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/ocfs2/file.c b/fs/ocfs2/file.c
index 1406c37..2c3a465 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -2236,9 +2236,9 @@
ocfs2_iocb_clear_sem_locked(iocb);
relock:
- /* to match setattr's i_mutex -> i_alloc_sem -> rw_lock ordering */
+ /* to match setattr's i_mutex -> rw_lock ordering */
if (direct_io) {
- down_read(&inode->i_alloc_sem);
+ atomic_inc(&inode->i_dio_count);
have_alloc_sem = 1;
/* communicate with ocfs2_dio_end_io */
ocfs2_iocb_set_sem_locked(iocb);
@@ -2290,7 +2290,7 @@
*/
if (direct_io && !can_do_direct) {
ocfs2_rw_unlock(inode, rw_level);
- up_read(&inode->i_alloc_sem);
+ inode_dio_done(inode);
have_alloc_sem = 0;
rw_level = -1;
@@ -2361,8 +2361,7 @@
/*
* deep in g_f_a_w_n()->ocfs2_direct_IO we pass in a ocfs2_dio_end_io
* function pointer which is called when o_direct io completes so that
- * it can unlock our rw lock. (it's the clustered equivalent of
- * i_alloc_sem; protects truncate from racing with pending ios).
+ * it can unlock our rw lock.
* Unfortunately there are error cases which call end_io and others
* that don't. so we don't have to unlock the rw_lock if either an
* async dio is going to do it in the future or an end_io after an
@@ -2379,7 +2378,7 @@
out_sems:
if (have_alloc_sem) {
- up_read(&inode->i_alloc_sem);
+ inode_dio_done(inode);
ocfs2_iocb_clear_sem_locked(iocb);
}
@@ -2531,8 +2530,8 @@
* need locks to protect pending reads from racing with truncate.
*/
if (filp->f_flags & O_DIRECT) {
- down_read(&inode->i_alloc_sem);
have_alloc_sem = 1;
+ atomic_inc(&inode->i_dio_count);
ocfs2_iocb_set_sem_locked(iocb);
ret = ocfs2_rw_lock(inode, 0);
@@ -2575,7 +2574,7 @@
bail:
if (have_alloc_sem) {
- up_read(&inode->i_alloc_sem);
+ inode_dio_done(inode);
ocfs2_iocb_clear_sem_locked(iocb);
}
if (rw_level != -1)
