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    SubjectRe: [PATCH 6/6] kill-the-BKL/reiserfs: release the write lock on flush_commit_list()

    * Frederic Weisbecker <fweisbec@gmail.com> wrote:

    > On Fri, May 01, 2009 at 07:42:47AM +0200, Ingo Molnar wrote:
    > >
    > > * Frederic Weisbecker <fweisbec@gmail.com> 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@suse.com>
    > > > Cc: Chris Mason <chris.mason@oracle.com>
    > > > Cc: Alexander Beregalov <a.beregalov@gmail.com>
    > > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > > > ---
    > > > 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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