netfs: Downgrade i_rwsem for a buffered write

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@redhat.com>
Link: https://lore.kernel.org/r/1317958.1729096113@warthog.procyon.org.uk
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Trond Myklebust <trondmy@kernel.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/netfs/locking.c b/fs/netfs/locking.c
index 21eab56ee2f94f2d9a87ac11f99888cbe1da83fc..2249ecd09d0a8dd5cb31dfec798cc24c3ea1d411 100644 (file)
--- 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);