From: Alan Stern <stern@rowland.harvard.edu> To: Manfred Spraul <manfred@colorfullife.com> Cc: paulmck@linux.vnet.ibm.com, <linux-kernel@vger.kernel.org>, <netfilter-devel@vger.kernel.org>, <netdev@vger.kernel.org>, <oleg@redhat.com>, <akpm@linux-foundation.org>, <mingo@redhat.com>, <dave@stgolabs.net>, <tj@kernel.org>, <arnd@arndb.de>, <linux-arch@vger.kernel.org>, <will.deacon@arm.com>, <peterz@infradead.org>, <parri.andrea@gmail.com>, <torvalds@linux-foundation.org>, Pablo Neira Ayuso <pablo@netfilter.org>, Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>, Florian Westphal <fw@strlen.de>, "David S. Miller" <davem@davemloft.net>, <coreteam@netfilter.org>, <1vier1@web.de> Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair Date: Mon, 3 Jul 2017 15:57:03 -0400 (EDT) [thread overview] Message-ID: <Pine.LNX.4.44L0.1707031543170.2027-100000@iolanthe.rowland.org> (raw) In-Reply-To: <53bbfa1a-2836-863d-3a5c-4f2f7f0baa40@colorfullife.com> On Mon, 3 Jul 2017, Manfred Spraul wrote: > >>> + /* 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(). > I was looking at nf_conntrack_all_unlock: > There is a smp_store_release() - which memory barrier does this pair with? > > nf_conntrack_all_unlock() > <arbitrary writes> > smp_store_release(a, false) > spin_unlock(b); > > nf_conntrack_lock() > spin_lock(c); > xx=read_once(a) > if (xx==false) > return > <arbitrary read> Ah, I see your point. Yes, I did wonder about what would happen when nf_conntrack_locks_all was set back to false. But I didn't think about it any further, because the relevant code wasn't in your patch. > I tried to pair the memory barriers: > nf_conntrack_all_unlock() contains a smp_store_release(). > What does that pair with? You are right, this does need to be smp_load_acquire() after all. Perhaps the preceding comment should mention that it pairs with the smp_store_release() from an earlier invocation of nf_conntrack_all_unlock(). (Alternatively, you could make nf_conntrack_all_unlock() do a lock+unlock on all the locks in the array, just like nf_conntrack_all_lock(). But of course, that would be a lot less efficient.) Alan Stern
WARNING: multiple messages have this Message-ID (diff)
From: Alan Stern <stern@rowland.harvard.edu> To: Manfred Spraul <manfred@colorfullife.com> Cc: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, netfilter-devel@vger.kernel.org, netdev@vger.kernel.org, oleg@redhat.com, akpm@linux-foundation.org, mingo@redhat.com, dave@stgolabs.net, tj@kernel.org, arnd@arndb.de, linux-arch@vger.kernel.org, will.deacon@arm.com, peterz@infradead.org, parri.andrea@gmail.com, torvalds@linux-foundation.org, Pablo Neira Ayuso <pablo@netfilter.org>, Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>, Florian Westphal <fw@strlen.de>, "David S. Miller" <davem@davemloft.net>, coreteam@netfilter.org, 1vier1@web.de Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair Date: Mon, 3 Jul 2017 15:57:03 -0400 (EDT) [thread overview] Message-ID: <Pine.LNX.4.44L0.1707031543170.2027-100000@iolanthe.rowland.org> (raw) In-Reply-To: <53bbfa1a-2836-863d-3a5c-4f2f7f0baa40@colorfullife.com> On Mon, 3 Jul 2017, Manfred Spraul wrote: > >>> + /* 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(). > I was looking at nf_conntrack_all_unlock: > There is a smp_store_release() - which memory barrier does this pair with? > > nf_conntrack_all_unlock() > <arbitrary writes> > smp_store_release(a, false) > spin_unlock(b); > > nf_conntrack_lock() > spin_lock(c); > xx=read_once(a) > if (xx==false) > return > <arbitrary read> Ah, I see your point. Yes, I did wonder about what would happen when nf_conntrack_locks_all was set back to false. But I didn't think about it any further, because the relevant code wasn't in your patch. > I tried to pair the memory barriers: > nf_conntrack_all_unlock() contains a smp_store_release(). > What does that pair with? You are right, this does need to be smp_load_acquire() after all. Perhaps the preceding comment should mention that it pairs with the smp_store_release() from an earlier invocation of nf_conntrack_all_unlock(). (Alternatively, you could make nf_conntrack_all_unlock() do a lock+unlock on all the locks in the array, just like nf_conntrack_all_lock(). But of course, that would be a lot less efficient.) Alan Stern
next prev parent reply other threads:[~2017-07-03 19:57 UTC|newest] Thread overview: 211+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-06-29 23:59 [PATCH RFC 0/26] Remove spin_unlock_wait() Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney [not found] ` <a6642feb-2f3a-980f-5ed6-2deb79563e6b@colorfullife.com> 2017-07-02 2:00 ` Paul E. McKenney 2017-07-03 14:39 ` Alan Stern 2017-07-03 14:39 ` Alan Stern 2017-07-03 17:14 ` Paul E. McKenney 2017-07-03 19:01 ` Manfred Spraul 2017-07-03 19:57 ` Alan Stern [this message] 2017-07-03 19:57 ` Alan Stern 2017-07-06 18:43 ` Manfred Spraul 2017-07-06 18:43 ` Manfred Spraul 2017-07-06 18:43 ` Manfred Spraul 2017-07-03 20:04 ` Alan Stern 2017-07-03 20:04 ` Alan Stern 2017-07-03 20:53 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 02/26] task_work: " Paul E. McKenney 2017-06-30 11:04 ` Oleg Nesterov 2017-06-30 12:50 ` Paul E. McKenney 2017-06-30 15:20 ` Oleg Nesterov 2017-06-30 16:16 ` Paul E. McKenney 2017-06-30 17:21 ` Paul E. McKenney 2017-06-30 19:21 ` Oleg Nesterov 2017-06-30 19:50 ` Alan Stern 2017-06-30 19:50 ` Alan Stern 2017-06-30 20:04 ` Paul E. McKenney 2017-06-30 20:02 ` Paul E. McKenney 2017-06-30 20:19 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 03/26] sched: " Paul E. McKenney 2017-06-30 10:31 ` Arnd Bergmann 2017-06-30 12:35 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 04/26] completion: " Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 05/26] exit: " Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 06/26] ipc: " Paul E. McKenney 2017-07-01 19:23 ` Manfred Spraul 2017-07-02 3:16 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 07/26] drivers/ata: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney 2017-06-30 9:19 ` Will Deacon 2017-06-30 12:38 ` Paul E. McKenney 2017-06-30 13:13 ` Will Deacon 2017-06-30 22:18 ` Paul E. McKenney 2017-07-03 13:15 ` Will Deacon 2017-07-03 16:18 ` Paul E. McKenney 2017-07-03 16:40 ` Linus Torvalds 2017-07-03 17:13 ` Will Deacon 2017-07-03 22:30 ` Paul E. McKenney 2017-07-03 22:49 ` Linus Torvalds 2017-07-04 0:39 ` Paul E. McKenney 2017-07-04 0:54 ` Paul E. McKenney 2017-07-03 21:10 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 09/26] alpha: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 10/26] arc: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 11/26] arm: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 12/26] arm64: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 9:20 ` Will Deacon 2017-06-30 9:20 ` Will Deacon 2017-06-30 17:29 ` Paul E. McKenney 2017-06-30 17:29 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 13/26] blackfin: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 14/26] hexagon: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 15/26] ia64: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 16/26] m32r: " Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 17/26] metag: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 18/26] mips: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 19/26] mn10300: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 20/26] parisc: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 21/26] powerpc: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-07-02 3:58 ` Boqun Feng 2017-07-05 23:57 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 22/26] s390: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 23/26] sh: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 24/26] sparc: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 25/26] tile: " Paul E. McKenney 2017-06-30 0:06 ` Linus Torvalds 2017-06-30 0:09 ` Paul E. McKenney 2017-06-30 0:14 ` Paul E. McKenney 2017-06-30 0:10 ` Linus Torvalds 2017-06-30 0:24 ` Paul E. McKenney 2017-06-30 0:01 ` [PATCH RFC 26/26] xtensa: " Paul E. McKenney 2017-06-30 0:01 ` Paul E. McKenney 2017-07-05 23:29 ` [PATCH v2 0/9] Remove spin_unlock_wait() Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 1/9] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() Paul E. McKenney 2017-07-05 23:31 ` Paul E. McKenney 2017-07-06 18:45 ` Manfred Spraul 2017-07-06 18:45 ` Manfred Spraul 2017-07-06 18:45 ` Manfred Spraul 2017-07-06 20:26 ` Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 3/9] sched: " Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 4/9] completion: " Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 5/9] exit: " Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 6/9] ipc: " Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 7/9] drivers/ata: " Paul E. McKenney 2017-07-05 23:31 ` Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 8/9] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney 2017-07-05 23:31 ` [PATCH v2 9/9] arch: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney 2017-07-06 14:12 ` [PATCH v2 0/9] Remove spin_unlock_wait() David Laight 2017-07-06 15:21 ` Paul E. McKenney 2017-07-06 15:21 ` Paul E. McKenney 2017-07-06 15:21 ` Paul E. McKenney 2017-07-06 16:10 ` Peter Zijlstra 2017-07-06 16:10 ` Peter Zijlstra 2017-07-06 16:10 ` Peter Zijlstra 2017-07-06 16:24 ` Paul E. McKenney 2017-07-06 16:24 ` Paul E. McKenney 2017-07-06 16:24 ` Paul E. McKenney 2017-07-06 16:41 ` Peter Zijlstra 2017-07-06 16:41 ` Peter Zijlstra 2017-07-06 16:41 ` Peter Zijlstra 2017-07-06 17:03 ` Paul E. McKenney 2017-07-06 17:03 ` Paul E. McKenney 2017-07-06 17:03 ` Paul E. McKenney 2017-07-06 16:49 ` Alan Stern 2017-07-06 16:54 ` Peter Zijlstra 2017-07-06 16:54 ` Peter Zijlstra 2017-07-06 19:37 ` Alan Stern 2017-07-06 19:37 ` Alan Stern 2017-07-06 16:05 ` Peter Zijlstra 2017-07-06 16:05 ` Peter Zijlstra 2017-07-06 16:05 ` Peter Zijlstra 2017-07-06 16:20 ` Paul E. McKenney 2017-07-06 16:20 ` Paul E. McKenney 2017-07-06 16:20 ` Paul E. McKenney 2017-07-06 16:50 ` Peter Zijlstra 2017-07-06 16:50 ` Peter Zijlstra 2017-07-06 16:50 ` Peter Zijlstra 2017-07-06 17:08 ` Will Deacon 2017-07-06 17:08 ` Will Deacon 2017-07-06 17:08 ` Will Deacon 2017-07-06 17:29 ` Paul E. McKenney 2017-07-06 17:29 ` Paul E. McKenney 2017-07-06 17:29 ` Paul E. McKenney 2017-07-06 17:18 ` Paul E. McKenney 2017-07-06 17:18 ` Paul E. McKenney 2017-07-06 17:18 ` Paul E. McKenney 2017-07-07 8:31 ` Ingo Molnar 2017-07-07 8:31 ` Ingo Molnar 2017-07-07 8:31 ` Ingo Molnar 2017-07-07 8:44 ` Peter Zijlstra 2017-07-07 8:44 ` Peter Zijlstra 2017-07-07 8:44 ` Peter Zijlstra 2017-07-07 10:33 ` Ingo Molnar 2017-07-07 10:33 ` Ingo Molnar 2017-07-07 10:33 ` Ingo Molnar 2017-07-07 11:23 ` Peter Zijlstra 2017-07-07 11:23 ` Peter Zijlstra 2017-07-07 11:23 ` Peter Zijlstra 2017-07-07 14:41 ` Paul E. McKenney 2017-07-07 14:41 ` Paul E. McKenney 2017-07-08 8:43 ` Ingo Molnar 2017-07-08 8:43 ` Ingo Molnar 2017-07-08 11:41 ` Paul E. McKenney 2017-07-08 11:41 ` Paul E. McKenney 2017-07-07 17:47 ` Manfred Spraul 2017-07-07 17:47 ` Manfred Spraul 2017-07-07 17:47 ` Manfred Spraul 2017-07-08 8:35 ` Ingo Molnar 2017-07-08 8:35 ` Ingo Molnar 2017-07-08 11:39 ` Paul E. McKenney 2017-07-08 11:39 ` Paul E. McKenney 2017-07-08 12:30 ` Ingo Molnar 2017-07-08 12:30 ` Ingo Molnar 2017-07-08 14:45 ` Paul E. McKenney 2017-07-08 14:45 ` Paul E. McKenney 2017-07-08 16:21 ` Alan Stern 2017-07-08 16:21 ` Alan Stern 2017-07-10 17:22 ` Manfred Spraul 2017-07-07 8:06 ` Ingo Molnar 2017-07-07 8:06 ` Ingo Molnar 2017-07-07 8:06 ` Ingo Molnar 2017-07-07 9:32 ` Ingo Molnar 2017-07-07 9:32 ` Ingo Molnar 2017-07-07 9:32 ` Ingo Molnar 2017-07-07 19:27 ` [PATCH v3 " Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 1/9] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() Paul E. McKenney 2017-07-07 19:28 ` Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 3/9] sched: " Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 4/9] completion: " Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 5/9] exit: " Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 6/9] ipc: " Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 7/9] drivers/ata: " Paul E. McKenney 2017-07-07 19:28 ` Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 8/9] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney 2017-07-07 19:28 ` [PATCH v3 9/9] arch: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney
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