vfs: Fix absolute RCU path walk failures due to uninitialized seq number

During RCU walk in path_lookupat and path_openat, the rcu lookup
frequently failed if looking up an absolute path, 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.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: stable@kernel.org (v2.6.38)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/namei.c b/fs/namei.c
index e6cd6113872ccd4db1dadfa5deff08f90a45c8a0..54fc993e3027d4dff98dcf4f14ebbb4d817cdb0f 100644 (file)
--- 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));
        }
 }