linux-kernel - Re: [PATCH 4/7] aio: remove the extra get_file/fput pair in io_submit

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Message-ID: <20180502220457.GB4116@magnolia>
Date:   Wed, 2 May 2018 15:04:57 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     viro@...iv.linux.org.uk, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] aio: remove the extra get_file/fput pair in
 io_submit_one

On Wed, May 02, 2018 at 11:14:45PM +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.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

LGTM,
Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>

--D

> ---
>  fs/aio.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 18507743757a..e5866e2fc331 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1515,16 +1515,19 @@ 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.
> +		 * 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 the 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);
>  			__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 +1602,6 @@ 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);
>  	switch (iocb->aio_lio_opcode) {
>  	case IOCB_CMD_PREAD:
>  		ret = aio_read(&req->common, iocb, false, compat);
> @@ -1618,8 +1620,13 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  		ret = -EINVAL;
>  		break;
>  	}
> -	fput(file);
>  
> +	/*
> +	 * If ret is -EIOCBQUEUED, ownership of the file reference acquired
> +	 * above passed to the file system, which at this point might have
> +	 * dropped the reference, so we must be careful to not reference it
> +	 * once we have called into the file system.
> +	 */
>  	if (ret && ret != -EIOCBQUEUED)
>  		goto out_put_req;
>  	return 0;
> -- 
> 2.17.0
> 
> --
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