* Chris Mason <chris.mason@xxxxxxxxxx> 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@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe reiserfs-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
