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 <manf...@colorfullife.com>
> >
> >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 <manf...@colorfullife.com>
> 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 <manf...@colorfullife.com>
> Cc: <sta...@vger.kernel.org>
> Cc: Alan Stern <st...@rowland.harvard.edu>
> Cc: Sasha Levin <sasha.le...@oracle.com>
> Cc: Pablo Neira Ayuso <pa...@netfilter.org>
> Cc: netfilter-de...@vger.kernel.org
> ---
> 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
>
