Re: bio linked list corruption.

From	Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups	linux.kernel
Subject	Re: bio linked list corruption.
Date	2016-12-06 18:00 +0100
Message-ID	<sLrfX-4AN-29@gated-at.bofh.it> (permalink)
References	(6 earlier) <sL5Iu-7AR-11@gated-at.bofh.it> <sL6XT-81-3@gated-at.bofh.it> <sL7TY-KS-7@gated-at.bofh.it> <sL8dj-QU-19@gated-at.bofh.it> <sLj8J-81i-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On Tue, Dec 6, 2016 at 12:16 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> Of course, I'm really hoping that this shmem.c use is the _only_ such
>> case.  But I doubt it.
>
> $ git grep DECLARE_WAIT_QUEUE_HEAD_ONSTACK | wc -l
> 28

Hmm. Most of them seem to be ok, because they use "wait_event()",
which will always remove itself from the wait-queue. And they do it
from the place that allocates the wait-queue.

IOW, the mm/shmem.c case really was fairly special, because it just
did "prepare_to_wait()", and then did a finish_wait() - and not in the
thread that allocated it on the stack.

So it's really that "some _other_ thread allocated the waitqueue on
the stack, and now we're doing a wait on it" that is bad.

So the normal pattern seems to be:

 - allocate wq on the stack
 - pass it on to a waker
 - wait for it

and that's ok, because as part of "wait for it" we will also be
cleaning things up.

The reason mm/shmem.c was buggy was that it did

 - allocate wq on the stack
 - pass it on to somebody else to wait for
 - wake it up

and *that* is buggy, because it's the waiter, not the waker, that
normally cleans things up.

Is there some good way to find this kind of pattern automatically, I wonder....

                     Linus

Thread

Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 00:10 +0100
  Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 18:20 +0100
    Re: bio linked list corruption. Dave Jones <davej@codemonkey.org.uk> - 2016-12-05 18:30 +0100
    Re: bio linked list corruption. Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-05 19:00 +0100
      Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 20:20 +0100
        Re: bio linked list corruption. Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-05 21:20 +0100
          Re: bio linked list corruption. Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-05 21:40 +0100
            Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 22:40 +0100
              Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-06 09:50 +0100
            Re: bio linked list corruption. Peter Zijlstra <peterz@infradead.org> - 2016-12-06 09:20 +0100
              Re: bio linked list corruption. Ingo Molnar <mingo@kernel.org> - 2016-12-06 09:40 +0100
              Re: bio linked list corruption. Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-06 18:00 +0100
        Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 21:20 +0100
  Re: bio linked list corruption. Andy Lutomirski <luto@kernel.org> - 2016-12-05 19:20 +0100
    Re: bio linked list corruption. Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-05 19:30 +0100
    Re: bio linked list corruption. Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-05 19:30 +0100

csiph-web