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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
