Ming Lei <ming....@canonical.com> writes:
> Each bio is always submitted to block device one by one,
> so it isn't necessary to increase the bio refcount by one
> each time with holding dio->bio_lock.
This patch opens up a race where a completion event can come in before
the refcount for the dio is incremented, resulting in refcount going
negative. I don't think that will actually cause problems, but it
certainly is ugly, and I doubt it was the intended design.
Before I dig into this any further, would you care to comment on why you
went down this path? Did you see spinlock contention here? And was
there a resultant performance improvement for some benchmark with the
patch applied?
Cheers,
Jeff
> Signed-off-by: Ming Lei <ming....@canonical.com>
> ---
> fs/direct-io.c | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 6fb00e3..57b8e73 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -79,6 +79,8 @@ struct dio_submit {
> get_block_t *get_block; /* block mapping function */
> dio_submit_t *submit_io; /* IO submition function */
>
> + long submitted_bio;
> +
> loff_t logical_offset_in_bio; /* current first logical block in bio */
> sector_t final_block_in_bio; /* current final block in bio + 1 */
> sector_t next_block_for_io; /* next block to be put under IO,
> @@ -121,7 +123,7 @@ struct dio {
> int is_async; /* is IO async ? */
> bool defer_completion; /* defer AIO completion to workqueue? */
> int io_error; /* IO error in completion path */
> - unsigned long refcount; /* direct_io_worker() and bios */
> + long refcount; /* direct_io_worker() and bios */
> struct bio *bio_list; /* singly linked via bi_private */
> struct task_struct *waiter; /* waiting task (NULL if none) */
>
> @@ -383,14 +385,9 @@ dio_bio_alloc(struct dio *dio, struct dio_submit *sdio,
> static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
> {
> struct bio *bio = sdio->bio;
> - unsigned long flags;
>
> bio->bi_private = dio;
>
> - spin_lock_irqsave(&dio->bio_lock, flags);
> - dio->refcount++;
> - spin_unlock_irqrestore(&dio->bio_lock, flags);
> -
> if (dio->is_async && dio->rw == READ)
> bio_set_pages_dirty(bio);
>
> @@ -403,15 +400,26 @@ static inline void dio_bio_submit(struct dio *dio,
> struct dio_submit *sdio)
> sdio->bio = NULL;
> sdio->boundary = 0;
> sdio->logical_offset_in_bio = 0;
> + sdio->submitted_bio++;
> }
>
> /*
> * Release any resources in case of a failure
> */
> -static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio)
> +static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio,
> + bool commit_refcount)
> {
> + unsigned long flags;
> +
> while (sdio->head < sdio->tail)
> page_cache_release(dio->pages[sdio->head++]);
> +
> + if (!commit_refcount)
> + return;
> +
> + spin_lock_irqsave(&dio->bio_lock, flags);
> + dio->refcount += (sdio->submitted_bio + 1);
> + spin_unlock_irqrestore(&dio->bio_lock, flags);
> }
>
> /*
> @@ -1215,7 +1223,6 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb,
> struct inode *inode,
> dio->i_size = i_size_read(inode);
>
> spin_lock_init(&dio->bio_lock);
> - dio->refcount = 1;
>
> sdio.iter = iter;
> sdio.final_block_in_request =
> @@ -1234,7 +1241,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb,
> struct inode *inode,
>
> retval = do_direct_IO(dio, &sdio, &map_bh);
> if (retval)
> - dio_cleanup(dio, &sdio);
> + dio_cleanup(dio, &sdio, false);
>
> if (retval == -ENOTBLK) {
> /*
> @@ -1267,7 +1274,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb,
> struct inode *inode,
> * It is possible that, we return short IO due to end of file.
> * In that case, we need to release all the pages we got hold on.
> */
> - dio_cleanup(dio, &sdio);
> + dio_cleanup(dio, &sdio, true);
>
> /*
> * All block lookups have been performed. For READ requests
--
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