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, 4 deletions

diff --git a/fs/attr.c b/fs/attr.c
index caf2aa521e2b..f177ac86fa48 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -233,16 +233,13 @@ int notify_change(struct dentry * dentry, struct iattr * attr)
                 return error;
 
         if (ia_valid & ATTR_SIZE)
-                down_write(&dentry->d_inode->i_alloc_sem);
+                inode_dio_wait(inode);
 
         if (inode->i_op->setattr)
                 error = inode->i_op->setattr(dentry, attr);
         else
                 error = simple_setattr(dentry, attr);
 
-        if (ia_valid & ATTR_SIZE)
-                up_write(&dentry->d_inode->i_alloc_sem);
-
         if (!error)
                 fsnotify_change(dentry, ia_valid);