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SubjectRe: [PATCH 4/6] kill-the-BKL/reiserfs: release the write lock inside get_neighbors()

* Chris Mason <chris.mason@oracle.com> wrote:

> On Fri, 2009-05-01 at 15:25 +0200, Frederic Weisbecker wrote:
> > On Fri, May 01, 2009 at 07:51:35AM +0200, Ingo Molnar wrote:
> > >
> > > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > >
> > > > get_neighbors() is used to get the left and/or right blocks
> > > > against a given one in order to balance a tree.
> > > >
> > > > sb_bread() is used to read the buffer of these neighors blocks and
> > > > while it waits for this operation, it might sleep.
> > > >
> > > > The bkl was released at this point, and then we can also release
> > > > the write lock before calling sb_bread().
> > > >
> > > > This is safe because if the filesystem is changed after this lock
> > > > release, the function returns REPEAT_SEARCH (aka SCHEDULE_OCCURRED
> > > > in the function header comments) in order to repeat the neighbhor
> > > > research.
> > > >
> > > > [ Impact: release the reiserfs write lock when it is not needed ]
> > >
> > > This should also be safe because under the BKL we _already_ dropped
> > > the lock when sb_bread() blocked (which it really would in the
> > > normal case).
> > >
> > > There's one special case to consider though: sb_read() maps to
> > > __bread() which can return without sleeping if the bh is already
> > > uptodate. So if the filesystem _knows_ that the bh is already
> > > uptodate and holds a reference to it (this is common pattern in
> > > filesystems), it can have a locking assumption on that.
> > >
>
> sb_bread calls __bread which calls __getblk which always calls
> might_sleep() before returning. So, the unlock isn't adding a
> schedule that wasn't there before.

ah, and might_sleep() can schedule under CONFIG_PREEMPT_VOLUNTARY=y.
Good point.

Ingo


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