smp: Fix SMP function call empty cpu mask race

I get the following warning every day with v3.7, once or
twice a day:

  [ 2235.186027] WARNING: at /mnt/sda7/kernel/linux/arch/x86/kernel/apic/ipi.c:109 default_send_IPI_mask_logical+0x2f/0xb8()

As explained by Linus as well:

 |
 | Once we've done the "list_add_rcu()" to add it to the
 | queue, we can have (another) IPI to the target CPU that can
 | now see it and clear the mask.
 |
 | So by the time we get to actually send the IPI, the mask might
 | have been cleared by another IPI.
 |

This patch also fixes a system hang problem, if the data->cpumask
gets cleared after passing this point:

        if (WARN_ONCE(!mask, "empty IPI mask"))
                return;

then the problem in commit 83d349f3 ("x86: don't send an IPI to
the empty set of CPU's") will happen again.
Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: peterz@infradead.org
Cc: mina86@mina86.org
Cc: srivatsa.bhat@linux.vnet.ibm.com
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/r/20130126075357.GA3205@udknight
[ Tidied up the changelog and the comment in the code. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/smp.c

@@ -33,6 +33,7 @@ struct call_function_data {
 	struct call_single_data	csd;
 	atomic_t		refs;
 	cpumask_var_t		cpumask;
+	cpumask_var_t		cpumask_ipi;
 };
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_function_data, cfd_data);
@@ -56,6 +57,9 @@ hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		if (!zalloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
 				cpu_to_node(cpu)))
 			return notifier_from_errno(-ENOMEM);
+		if (!zalloc_cpumask_var_node(&cfd->cpumask_ipi, GFP_KERNEL,
+				cpu_to_node(cpu)))
+			return notifier_from_errno(-ENOMEM);
 		break;
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -65,6 +69,7 @@ hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
 		free_cpumask_var(cfd->cpumask);
+		free_cpumask_var(cfd->cpumask_ipi);
 		break;
 #endif
 	};
@@ -526,6 +531,12 @@ void smp_call_function_many(const struct cpumask *mask,
 		return;
 	}
 
+	/*
+	 * After we put an entry into the list, data->cpumask
+	 * may be cleared again when another CPU sends another IPI for
+	 * a SMP function call, so data->cpumask will be zero.
+	 */
+	cpumask_copy(data->cpumask_ipi, data->cpumask);
 	raw_spin_lock_irqsave(&call_function.lock, flags);
 	/*
 	 * Place entry at the _HEAD_ of the list, so that any cpu still
@@ -549,7 +560,7 @@ void smp_call_function_many(const struct cpumask *mask,
 	smp_mb();
 
 	/* Send a message to all CPUs in the map */
-	arch_send_call_function_ipi_mask(data->cpumask);
+	arch_send_call_function_ipi_mask(data->cpumask_ipi);
 
 	/* Optionally wait for the CPUs to complete */
 	if (wait)