On Mon, Dec 02, 2013 at 08:00:38AM -0800, Linus Torvalds wrote:
> On Sat, Nov 30, 2013 at 1:08 PM, Linus Torvalds
> <torva...@linux-foundation.org> wrote:
> >
> > I still don't see what could be wrong with the pipe_inode_info thing,
> > but the fact that it's been so consistent in your traces does make me
> > suspect it really is *that* particular slab.
>
> I think I finally found it.
>
> I've spent waaayy too much time looking at and thinking about that
> code without seeing anything wrong, but this morning I woke up and
> thought to myself "What if.."
>
> And looking at the code again, I went "BINGO".
>
> All our reference counting etc seems right, but we have one very
> subtle bug: on the freeing path, we have a pattern like this:
>
> spin_lock(&inode->i_lock);
> if (!--pipe->files) {
> inode->i_pipe = NULL;
> kill = 1;
> }
> spin_unlock(&inode->i_lock);
> __pipe_unlock(pipe);
> if (kill)
> free_pipe_info(pipe);
>
> which on the face of it is trying to be very careful in not accessing
> the pipe-info after it is released by having that "kill" flag, and
> doing the release last.
>
> And it's complete garbage.
>
> Why?
>
> Because the thread that decrements "pipe->files" *without* releasing
> it, will very much access it after it has been dropped: that
> "__pipe_unlock(pipe)" happens *after* we've decremented the pipe
> reference count and dropped the inode lock. So another CPU can come in
> and free the structure concurrently with that __pipe_unlock(pipe).
>
> This happens in two places, and we don't actually need or want the
> pipe lock for the pipe->files accesses (since pipe->files is protected
> by inode->i_lock, not the pipe lock), so the solution is to just do
> the __pipe_unlock() before the whole dance about the pipe->files
> reference count.
>
> Patch appended. And no wonder nobody has ever seen it, because the
> race is unlikely as hell to ever happen. Simon, I assume it will be
> another few months before we can say "yeah, that fixed it", but I
> really think this is it. It explains all the symptoms, including
> "DEBUG_PAGEALLOC didn't catch it" (because the access happens just as
> it is released, and DEBUG_PAGEALLOC takes too long to actually free
> unmap the page etc).
Nice catch. I'd probably add
/* we are holding inode->i_pipe->files */
static void drop_pipe(struct inode *inode, struct pipe_inode_info *pipe)
{
int kill = 0;
spin_lock(&inode->i_lock);
WARN_ON(pipe != inode->i_pipe);
if (!--pipe->files) {
inode->i_pipe = NULL;
kill = 1;
}
spin_unlock(&inode->i_lock);
if (kill)
free_pipe_info(pipe);
}
and use it in both places...
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