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

[Ingo Molnar <mingo@elte.hu>]

* 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.

No such assumption seems to be present here though.

	Ingo