[PATCH] vfs: Fix RCU path walk failiures due to uninitializednameidata seq number for root directory

[Tim Chen]

During RCU walk in path_lookupat and path_openat, the rcu lookup
frequently failed because when root directory was looked up, seq number
was not properly set in nameidata.  We dropped out of RCU walk in
nameidata_drop_rcu due to mismatch in directory entry's seq number.  We
reverted to slow path walk that need to take references.

With the following patch, I saw a 50% increase in an exim mail server
benchmark throughput on a 4-socket Nehalem-EX system.

Thanks.

Tim

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
diff --git a/fs/namei.c b/fs/namei.c
index 3cb616d..e4b27a6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -697,6 +697,7 @@ static __always_inline void set_root_rcu(struct nameidata *nd)
 		do {
 			seq = read_seqcount_begin(&fs->seq);
 			nd->root = fs->root;
+			nd->seq = __read_seqcount_begin(&nd->root.dentry->d_seq);
 		} while (read_seqcount_retry(&fs->seq, seq));
 	}
 }


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