Re: Deadlock in do_page_fault() on ARM (old kernel)

[Michal Hocko]

On Wed 15-01-14 20:13:04, Alan Ott wrote:
[...]
> 2. __copy_to_user_memcpy() takes a read lock (down_read()) on

This looks like a bug. copy_to_user_* shouldn't take mmap_sem at all
Check the might_fault annotation used in generic code. Arm version of
copy_to_user* doesn't seem to use the annotation and I do not see a good
reason for that.

> mm->mmap_sem. While that lock is held, __copy_to_user_memcpy() can
> generate a page fault, causing do_page_fault() to get called, which
> will also try to get a read lock (down_read()) on mm->mmap_sem.
> Multiple read locks can be taken on an rw_semaphore, but deadlock
> will occur if another thread tries to get a write lock
> (down_write()) in between. For example:
>     Task 1:         Task 2:
>     down_read(sem)
>                     down_write(sem)    <-- Goes to sleep
>     down_read(sem)                     <-- Goes to sleep
> 
> There is a thread from 2005[3] which seems to discuss the same
> concept of recursive rw_semaphores, but for futexes.
> 
> Other comments:
> 1. My analysis of this probably wrong. Otherwise it seems many
> others would have the same problem, and they don't seem to. I'm
> hoping this email will help to correct my understanding.
> 2. I looked through the git logs for recent (since 2.6.37 time
> frame) and nothing else jumped out at me as being an obvious fix for
> this situation.
> 
> Thanks for any insight you can give,
> 
> Alan.
> 
> [1] http://www.signal11.us/~alan/show-all-tasks-deadlock.txt
> 
> [2] Some websites/bugtrackers mention this commit with a similar
> issue, but I'm not entirely sure how it's related:
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8878a539ff19a43cf3729e7562cd528f490246ae
> 
> This one seems obviously related, but has no effect on my system:
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=435a7ef52db7d86e67a009b36cac1457f8972391
> 
> [3] http://thread.gmane.org/gmane.linux.kernel/280900
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-- 
Michal Hocko
SUSE Labs
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