Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head | Date | Tue, 12 Apr 2022 18:20:40 +0200 |
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On Mon, Apr 11 2022 at 19:51, Nico Pache wrote: > On 4/8/22 09:54, Thomas Gleixner wrote: >> 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.
The pthread library functions are just conveniance so I did not have to hand code the futex and robust list handling.
> 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.
They don't share a robust list, each thread has it's own.
The shared mutex mut_s is initialized in the parent before fork and it's the same address in the child and it's not COWed because the mapping is MAP_SHARED.
The child allocates private memory and initializes the private mutex in that private mapping.
So now child does:
pthread_mutex_lock(mut_s);
That's the mutex in the memory shared with the parent. After that the childs robusts list head points to mut_s::robust_list.
Now it does:
pthread_mutex_lock(mut_p);
after that the childs robust list head points to mut_p::robust_list and mut_p::robust_list points to mut_s::robust_list.
So now the child unmaps the private memory and exists.
The kernel tries to walk the robust list pointer and faults when trying to access mut_p. End of walk and mut_s stays locked.
So now think about the OOM case. The killed process has a shared mapping with some other unrelated process (file, shmem) where mut_p sits.
It gets killed after: pthread_mutex_lock(mut_s); pthread_mutex_lock(mut_p);
So the OOM reaper rips the VMA which contains mut_p and therefore breaks the chain which is necessary to reach mut_p.
See?
Thanks,
tglx
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