[PATCH net-next] af_unix: fix a fatal race with bit fields

[Eric Dumazet]

On Fri, 2013-05-03 at 11:01 +0930, Alan Modra wrote:
> On Tue, Apr 30, 2013 at 10:04:32PM -0700, Eric Dumazet wrote:
> > These kind of errors are pretty hard to find, its a pity to spend time
> > on them.
> 
> Well, yes.  From the first comment in gcc PR52080.  "For the following
> testcase we generate a 8 byte RMW cycle on IA64 which causes locking
> problems in the linux kernel btrfs filesystem."
> 
> Did someone fix btrfs, but not check other kernel locks?  Having now
> hit the same problem again, have you checked that other kernel locks
> don't have adjacent bit fields in the same 64-bit word?  And comment
> the struct to ensure someone doesn't optimize those unsigned chars
> back to bit fields.

Not only spinlock, but atomic_t followed by bit fields.

BTW, if a spinlock is followed by bit fields, but bit fields
only changed when this spinlock is held, there is no problem, unless
spinlock is a ticket spinlock.

In af_unix, bug happens because the bit fields were changed without
spinlock being held (another global spinlock is used instead)

(ppc64 doesnt use ticket spinlocks yet)