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fs: kill i_alloc_sem

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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>
This commit is contained in:
Christoph Hellwig
2011-06-24 14:29:43 -04:00
committed by Al Viro
parent f9b5570d7f
commit bd5fe6c5eb
13 changed files with 78 additions and 53 deletions
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15
fs/ocfs2/file.c
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@ -2236,9 +2236,9 @@ static ssize_t ocfs2_file_aio_write(struct kiocb *iocb,
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 @@ relock:
*/
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 @@ out_dio:
/*
* 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:
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 @@ static ssize_t ocfs2_file_aio_read(struct kiocb *iocb,
* 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 @@ static ssize_t ocfs2_file_aio_read(struct kiocb *iocb,
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)