On Tue, Nov 08, 2016 at 12:08:09PM +0100, Jan Kara wrote:
> Implement basic iomap_begin function that handles reading and use it for
> DAX reads.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/ext4/ext4.h | 2 ++
> fs/ext4/file.c | 38 +++++++++++++++++++++++++++++++++++++-
> fs/ext4/inode.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 93 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 282a51b07c57..098b39910001 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3271,6 +3271,8 @@ static inline bool ext4_aligned_io(struct inode *inode, loff_t off, loff_t len)
> return IS_ALIGNED(off, blksize) && IS_ALIGNED(len, blksize);
> }
>
> +extern struct iomap_ops ext4_iomap_ops;
> +
> #endif /* __KERNEL__ */
>
> #define EFSBADCRC EBADMSG /* Bad CRC detected */
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 9facb4dc5c70..1f25c644cb12 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -31,6 +31,42 @@
> #include "xattr.h"
> #include "acl.h"
>
> +#ifdef CONFIG_FS_DAX
> +static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> + struct inode *inode = file_inode(iocb->ki_filp);
> + ssize_t ret;
> +
> + inode_lock_shared(inode);
> + /*
> + * Recheck under inode lock - at this point we are sure it cannot
> + * change anymore
> + */
> + if (!IS_DAX(inode)) {
> + inode_unlock_shared(inode);
> + /* Fallback to buffered IO in case we cannot support DAX */
> + return generic_file_read_iter(iocb, to);
Is this not also racy, since we've just dropped the inode lock? What's to
prevent this sequence?
Thread 0 Thread 1
-------- --------
ext4_file_read_iter()
IS_DAX() returns true
changes S_DAX to false
ext4_dax_read_iter()
inode_lock_shared()
IS_DAX() returns false
inode_unlock_shared()
changes S_DAX to true
generic_file_read_iter() on a DAX inode
Or are we okay in this scenario?
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