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>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index cd0e57f3a00f..cc3a09db41a9 100644
--- a/fs/nfs/inode.c
+++ b/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.