Messages in this thread | | | Date | Thu, 26 Jun 2008 08:49:11 -0400 | From | Christoph Hellwig <> | Subject | Re: [PATCH 1/6] Extend completions to provide XFS object flush requirements |
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On Thu, Jun 26, 2008 at 06:40:09AM -0600, Matthew Wilcox wrote: > On Thu, Jun 26, 2008 at 10:21:12PM +1000, Dave Chinner wrote: > > On Thu, Jun 26, 2008 at 05:42:42AM -0600, Matthew Wilcox wrote: > > > Then let's leave it as a semaphore. You can get rid of the sema_t if > > > you like, but I don't think that turning completions into semaphores is > > > a good idea (because it's confusing). > > > > So remind me what the point of the semaphore removal tree is again? > > To remove the semaphores which don't need to be semaphores any more. > > > As Christoph suggested, I can put this under another API that > > is implemented using completions. If I have to do that in XFS, > > so be it.... > > You could, yes. But you could just use completions directly ... > > > The main reason for this that we've just uncovered the fact that the > > way XFS uses semaphores is completely unsafe [*] on x86/x86_64 for > > kernels prior to the new generic semaphores. > > > > [*] 2.6.20 panics in up() because of this race when I/O completion > > (the up call) races with a simultaneous down() (iowaiter): > > > > T1 T2 > > up() down() > > kmem_free() > > > > When the down() call completes, the up() call can still be > > referencing the semaphore, and hence if we free the structure after > > the down call then the up() will reference freed memory. This is > > probably the cause of many unexplained log replay or unmount panics > > that we've been hitting for years with buffers that been freed while > > apparently still in use.... > > This is exactly the kind of thing completions were supposed to be used > for. T1 should be calling complete() and T2 should be calling > wait_for_completion().
Please read Dave's introductionary mail. What XFS wants if completions with a little bit extra, so he implemented the little bit extra. This little bit extra is pretty well described in the mail starting this thread.
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