lkml.org 
[lkml]   [2009]   [May]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/6] kill-the-BKL/reiserfs: release the write lock inside get_neighbors()

* 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


\
 
 \ /
  Last update: 2009-05-01 07:55    [from the cache]
©2003-2011 Jasper Spaans