On Mon, Feb 04, 2013 at 07:35:09PM -0600, Alex Elder wrote: > On 02/04/2013 05:06 PM, Dave Chinner wrote: > > On Mon, Feb 04, 2013 at 10:13:11AM -0600, Alex Elder wrote: > >> In xfs_ifunlock() there is a call to wake_up_bit() after clearing > >> the flush lock on the xfs inode. This is not guaranteed to be safe, > >> as noted in the comments above wake_up_bit() beginning with: > >> > >> In order for this to function properly, as it uses > >> waitqueue_active() internally, some kind of memory > >> barrier must be done prior to calling this. > >> > >> I claim no mastery of the details and subtlety of memory barrier > >> use, but I believe the issue is that the call to waitqueue_active() > >> in __wake_up_bit(), could be operating on a value of "wq" that is > >> out of date. This patch fixes this by inserting a call to smp_mb() > >> in xfs_iunlock before calling wake_up_bit(), along the lines of > >> what's done in unlock_new_inode(). A litte more explanation > >> follows. > >> > >> > >> In __xfs_iflock(), prepare_to_wait_exclusive() adds a wait queue > >> entry to the end of a bit wait queue before setting the current task > >> state to UNINTERRUPTIBLE. And although setting the task state > >> issues a full smp_mb() (which ensures changes made are visible to > >> the rest of the system at that point) that alone does not guarantee > >> that other CPUs will instantly avail themselves of the updated > >> value. A separate CPU needs to issue at least a read barrier in > >> order to ensure the wq value it uses to determine whether there are > >> waiters is up-to-date, and waitqueue_active() does not do that. > > > > You can probably trim most of this and simply point at the comment > > describing wake_up_bit().... > > Yeah, I know. I just wanted to sort of say what I was > thinking to get confirmation (or correction). I now > have a much better understanding of barriers than I did > before, but there are still corners I haven't wrapped > my head around. > > Ben, please feel free do trim off this stuff as you > see fit. Applied. Thanks Alex! -Ben _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
