Re: [PATCH 2/2] af_unix: split 'u->readlock' into two: 'iolock' and 'bindlock'

On 02.09.2016 20:13, Linus Torvalds wrote:

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 1 Sep 2016 14:43:53 -0700
Subject: [PATCH 2/2] af_unix: split 'u->readlock' into two: 'iolock' and 'bindlock'

Right now we use the 'readlock' both for protecting some of the af_unix
IO path and for making the bind be single-threaded.

The two are independent, but using the same lock makes for a nasty
deadlock due to ordering with regards to filesystem locking.  The bind
locking would want to nest outside the VSF pathname locking, but the IO
locking wants to nest inside some of those same locks.

We tried to fix this earlier with commit c845acb324aa ("af_unix: Fix
splice-bind deadlock") which moved the readlock inside the vfs locks,
but that caused problems with overlayfs that will then call back into
filesystem routines that take the lock in the wrong order anyway.

Splitting the locks means that we can go back to having the bind lock be
the outermost lock, and we don't have any deadlocks with lock ordering.

Acked-by: Rainer Weikusat <redacted>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

This patch is really trivial, and I've tried to be careful and look at the 
locking, but somebody who really knows the AF_UNIX code should definitely 
take a second look.

Note that I did the revert (that re-introduces the original splice 
deadlock) first, because that made the whole series much easier to 
explain. Doing it in the other order made the revert nastier because this 
patch obviously touches the same code that the revert in 1/2 does.

So this way the series ends up being "go back to the original code with 
the original deadlock, and then fix that original deadlock by splitting 
the bind lock".

Acked-by: Hannes Frederic Sowa <redacted>