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

[Thomas Gleixner <tglx@linutronix.de>]

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,

        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;
}