Hi Ryusuke,
On Sat, 2013-07-27 at 12:02 +0900, Ryusuke Konishi wrote:
> > From: Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
> > Subject: [PATCH v2] nilfs2: remove double bio_put() in nilfs_end_bio_write() for BIO_EOPNOTSUPP error
> >
> > This patch removes double call of bio_put() in nilfs_end_bio_write()
> > for the case of BIO_EOPNOTSUPP error detection. The issue was found
> > by Dan Carpenter and he suggests first version of the fix too.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
> > CC: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxxxxxx>
> > ---
> > fs/nilfs2/segbuf.c | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/nilfs2/segbuf.c b/fs/nilfs2/segbuf.c
> > index dc9a913..0b09ec9 100644
> > --- a/fs/nilfs2/segbuf.c
> > +++ b/fs/nilfs2/segbuf.c
> > @@ -345,8 +345,7 @@ static void nilfs_end_bio_write(struct bio *bio, int err)
> >
> > if (err == -EOPNOTSUPP) {
> > set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
> > - bio_put(bio);
> > - /* to be detected by submit_seg_bio() */
> > + /* to be detected by nilfs_segbuf_submit_bio() */
> > }
> >
> > if (!uptodate)
> > @@ -377,12 +376,13 @@ static int nilfs_segbuf_submit_bio(struct nilfs_segment_buffer *segbuf,
> > bio->bi_private = segbuf;
> > bio_get(bio);
> > submit_bio(mode, bio);
> > + segbuf->sb_nbio++;
> > if (bio_flagged(bio, BIO_EOPNOTSUPP)) {
>
> > + segbuf->sb_nbio--;
>
> This decrement looks wrong.
>
I tried to understand your vision but I am thinking that my patch is
correct. Maybe, I missed something in my considerations. I describe my
vision in more details.
> Otherwise, your change of nilfs_segbuf_submit_bio() is just a
> equivalent transformation and doesn't fix problem, that is, a mismatch
> of the number of calls between complete() and wait_for_completion().
>
> In your patch, nilfs_end_bio_write() function calls the complete()
> routine as before even if it received an EOPNOTSUPP error.
>
> In that case, segbuf->sb_nbio must be incremented to call
> wait_for_completion() the right number of times.
>
I moved incrementing of sb_nbio nearly after submit_bio(mode, bio). So,
firstly, we call submit_bio(mode, bio) and segbuf->sb_nbio++. Then,
because of asynchronous nature of nilfs_end_bio_write() call, we have
two alternatives: (1) nilfs_end_bio_write() detects that BIO_EOPNOTSUPP
flag is set; (2) BIO_EOPNOTSUPP flag isn't set because bio is not
processed yet or we haven't error.
If nilfs_end_bio_write() method would detect -EOPNOTSUPP error then it
set BIO_EOPNOTSUPP flag, increment sb_err field and to call complete().
So, if we detect that BIO_EOPNOTSUPP flag is set in
nilfs_segbuf_submit_bio() then we decrement sb_nbio and to return from
nilfs_segbuf_submit_bio() with error. And if we had submitted
successfully some bios before then it will need to wait theirs
completion.
Otherwise, if BIO_EOPNOTSUPP flag is not set yet during check in
nilfs_segbuf_submit_bio() then sb_nbio is remained incremented. And, for
example, nilfs_segbuf_wait() can end correctly the cycle with
wait_for_completion() call. We will know about error during bio
processing because of sb_err was incremented in nilfs_end_bio_write().
So, I suppose that my logic is right. Please, correct me if I am wrong.
What do you think?
By the way, why sb_err has atomic_t type but sb_nbio is simple int type?
Maybe, sb_nbio should be atomic_t type too?
With the best regards,
Vyacheslav Dubeyko.
> Note that wait_for_completion() is called based on the count of
> segbuf->sb_nbio even if nilfs_segbuf_submit_bio() returns an error.
> This is performed through the following path:
>
> nilfs_segbuf_submit_bio
> nilfs_segbuf_submit_bh
> nilfs_segbuf_write
> nilfs_write_logs
> nilfs_segctor_write
> nilfs_segctor_do_construct
> nilfs_segctor_abort_construction
> nilfs_wait_on_logs
> nilfs_segbuf_wait
> wait_for_completion
> (repeated for the number of times of segbuf->sb_nbio)
>
>
> If you think this is a separate problem, then it should be fixed in
> another patch and nilfs_segbuf_submit_bio() should not be touched in
> this patch.
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