Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head

[Nico Pache]

On 4/8/22 09:54, Thomas Gleixner wrote:
> On Fri, Apr 08 2022 at 04:41, Nico Pache wrote:
>> On 4/8/22 04:15, Peter Zijlstra wrote:
>>>>
>>>> The following case can still fail:
>>>> robust head (skipped) -> private lock (reaped) -> shared lock (skipped)
>>>
>>> This is still all sorts of confused.. it's a list head, the entries can
>>> be in any random other VMA. You must not remove *any* user memory before
>>> doing the robust thing. Not removing the VMA that contains the head is
>>> pointless in the extreme.
>> Not sure how its pointless if it fixes all the different reproducers we've
>> written for it. As for the private lock case we stated here, we havent been able
>> to reproduce it, but I could see how it can be a potential issue (which is why
>> its noted).
> 
> The below reproduces the problem nicely, i.e. the lock() in the parent
> times out. So why would the OOM killer fail to cause the same problem
> when it reaps the private anon mapping where the private futex sits?
> 
> If you revert the lock order in the child the robust muck works.

Thanks for the reproducer Thomas :)

I think I need to re-up my knowledge around COW and how it effects that stack.
There are increased oddities when you add the pthread library that I cant fully
wrap my head around at the moment.

My confusion lies in how the parent/child share a robust list here, but they
obviously do. In my mind the mut_s would be different in the child/parent after
the fork and pthread_mutex_init (and friends) are done in the child.

Thanks!
-- Nico
> 
> Thanks,
> 
>         tglx
> ---
> #include <errno.h>
> #include <fcntl.h>
> #include <pthread.h>
> #include <time.h>
> #include <stdio.h>
> #include <string.h>
> #include <unistd.h>
> 
> #include <sys/types.h>
> #include <sys/mman.h>
> 
> static char n[4096];
> 
> int main(void)
> {
> 	pthread_mutexattr_t mat_s, mat_p;
> 	pthread_mutex_t *mut_s, *mut_p;
> 	pthread_barrierattr_t ba;
> 	pthread_barrier_t *b;
> 	struct timespec to;
> 	void *pri, *shr;
> 	int r;
> 
> 	shr = mmap(NULL, sizeof(n), PROT_READ | PROT_WRITE,
> 		   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> 
> 	pthread_mutexattr_init(&mat_s);
> 	pthread_mutexattr_setrobust(&mat_s, PTHREAD_MUTEX_ROBUST);
> 	mut_s = shr;
> 	pthread_mutex_init(mut_s, &mat_s);
> 
> 	pthread_barrierattr_init(&ba);
> 	pthread_barrierattr_setpshared(&ba, PTHREAD_PROCESS_SHARED);
> 	b = shr + 1024;
> 	pthread_barrier_init(b, &ba, 2);
> 
> 	if (!fork()) {
> 		pri = mmap(NULL, 1<<20, PROT_READ | PROT_WRITE,
> 			   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 		pthread_mutexattr_init(&mat_p);
> 		pthread_mutexattr_setpshared(&mat_p, PTHREAD_PROCESS_PRIVATE);
> 		pthread_mutexattr_setrobust(&mat_p, PTHREAD_MUTEX_ROBUST);
> 		mut_p = pri;
> 		pthread_mutex_init(mut_p, &mat_p);
> 
> 		// With lock order s, p parent gets timeout
> 		// With lock order p, s parent gets owner died
> 		pthread_mutex_lock(mut_s);
> 		pthread_mutex_lock(mut_p);
> 		// Remove unmap and lock order does not matter
> 		munmap(pri, sizeof(n));
> 		pthread_barrier_wait(b);
> 		printf("child gone\n");
> 	} else {
> 		pthread_barrier_wait(b);
> 		printf("parent lock\n");
> 		clock_gettime(CLOCK_REALTIME, &to);
> 		to.tv_sec += 1;
> 		r = pthread_mutex_timedlock(mut_s, &to);
> 		printf("parent lock returned: %s\n", strerror(r));
> 	}
> 	return 0;
> }
>