NFS: Prevent nfs_getattr() hang during heavy write workloads

POSIX requires that ctime and mtime, as reported by the stat(2) call,
reflect the activity of the most recent write(2).  To that end, nfs_getattr()
flushes pending dirty writes to a file before doing a GETATTR to allow the
NFS server to set the file's size, ctime, and mtime properly.

However, nfs_getattr() can be starved when a constant stream of application
writes to a file prevents nfs_wb_nocommit() from completing.  This usually
results in hangs of programs doing a stat against an NFS file that is being
written.  "ls -l" is a common victim of this behavior.

To prevent starvation, hold the file's i_mutex in nfs_getattr() to
freeze applications writes temporarily so the client can more quickly obtain
clean values for a file's size, mtime, and ctime.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -461,9 +461,18 @@ int nfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
 	int need_atime = NFS_I(inode)->cache_validity & NFS_INO_INVALID_ATIME;
 	int err;
 
-	/* Flush out writes to the server in order to update c/mtime */
-	if (S_ISREG(inode->i_mode))
+	/*
+	 * Flush out writes to the server in order to update c/mtime.
+	 *
+	 * Hold the i_mutex to suspend application writes temporarily;
+	 * this prevents long-running writing applications from blocking
+	 * nfs_wb_nocommit.
+	 */
+	if (S_ISREG(inode->i_mode)) {
+		mutex_lock(&inode->i_mutex);
 		nfs_wb_nocommit(inode);
+		mutex_unlock(&inode->i_mutex);
+	}
 
 	/*
 	 * We may force a getattr if the user cares about atime.