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. > > No such assumption seems to be present here though. > > Ingo Yeah, fortunately it doesn't base its check on the state of the buffer but on the tree number of rebalancing. But I have to remember this pattern, it could be present elsewhere in reiserfs. Thanks. -- 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
