From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752755AbdGCUER (ORCPT ); Mon, 3 Jul 2017 16:04:17 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:44004 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752267AbdGCUEP (ORCPT ); Mon, 3 Jul 2017 16:04:15 -0400 Date: Mon, 3 Jul 2017 16:04:14 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "Paul E. McKenney" cc: Manfred Spraul , , , , , , , , , , , , , , , Pablo Neira Ayuso , Jozsef Kadlecsik , Florian Westphal , "David S. Miller" , Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair In-Reply-To: <20170703171420.GC2393@linux.vnet.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 3 Jul 2017, Paul E. McKenney wrote: > On Mon, Jul 03, 2017 at 10:39:49AM -0400, Alan Stern wrote: > > On Sat, 1 Jul 2017, Manfred Spraul wrote: > > > > > 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 > > > Cc: > > > Cc: Sasha Levin > > > Cc: Pablo Neira Ayuso > > > Cc: netfilter-devel@vger.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; > > > > As far as I can tell, this read does not need to have ACQUIRE > > semantics. > > > > You need to guarantee that two things can never happen: > > > > (1) We read nf_conntrack_locks_all == false, and this routine's > > critical section for nf_conntrack_locks[i] runs after the > > (empty) critical section for that lock in > > nf_conntrack_all_lock(). > > > > (2) We read nf_conntrack_locks_all == true, and this routine's > > critical section for nf_conntrack_locks_all_lock runs before > > the critical section in nf_conntrack_all_lock(). > > > > In fact, neither one can happen even if smp_load_acquire() is replaced > > with READ_ONCE(). The reason is simple enough, using this property of > > spinlocks: > > > > If critical section CS1 runs before critical section CS2 (for > > the same lock) then: (a) every write coming before CS1's > > spin_unlock() will be visible to any read coming after CS2's > > spin_lock(), and (b) no write coming after CS2's spin_lock() > > will be visible to any read coming before CS1's spin_unlock(). > > > > Thus for (1), assuming the critical sections run in the order mentioned > > above, since nf_conntrack_all_lock() writes to nf_conntrack_locks_all > > before releasing nf_conntrack_locks[i], and since nf_conntrack_lock() > > acquires nf_conntrack_locks[i] before reading nf_conntrack_locks_all, > > by (a) the read will always see the write. > > > > Similarly for (2), since nf_conntrack_all_lock() acquires > > nf_conntrack_locks_all_lock before writing to nf_conntrack_locks_all, > > and since nf_conntrack_lock() reads nf_conntrack_locks_all before > > releasing nf_conntrack_locks_all_lock, by (b) the read cannot see the > > write. > > And the Linux kernel memory model (https://lwn.net/Articles/718628/ > and https://lwn.net/Articles/720550/) agrees with Alan. Here is > a litmus test, which emulates spin_lock() with xchg_acquire() and > spin_unlock() with smp_store_release(): > > ------------------------------------------------------------------------ > > C C-ManfredSpraul-L1G1xchgnr.litmus > > (* Expected result: Never. *) > > { > } > > P0(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held) > { > /* Acquire local lock. */ > r10 = xchg_acquire(lcl, 1); > r1 = READ_ONCE(*nfcla); > if (r1) { > smp_store_release(lcl, 0); > r11 = xchg_acquire(gbl, 1); > r12 = xchg_acquire(lcl, 1); > smp_store_release(gbl, 0); > } > r2 = READ_ONCE(*gbl_held); > WRITE_ONCE(*lcl_held, 1); > WRITE_ONCE(*lcl_held, 0); > smp_store_release(lcl, 0); > } > > P1(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held) > { > /* Acquire global lock. */ > r10 = xchg_acquire(gbl, 1); > WRITE_ONCE(*nfcla, 1); > r11 = xchg_acquire(lcl, 1); > smp_store_release(lcl, 0); > r2 = READ_ONCE(*lcl_held); > WRITE_ONCE(*gbl_held, 1); This litmus test is incomplete, because it omits the assignment setting nf_conntrack_locks_all back to false when the global lock is released. You should insert smp_store_release(*nfcla, 0); right here. > WRITE_ONCE(*gbl_held, 0); > smp_store_release(gbl, 0); > } > > exists > ((0:r2=1 \/ 1:r2=1) /\ 0:r10=0 /\ 0:r11=0 /\ 0:r12=0 /\ 1:r10=0 /\ 1:r11=0) With that addition, the litmus test fails unless the read of nfcla in P0 is an smp_load_acquire. So Manfred's patch should not be changed. Alan Stern