* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 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.
No, it was safe prior. This was the prior code:
fs/reiserfs/super.c: ll_rw_block(READ, 1, &(SB_BUFFER_WITH_SB(s)));
The argument is passed to ll_rw_block _before_ the scheduling. So
the dereference always happens with the BKL held, and is safe.
It is true that ll_rw_block() can sleep (and will most likely sleep
for a READ command), but that's not the issue: the issue is the
former dereference which can now get out from under the lock. (it
might still be safe in special circumstances - but has to be
reviewed carefully.)
Ingo
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