On Sat, Jul 14, 2012 at 04:14:43AM -0600, Mike Galbraith wrote:
> On Fri, 2012-07-13 at 08:50 -0400, Chris Mason wrote:
> > On Wed, Jul 11, 2012 at 11:47:40PM -0600, Mike Galbraith wrote:
> > > Greetings,
> >
> > [ deadlocks with btrfs and the recent RT kernels ]
> >
> > I talked with Thomas about this and I think the problem is the
> > single-reader nature of the RW rwlocks. The lockdep report below
> > mentions that btrfs is calling:
> >
> > > [ 692.963099] [<ffffffff811fabd2>] btrfs_clear_path_blocking+0x32/0x70
> >
> > In this case, the task has a number of blocking read locks on the btrfs buffers,
> > and we're trying to turn them back into spinning read locks. Even
> > though btrfs is taking the read rwlock, it doesn't think of this as a new
> > lock operation because we were blocking out new writers.
> >
> > If the second task has taken the spinning read lock, it is going to
> > prevent that clear_path_blocking operation from progressing, even though
> > it would have worked on a non-RT kernel.
> >
> > The solution should be to make the blocking read locks in btrfs honor the
> > single-reader semantics. This means not allowing more than one blocking
> > reader and not allowing a spinning reader when there is a blocking
> > reader. Strictly speaking btrfs shouldn't need recursive readers on a
> > single lock, so I wouldn't worry about that part.
> >
> > There is also a chunk of code in btrfs_clear_path_blocking that makes
> > sure to strictly honor top down locking order during the conversion. It
> > only does this when lockdep is enabled because in non-RT kernels we
> > don't need to worry about it. For RT we'll want to enable that as well.
> >
> > I'll give this a shot later today.
>
> I took a poke at it. Did I do something similar to what you had in
> mind, or just hide behind performance stealing paranoid trylock loops?
> Box survived 1000 x xfstests 006 and dbench [-s] massive right off the
> bat, so it gets posted despite skepticism.
Great, thanks! I got stuck in bug land on Friday. You mentioned
performance problems earlier on Saturday, did this improve performance?
One other question:
> again:
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + while (atomic_read(&eb->blocking_readers))
> + cpu_chill();
> + while(!read_trylock(&eb->lock))
> + cpu_chill();
> + if (atomic_read(&eb->blocking_readers)) {
> + read_unlock(&eb->lock);
> + goto again;
> + }
Why use read_trylock() in a loop instead of just trying to take the
lock? Is this an RTism or are there other reasons?
-chris
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