On Fri, May 01, 2009 at 07:42:47AM +0200, Ingo Molnar wrote:
>
> * Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
>
> > flush_commit_list() uses ll_rw_block() to commit the pending log blocks.
> > ll_rw_block() might sleep, and the bkl was released at this point. Then
> > we can also relax the write lock at this point.
> >
> > [ Impact: release the reiserfs write lock when it is not needed ]
> >
> > Cc: Jeff Mahoney <jeffm@xxxxxxxx>
> > Cc: Chris Mason <chris.mason@xxxxxxxxxx>
> > Cc: Alexander Beregalov <a.beregalov@xxxxxxxxx>
> > Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > ---
> > fs/reiserfs/journal.c | 7 +++++--
> > 1 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c
> > index 373d080..b1ebd5a 100644
> > --- a/fs/reiserfs/journal.c
> > +++ b/fs/reiserfs/journal.c
> > @@ -1120,8 +1120,11 @@ static int flush_commit_list(struct super_block *s,
> > SB_ONDISK_JOURNAL_SIZE(s);
> > tbh = journal_find_get_block(s, bn);
> > if (tbh) {
> > - if (buffer_dirty(tbh))
> > - ll_rw_block(WRITE, 1, &tbh) ;
> > + if (buffer_dirty(tbh)) {
> > + reiserfs_write_unlock(s);
> > + ll_rw_block(WRITE, 1, &tbh);
> > + reiserfs_write_lock(s);
> > + }
> > put_bh(tbh) ;
> > }
> > }
>
> there's 7 other instances of ll_rw_block():
>
> fs/reiserfs/journal.c- spin_unlock(lock);
> fs/reiserfs/journal.c: ll_rw_block(WRITE, 1, &bh);
> fs/reiserfs/journal.c- spin_lock(lock);
> --
> fs/reiserfs/journal.c- reiserfs_write_unlock(s);
> fs/reiserfs/journal.c: ll_rw_block(WRITE, 1, &tbh);
> fs/reiserfs/journal.c- reiserfs_write_lock(s);
> --
> fs/reiserfs/journal.c- /* read in the log blocks, memcpy to the corresponding real block */
> fs/reiserfs/journal.c: ll_rw_block(READ, get_desc_trans_len(desc), log_blocks);
> fs/reiserfs/journal.c- for (i = 0; i < get_desc_trans_len(desc); i++) {
> --
> fs/reiserfs/journal.c- set_buffer_dirty(real_blocks[i]);
> fs/reiserfs/journal.c: ll_rw_block(SWRITE, 1, real_blocks + i);
> fs/reiserfs/journal.c- }
> --
> fs/reiserfs/journal.c- }
> fs/reiserfs/journal.c: ll_rw_block(READ, j, bhlist);
> fs/reiserfs/journal.c- for (i = 1; i < j; i++)
> --
> fs/reiserfs/stree.c- if (!buffer_uptodate(bh[j]))
> fs/reiserfs/stree.c: ll_rw_block(READA, 1, bh + j);
> fs/reiserfs/stree.c- brelse(bh[j]);
> --
> fs/reiserfs/stree.c- reada_blocks, reada_count);
> fs/reiserfs/stree.c: ll_rw_block(READ, 1, &bh);
> fs/reiserfs/stree.c- reiserfs_write_unlock(sb);
> --
> fs/reiserfs/super.c-{
> fs/reiserfs/super.c: ll_rw_block(READ, 1, &(SB_BUFFER_WITH_SB(s)));
> fs/reiserfs/super.c- reiserfs_write_unlock(s);
>
> in particular the second stree.c one and the super.c has a
> write-unlock straight before the lock-drop.
>
> I think the stree.c unlock could be moved to before the
> ll_rw_block() call straight away.
Indeed.
>
> The super.c one needs more care: first put &(SB_BUFFER_WITH_SB(s))
> into a local variable, then unlock the wite-lock, then call
> ll_rw_block(). (This is important because &(SB_BUFFER_WITH_SB(s)) is
> global filesystem state that has to be read with the lock held.)
Indeed &(SB_BUFFER_WITH_SB(s)) is a pointer to blocks that
reflect the state of the filesystem but it was already not
safe on the old code.
ll_rw_block() may sleep, and wait_on_buffer() too. And this
pointer could have changed already during these sleeps.
If we put it in a local variable, it prevents from a change of the
pointer value, but not from its content, like in the older scheme.
And I guess this pointer is unlikely to change, this is about the superblock
and the bitmap...
But I'm not sure. I guess it's indeed better to put it in a local variable.
> ll_rw_block() generally always has a chance to block (especially on
> READ) - so the other places could be converted to drop the
> write-lock too. Most seem straightforward - some need similar
> local-variable treatment as super.c.
Ok, thanks!
> Ingo
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