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SubjectRe: [PATCH 4/7] aio: remove the extra get_file/fput pair in io_submit_one
On Sun, Apr 15, 2018 at 05:01:05PM +0200, Christoph Hellwig wrote:
> If we release the lockdep write protection token before calling into
> ->write_iter and thus never access the file pointer after an -EIOCBQUEUED
> return from ->write_iter or ->read_iter we don't need this extra
> reference.

Hmm, subtleties lurk to this unfamiliar reader...

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/aio.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 18507743757a..d7be32cdd1db 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1515,16 +1515,17 @@ static ssize_t aio_write(struct kiocb *req, struct iocb *iocb, bool vectored,
> return ret;
> ret = rw_verify_area(WRITE, file, &req->ki_pos, iov_iter_count(&iter));
> if (!ret) {
> - req->ki_flags |= IOCB_WRITE;
> - file_start_write(file);
> - ret = aio_ret(req, call_write_iter(file, req, &iter));
> /*
> * We release freeze protection in aio_complete(). Fool lockdep
> * by telling it the lock got released so that it doesn't
> * complain about held lock when we return to userspace.
> */
> - if (S_ISREG(file_inode(file)->i_mode))
> + if (S_ISREG(file_inode(file)->i_mode)) {
> + __sb_start_write(file_inode(file)->i_sb, SB_FREEZE_WRITE, true);

It took me a while to figure out that this ^^^ is the same as the
file_start_write call that you remove above, can you please update the
comment to note that we take freeze protection for the file before
screwing with lockdep? e.g.,

/*
* Open-code file_start_write here to grab freeze protection, which will
* be released by another thread in aio_complete(). Fool lockdep by
* telling it the lock got released so that it doesn't complain about
* held lock when we return to userspace.
*/

> __sb_writers_release(file_inode(file)->i_sb, SB_FREEZE_WRITE);
> + }
> + req->ki_flags |= IOCB_WRITE;
> + ret = aio_ret(req, call_write_iter(file, req, &iter));
> }
> kfree(iovec);
> return ret;
> @@ -1599,7 +1600,6 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
> req->ki_user_iocb = user_iocb;
> req->ki_user_data = iocb->aio_data;
>
> - get_file(file);

Here we have a reference to *file, but...

> switch (iocb->aio_lio_opcode) {
> case IOCB_CMD_PREAD:
> ret = aio_read(&req->common, iocb, false, compat);
> @@ -1618,7 +1618,6 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
> ret = -EINVAL;
> break;
> }
> - fput(file);

...by the time we get to here the reference may have gone away, but
you'd have to dig through aio_{read,write} -> call_{r,w}_iter ->
{r,w}_iter in order to figure out that the reference isn't valid
anymore on a EIOCBQUEUED return.

That's a little subtle, can you add a comment about that?

/*
* If ret is EIOCBQUEUED here, the ->read_iter/->write_iter dropped the
* reference on *file. We don't ourselves ensure a reference to the
* file, so we must be careful about that here and in the subfunctions.
*/
--D

>
> if (ret && ret != -EIOCBQUEUED)
> goto out_put_req;
> --
> 2.17.0
>

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