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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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7
fs/ocfs2/aops.c
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@@ -551,9 +551,8 @@ bail:
/*
* ocfs2_dio_end_io is called by the dio core when a dio is finished. We're
* particularly interested in the aio/dio case. Like the core uses
* i_alloc_sem, we use the rw_lock DLM lock to protect io on one node from
* truncation on another.
* particularly interested in the aio/dio case. We use the rw_lock DLM lock
* to protect io on one node from truncation on another.
*/
static void ocfs2_dio_end_io(struct kiocb *iocb,
loff_t offset,
@@ -569,7 +568,7 @@ static void ocfs2_dio_end_io(struct kiocb *iocb,
BUG_ON(!ocfs2_iocb_is_rw_locked(iocb));
if (ocfs2_iocb_is_sem_locked(iocb)) {
up_read(&inode->i_alloc_sem);
inode_dio_done(inode);
ocfs2_iocb_clear_sem_locked(iocb);
}