does is
to send a fatal signal to the victim. Why is this any different from
sending SIGKILL from the userspace?
> > 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
--
Michal Hocko
SUSE Labs
