[PATCH] netfs: Downgrade i_rwsem for a buffered write

[David Howells <dhowells@xxxxxxxxxx>]

In the I/O locking code borrowed from NFS into netfslib, i_rwsem is held
locked across a buffered write - but this causes a performance regression
in cifs as it excludes buffered reads for the duration (cifs didn't use any
locking for buffered reads).

Mitigate this somewhat by downgrading the i_rwsem to a read lock across the
buffered write.  This at least allows parallel reads to occur whilst
excluding other writes, DIO, truncate and setattr.

Note that this shouldn't be a problem for a buffered write as a read
through an mmap can circumvent i_rwsem anyway.

Also note that we might want to make this change in NFS also.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Steve French <sfrench@xxxxxxxxx>
cc: Paulo Alcantara <pc@xxxxxxxxxxxxx>
cc: Trond Myklebust <trondmy@xxxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: netfs@xxxxxxxxxxxxxxx
cc: linux-cifs@xxxxxxxxxxxxxxx
cc: linux-nfs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/netfs/locking.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/locking.c b/fs/netfs/locking.c
index 21eab56ee2f9..2249ecd09d0a 100644
--- a/fs/netfs/locking.c
+++ b/fs/netfs/locking.c
@@ -109,6 +109,7 @@ int netfs_start_io_write(struct inode *inode)
 		up_write(&inode->i_rwsem);
 		return -ERESTARTSYS;
 	}
+	downgrade_write(&inode->i_rwsem);
 	return 0;
 }
 EXPORT_SYMBOL(netfs_start_io_write);
@@ -123,7 +124,7 @@ EXPORT_SYMBOL(netfs_start_io_write);
 void netfs_end_io_write(struct inode *inode)
 	__releases(inode->i_rwsem)
 {
-	up_write(&inode->i_rwsem);
+	up_read(&inode->i_rwsem);
 }
 EXPORT_SYMBOL(netfs_end_io_write);