On Thu, Jul 06, 2017 at 08:45:59PM +0200, Manfred Spraul wrote: > Hi Paul, > > On 07/06/2017 01:31 AM, Paul E. McKenney wrote: > >From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> > > > >As we want to remove spin_unlock_wait() and replace it with explicit > >spin_lock()/spin_unlock() calls, we can use this to simplify the > >locking. > > > >In addition: > >- Reading nf_conntrack_locks_all needs ACQUIRE memory ordering. > >- The new code avoids the backwards loop. > > > >Only slightly tested, I did not manage to trigger calls to > >nf_conntrack_all_lock(). > > If you want: > Attached would be V2, with adapted comments. I do like the improved comments, thank you! Queued, and will be part of a later v3 of the series. Thanx, Paul > -- > Manfred > >From e3562faa1bc96e883108505e05deecaf38c87a26 Mon Sep 17 00:00:00 2001 > From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> > Date: Sun, 21 Aug 2016 07:17:55 +0200 > Subject: [PATCH 1/2] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() > > As we want to remove spin_unlock_wait() and replace it with explicit > spin_lock()/spin_unlock() calls, we can use this to simplify the > locking. > > In addition: > - Reading nf_conntrack_locks_all needs ACQUIRE memory ordering. > - The new code avoids the backwards loop. > > Only slightly tested, I did not manage to trigger calls to > nf_conntrack_all_lock(). > > V2: With improved comments, to clearly show how the barriers > pair. > > Fixes: b16c29191dc8 > Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Cc: Sasha Levin <sasha.levin@xxxxxxxxxx> > Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> > Cc: netfilter-devel@xxxxxxxxxxxxxxx > --- > net/netfilter/nf_conntrack_core.c | 52 ++++++++++++++++++++++----------------- > 1 file changed, 29 insertions(+), 23 deletions(-) > > diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c > index 9979f46..51390fe 100644 > --- a/net/netfilter/nf_conntrack_core.c > +++ b/net/netfilter/nf_conntrack_core.c > @@ -96,19 +96,26 @@ static struct conntrack_gc_work conntrack_gc_work; > > void nf_conntrack_lock(spinlock_t *lock) __acquires(lock) > { > + /* 1) Acquire the 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); > - spin_lock(lock); > - } > + /* 2) read nf_conntrack_locks_all, with ACQUIRE semantics > + * It pairs with the smp_store_release() in nf_conntrack_all_unlock() > + */ > + if (likely(smp_load_acquire(&nf_conntrack_locks_all) == false)) > + return; > + > + /* fast path failed, unlock */ > + spin_unlock(lock); > + > + /* Slow path 1) get global lock */ > + spin_lock(&nf_conntrack_locks_all_lock); > + > + /* Slow path 2) get the lock we want */ > + spin_lock(lock); > + > + /* Slow path 3) release the global lock */ > + spin_unlock(&nf_conntrack_locks_all_lock); > } > EXPORT_SYMBOL_GPL(nf_conntrack_lock); > > @@ -149,28 +156,27 @@ static void nf_conntrack_all_lock(void) > int i; > > 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) */ > + nf_conntrack_locks_all = true; > > for (i = 0; i < CONNTRACK_LOCKS; i++) { > - spin_unlock_wait(&nf_conntrack_locks[i]); > + spin_lock(&nf_conntrack_locks[i]); > + > + /* This spin_unlock provides the "release" to ensure that > + * nf_conntrack_locks_all==true is visible to everyone that > + * acquired spin_lock(&nf_conntrack_locks[]). > + */ > + spin_unlock(&nf_conntrack_locks[i]); > } > } > > static void nf_conntrack_all_unlock(void) > { > - /* > - * All prior stores must be complete before we clear > + /* All prior stores must be complete before we clear > * 'nf_conntrack_locks_all'. Otherwise nf_conntrack_lock() > * might observe the false value but not the entire > - * critical section: > + * critical section. > + * It pairs with the smp_load_acquire() in nf_conntrack_lock() > */ > smp_store_release(&nf_conntrack_locks_all, false); > spin_unlock(&nf_conntrack_locks_all_lock); > -- > 2.9.4 >