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Message-Id: <20170702020045.GR2393@linux.vnet.ibm.com>
Date: Sat, 1 Jul 2017 19:00:45 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
netdev@...r.kernel.org, oleg@...hat.com, akpm@...ux-foundation.org,
mingo@...hat.com, dave@...olabs.net, tj@...nel.org, arnd@...db.de,
linux-arch@...r.kernel.org, will.deacon@....com,
peterz@...radead.org, stern@...land.harvard.edu,
parri.andrea@...il.com, torvalds@...ux-foundation.org,
Pablo Neira Ayuso <pablo@...filter.org>,
Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
Florian Westphal <fw@...len.de>,
"David S. Miller" <davem@...emloft.net>, coreteam@...filter.org
Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with
lock/unlock pair
On Sat, Jul 01, 2017 at 09:44:12PM +0200, Manfred Spraul wrote:
> Hi Paul,
>
> On 06/30/2017 02:01 AM, Paul E. McKenney wrote:
> >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@...ux.vnet.ibm.com>
> >Cc: Pablo Neira Ayuso<pablo@...filter.org>
> >Cc: Jozsef Kadlecsik<kadlec@...ckhole.kfki.hu>
> >Cc: Florian Westphal<fw@...len.de>
> >Cc: "David S. Miller"<davem@...emloft.net>
> >Cc:<netfilter-devel@...r.kernel.org>
> >Cc:<coreteam@...filter.org>
> >Cc:<netdev@...r.kernel.org>
> >Cc: Will Deacon<will.deacon@....com>
> >Cc: Peter Zijlstra<peterz@...radead.org>
> >Cc: Alan Stern<stern@...land.harvard.edu>
> >Cc: Andrea Parri<parri.andrea@...il.com>
> >Cc: Linus Torvalds<torvalds@...ux-foundation.org>
> >---
> > 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)) {
> I think here an ACQUIRE is missing.
> > 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. */
> > }
> > }
> As far as I see, nf_conntrack_locks[] nests inside
> nf_conntrack_lock_all_lock.
> So
> spin_lock(&nf_conntrack_locks_all_lock);
> spin_lock(lock);
> spin_unlock(&nf_conntrack_locks_all_lock);
>
> can replace the retry logic.
>
> Correct? Then what about the attached patch?
At first glance, it looks correct to me, thank you! I have replaced my
patch with this one for testing and further review.
Thanx, Paul
> --
> Manfred
>
>
> >From 453e7a77f3756d939c754031b092cbdfbd149559 Mon Sep 17 00:00:00 2001
> From: Manfred Spraul <manfred@...orfullife.com>
> Date: Sun, 21 Aug 2016 07:17:55 +0200
> Subject: [PATCH] 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().
>
> Fixes: b16c29191dc8
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
> Cc: <stable@...r.kernel.org>
> Cc: Sasha Levin <sasha.levin@...cle.com>
> Cc: Pablo Neira Ayuso <pablo@...filter.org>
> Cc: netfilter-devel@...r.kernel.org
> ---
> net/netfilter/nf_conntrack_core.c | 44 +++++++++++++++++++++------------------
> 1 file changed, 24 insertions(+), 20 deletions(-)
>
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index e847dba..1193565 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -96,19 +96,24 @@ 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 */
> + 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);
>
> @@ -149,18 +154,17 @@ 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]);
> }
> }
>
> --
> 2.9.4
>
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