[PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair

[Paul E. McKenney]

There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair.  This commit therefore replaces the spin_unlock_wait() calls
in nf_conntrack_lock() and nf_conntrack_all_lock() with spin_lock()
followed immediately by spin_unlock().  These functions do not appear
to be invoked on any fastpaths.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxxxxxx>
Cc: Florian Westphal <fw@xxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: <netfilter-devel@xxxxxxxxxxxxxxx>
Cc: <coreteam@xxxxxxxxxxxxx>
Cc: <netdev@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 net/netfilter/nf_conntrack_core.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index e847dbaa0c6b..9f997859d160 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -99,15 +99,11 @@ void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
 	spin_lock(lock);
 	while (unlikely(nf_conntrack_locks_all)) {
 		spin_unlock(lock);
-
-		/*
-		 * Order the 'nf_conntrack_locks_all' load vs. the
-		 * spin_unlock_wait() loads below, to ensure
-		 * that 'nf_conntrack_locks_all_lock' is indeed held:
-		 */
-		smp_rmb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
-		spin_unlock_wait(&nf_conntrack_locks_all_lock);
+		/* Wait for nf_conntrack_locks_all_lock holder to release ... */
+		spin_lock(&nf_conntrack_locks_all_lock);
+		spin_unlock(&nf_conntrack_locks_all_lock);
 		spin_lock(lock);
+		/* ... and retry. */
 	}
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_lock);
@@ -150,17 +146,11 @@ static void nf_conntrack_all_lock(void)
 
 	spin_lock(&nf_conntrack_locks_all_lock);
 	nf_conntrack_locks_all = true;
-
-	/*
-	 * Order the above store of 'nf_conntrack_locks_all' against
-	 * the spin_unlock_wait() loads below, such that if
-	 * nf_conntrack_lock() observes 'nf_conntrack_locks_all'
-	 * we must observe nf_conntrack_locks[] held:
-	 */
-	smp_mb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
-
 	for (i = 0; i < CONNTRACK_LOCKS; i++) {
-		spin_unlock_wait(&nf_conntrack_locks[i]);
+		/* Wait for any current holder to release lock. */
+		spin_lock(&nf_conntrack_locks[i]);
+		spin_unlock(&nf_conntrack_locks[i]);
+		/* Next acquisition will see nf_conntrack_locks_all == true. */
 	}
 }
 
-- 
2.5.2