Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753133Ab1BAWBD (ORCPT ); Tue, 1 Feb 2011 17:01:03 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:34260 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752118Ab1BAWBB (ORCPT ); Tue, 1 Feb 2011 17:01:01 -0500 Date: Tue, 1 Feb 2011 14:00:26 -0800 From: "Paul E. McKenney" To: Milton Miller Cc: Peter Zijlstra , akpm@linux-foundation.org, Anton Blanchard , xiaoguangrong@cn.fujitsu.com, mingo@elte.hu, jaxboe@fusionio.com, npiggin@gmail.com, rusty@rustcorp.com.au, efault@gmx.de, torvalds@linux-foundation.org, benh@kernel.crashing.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3 v2] call_function_many: fix list delete vs add race Message-ID: <20110201220026.GD2142@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110112150740.77dde58c@kryten> <1295288253.30950.280.camel@laptop> <1296145360.15234.234.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8113 Lines: 208 On Tue, Feb 01, 2011 at 01:12:18AM -0600, Milton Miller wrote: > Peter pointed out there was nothing preventing the list_del_rcu in > smp_call_function_interrupt from running before the list_add_rcu in > smp_call_function_many. Fix this by not setting refs until we > have gotten the lock for the list. Take advantage of the wmb in > list_add_rcu to save an explicit additional one. Finally getting a chance to give this full attention... I don't know how to do this patch-by-patch, so I applied these three patches to mainline and looked at the result. This probably gets some comments that are irrelevant to these patches, but so it goes. Starting with smp_call_function_many(): o The check for refs is redundant: /* some callers might race with other cpus changing the mask */ if (unlikely(!refs)) { csd_unlock(&data->csd); return; } The memory barriers and atomic functions in generic_smp_call_function_interrupt() prevent the callback from being reused before the cpumask bits have all been cleared, right? Furthermore, csd_lock() contains a full memory barrier that pairs with the full memory barrier in csd_unlock(), so if csd_lock() returns, we are guaranteed to see the effects of all accesses to the prior incarnation of this structure. Yes, there might be another CPU that got a pointer to this callback just as another CPU removed it. That CPU will see the callback as having their bit of the CPU mask zero until we set it, and they will further see ->refs as being zero until we set it. And we don't set ->refs until after we re-insert the callback. So I we can drop this "if" statement entirely. o The smp_mb() below look extraneous. The comment in generic_exec_single() agrees -- it says that the IPI code is required to impose ordering. Besides, there is a lock in arch_send_call_function_ipi_mask(), and in conjunction with the unlock below, this makes a full memory barrier. So I believe that we can drop the smp_mb() shown below. (But not the comment!!!) raw_spin_unlock_irqrestore(&call_function.lock, flags); /* * Make the list addition visible before sending the ipi. * (IPIs must obey or appear to obey normal Linux cache * coherency rules -- see comment in generic_exec_single). */ smp_mb(); /* Send a message to all CPUs in the map */ arch_send_call_function_ipi_mask(data->cpumask); Next up: generic_smp_call_function_interrupt() o Shouldn't the smp_mb() at the top of the function be supplied in arch-specific code for those architectures that require it? o So, the check for the current CPU's bit in the mask... o If the bit is clear, then we are looking at an callback that this CPU already processed or that was not intended for this CPU in the first place. Of course, this callback might be reused immediately after we do the check. In that case, there should be an IPI waiting for us shortly. If the IPI beat the callback to us, that seems to me to be a bug in the architecture rather than in this code. o If the bit is set, then we need to process this callback. IRQs are disabled, so we cannot race with ourselves -- our bit will remain set until we clear it. The list_add_rcu() in smp_call_function_many() in conjunction with the list_for_each_entry_rcu() in generic_smp_call_function_interrupt() guarantees that all of the field except for ->refs will be seen as initialized in the common case where we are looking at an callback that has just been enqueued. In the uncommon case where we picked up the pointer in list_for_each_entry_rcu() just before the last CPU removed the callback and when someone else immediately recycled it, all bets are off. We must ensure that we see all initialization via some other means. OK, so where is the memory barrier that pairs with the smp_rmb() between the ->cpumask and ->refs checks? It must be before the assignment to ->cpumask. One candidate is the smp_mb() in csd_lock(), but that does not make much sense. What we need to do is to ensure that if we see our bit in ->cpumask, that we also see the atomic decrement that previously zeroed ->refs. But some third CPU did that atomic decrement, which brings transitivity into play, which leads me to believe that this smp_rmb() might need to be upgraded to a full smp_mb(). o After we verify that the ->refs field is non-zero, we pick up the ->csd.func and ->csd.info fields, but with no intervening memory barrier. There needs to be at least an smp_rmb() following the test of the ->refs field. Since there is no transitivity required, smp_rmb() should do the trick. If this seems unnecessary, please keep in mind that there is nothing stopping the compiler and the CPU from reordering the statements in smp_call_function_many() that initialize ->csd.func, ->csd.info, and ->cpumask. In contrast, given the suggested smp_rmb(), we are guaranteed of the identity of the callback from the time we test ->refs until the time someone calls csd_unlock(), which cannot precede the time that we atomically decrement ->refs. o It seems silly to pick up the ->csd.func field twice. Why not use the local variable? o The cpumask_test_and_clear_cpu() and the atomic_dec_return() are both atomic operations that return values, so they act as full memory barriers. The cpumask_test_and_clear_cpu() needs to be atomic despite the identity guarantee because CPUs might be concurrently trying to clear bits in the same word. Here is the corresponding (untested) patch. Thoughts? Thanx, Paul ------------------------------------------------------------------------ smp_call_function: additional memory-order tightening. The csd_lock() and csd_unlock() interaction guarantees that the smp_call_function_many() function sees the results of interactions with prior incarnations of the callback, so the check is not needed. Instead, tighter memory ordering is required in the companion generic_smp_call_function_interrupt() function to ensure proper interaction with partially initialized callbacks. Signed-off-by: Paul E. McKenney diff --git a/kernel/smp.c b/kernel/smp.c index 064bb6e..4c8b005 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -209,13 +209,19 @@ void generic_smp_call_function_interrupt(void) if (!cpumask_test_cpu(cpu, data->cpumask)) continue; - smp_rmb(); + smp_mb(); /* If we see our bit set above, we need to see */ + /* all the processing associated with the prior */ + /* incarnation of this callback. */ if (atomic_read(&data->refs) == 0) continue; + smp_rmb(); /* If we see non-zero ->refs, we need to see all */ + /* other initialization for this incarnation of */ + /* this callback. */ + func = data->csd.func; /* for later warn */ - data->csd.func(data->csd.info); + func(data->csd.info); /* * If the cpu mask is not still set then func enabled @@ -492,12 +498,6 @@ void smp_call_function_many(const struct cpumask *mask, cpumask_clear_cpu(this_cpu, data->cpumask); refs = cpumask_weight(data->cpumask); - /* some callers might race with other cpus changing the mask */ - if (unlikely(!refs)) { - csd_unlock(&data->csd); - return; - } - /* * We reuse the call function data without waiting for any grace * period after some other cpu removes it from the global queue. @@ -527,8 +527,9 @@ void smp_call_function_many(const struct cpumask *mask, * Make the list addition visible before sending the ipi. * (IPIs must obey or appear to obey normal Linux cache * coherency rules -- see comment in generic_exec_single). + * The unlock above combined with the lock in the IPI + * code covers this requirement. */ - smp_mb(); /* Send a message to all CPUs in the map */ arch_send_call_function_ipi_mask(data->cpumask); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/