On 12/7/21 18:47, Andrew Morton wrote:
> (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?
If I understood your question correctly, there is a way to recover the system by
killing the process that is utilizing the futex; however, the purpose of robust
futexes is to avoid having to do this.
>From my work with Joel on this it seems like a race is occurring between the
oom_reaper and the exit signal sent to the OMM'd process. By setting the
futex_exit_release before these signals are sent we avoid this.
>
>> 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
>
