[PATCH] fs/dcache.c: avoid race when updating nr_unused count of unused dentries.

[PATCH] fs/dcache.c: avoid race when updating nr_unused count of unused dentries.

From: NeilBrown <neilb@cse.unsw.edu.au>

d_count == 1 is no guarantee that dentry is on the dentry_unused list, even
if it has just been incremented inside dcache_lock, as dput can decrement
at any time.

This test from Greg Banks is much safer, and is more transparently correct.

Cc: Maneesh Soni <maneesh@in.ibm.com>, Dipankar Sarma <dipankar@in.ibm.com>
From: Greg Banks <gnb@melbourne.sgi.com>
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>