Commit 3fb962bd authored by Nathan Scott's avatar Nathan Scott
Browse files

Fix a direct I/O locking issue revealed by the new mutex code. 

Affects only XFS (i.e. DIO_OWN_LOCKING case) - currently it is
not possible to get i_mutex locking correct when using DIO_OWN
direct I/O locking in a filesystem due to indeterminism in the
possible return code/lock/unlock combinations.  This can cause
a direct read to attempt a double i_mutex unlock inside XFS.

We're now ensuring __blockdev_direct_IO always exits with the
inode i_mutex (still) held for a direct reader.

Tested with the three different locking modes (via direct block
device access, ext3 and XFS) - both reading and writing; cannot
find any regressions resulting from this change, and it clearly
fixes the mutex_unlock warning originally reported here:
http://marc.theaimsgroup.com/?l=linux-kernel&m=114189068126253&w=2



Signed-off-by: default avatarNathan Scott <nathans@sgi.com>
Acked-by: default avatarChristoph Hellwig <hch@lst.de>
parent 3759fa9c

fs/direct-io.c

+12 −9
Original line number Diff line number Diff line
@@ -1155,15 +1155,16 @@ direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode, 
 * For writes, i_mutex is not held on entry; it is never taken.

 *

 * DIO_LOCKING (simple locking for regular files)

 * For writes we are called under i_mutex and return with i_mutex held, even though

 * it is internally dropped.

 * For writes we are called under i_mutex and return with i_mutex held, even

 * though it is internally dropped.

 * For reads, i_mutex is not held on entry, but it is taken and dropped before

 * returning.

 *

 * DIO_OWN_LOCKING (filesystem provides synchronisation and handling of

 *	uninitialised data, allowing parallel direct readers and writers)

 * For writes we are called without i_mutex, return without it, never touch it.

 * For reads, i_mutex is held on entry and will be released before returning.

 * For reads we are called under i_mutex and return with i_mutex held, even

 * though it may be internally dropped.

 *

 * Additional i_alloc_sem locking requirements described inline below.

 */
@@ -1182,7 +1183,8 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 
	ssize_t retval = -EINVAL;

	loff_t end = offset;

	struct dio *dio;

	int reader_with_isem = (rw == READ && dio_lock_type == DIO_OWN_LOCKING);

	int release_i_mutex = 0;

	int acquire_i_mutex = 0;



	if (rw & WRITE)

		current->flags |= PF_SYNCWRITE;
@@ -1225,7 +1227,6 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 
	 *	writers need to grab i_alloc_sem only (i_mutex is already held)

	 * For regular files using DIO_OWN_LOCKING,

	 *	neither readers nor writers take any locks here

	 *	(i_mutex is already held and release for writers here)

	 */

	dio->lock_type = dio_lock_type;

	if (dio_lock_type != DIO_NO_LOCKING) {
@@ -1236,7 +1237,7 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 
			mapping = iocb->ki_filp->f_mapping;

			if (dio_lock_type != DIO_OWN_LOCKING) {

				mutex_lock(&inode->i_mutex);

				reader_with_isem = 1;

				release_i_mutex = 1;

			}



			retval = filemap_write_and_wait_range(mapping, offset,
@@ -1248,7 +1249,7 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 


			if (dio_lock_type == DIO_OWN_LOCKING) {

				mutex_unlock(&inode->i_mutex);

				reader_with_isem = 0;

				acquire_i_mutex = 1;

			}

		}
@@ -1269,11 +1270,13 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 
				nr_segs, blkbits, get_blocks, end_io, dio);



	if (rw == READ && dio_lock_type == DIO_LOCKING)

		reader_with_isem = 0;

		release_i_mutex = 0;



out:

	if (reader_with_isem)

	if (release_i_mutex)

		mutex_unlock(&inode->i_mutex);

	else if (acquire_i_mutex)

		mutex_lock(&inode->i_mutex);

	if (rw & WRITE)

		current->flags &= ~PF_SYNCWRITE;

	return retval;

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