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>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/truncate.c b/mm/truncate.c
index e13f22efaad7..003c6c685fc8 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -622,12 +622,11 @@ int vmtruncate_range(struct inode *inode, loff_t offset, loff_t end)
 		return -ENOSYS;
 
 	mutex_lock(&inode->i_mutex);
-	down_write(&inode->i_alloc_sem);
+	inode_dio_wait(inode);
 	unmap_mapping_range(mapping, offset, (end - offset), 1);
 	inode->i_op->truncate_range(inode, offset, end);
 	/* unmap again to remove racily COWed private pages */
 	unmap_mapping_range(mapping, offset, (end - offset), 1);
-	up_write(&inode->i_alloc_sem);
 	mutex_unlock(&inode->i_mutex);
 
 	return 0;