fs: kill i_alloc_sem
i_alloc_sem is a rather special rw_semaphore. It's the last one that may
be released by a non-owner, and it's write side is always mirrored by
real exclusion. It's intended use it to wait for all pending direct I/O
requests to finish before starting a truncate.
Replace it with a hand-grown construct:
- exclusion for truncates is already guaranteed by i_mutex, so it can
simply fall way
- the reader side is replaced by an i_dio_count member in struct inode
that counts the number of pending direct I/O requests. Truncate can't
proceed as long as it's non-zero
- when i_dio_count reaches non-zero we wake up a pending truncate using
wake_up_bit on a new bit in i_flags
- new references to i_dio_count can't appear while we are waiting for
it to read zero because the direct I/O count always needs i_mutex
(or an equivalent like XFS's i_iolock) for starting a new operation.
This scheme is much simpler, and saves the space of a spinlock_t and a
struct list_head in struct inode (typically 160 bits on a non-debug 64-bit
system).
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ntfs/inode.c b/fs/ntfs/inode.c
index c05d6dc..1371487 100644
--- a/fs/ntfs/inode.c
+++ b/fs/ntfs/inode.c
@@ -2357,12 +2357,7 @@
*
* Returns 0 on success or -errno on error.
*
- * Called with ->i_mutex held. In all but one case ->i_alloc_sem is held for
- * writing. The only case in the kernel where ->i_alloc_sem is not held is
- * mm/filemap.c::generic_file_buffered_write() where vmtruncate() is called
- * with the current i_size as the offset. The analogous place in NTFS is in
- * fs/ntfs/file.c::ntfs_file_buffered_write() where we call vmtruncate() again
- * without holding ->i_alloc_sem.
+ * Called with ->i_mutex held.
*/
int ntfs_truncate(struct inode *vi)
{
@@ -2887,8 +2882,7 @@
* We also abort all changes of user, group, and mode as we do not implement
* the NTFS ACLs yet.
*
- * Called with ->i_mutex held. For the ATTR_SIZE (i.e. ->truncate) case, also
- * called with ->i_alloc_sem held for writing.
+ * Called with ->i_mutex held.
*/
int ntfs_setattr(struct dentry *dentry, struct iattr *attr)
{