[PATCH 3/4 v3] smp_call_function_many: handle concurrent clearing of mask

[Milton Miller]

Mike Galbraith reported finding a lockup ("perma-spin bug") where the
cpumask passed to smp_call_function_many was cleared by other cpu(s)
while a cpu was preparing its call_data block, resulting in no cpu to
clear the last ref and unlock the block.

Having cpus clear their bit asynchronously could be useful on a mask of
cpus that might have a translation context, or cpus that need a push to
complete an rcu window.

Instead of adding a BUG_ON and requiring yet another cpumask copy, just
detect the race and handle it.

Note: arch_send_call_function_ipi_mask must still handle an empty
cpumask because the data block is globally visible before the that
arch callback is made.  And (obviously) there are no guarantees to
which cpus are notified if the mask is changed during the call;
only cpus that were online and had their mask bit set during the
whole call are guaranteed to be called.

Reported-by: Mike Galbraith <efault@xxxxxx>
Reported-by: Jan Beulich <JBeulich@xxxxxxxxxx>
Signed-off-by: Milton Miller <miltonm@xxxxxxx>
---
v3: try to clarify which mask in comment
v2: rediff for v2 of call_function_many: fix list delete vs add race

The arch code not expecting the race to empty the mask is the cause
of https://bugzilla.kernel.org/show_bug.cgi?id=23042 that Andrew pointed
out.


Index: common/kernel/smp.c
===================================================================
--- common.orig/kernel/smp.c	2011-03-15 05:22:26.000000000 -0500
+++ common/kernel/smp.c	2011-03-15 06:22:26.000000000 -0500
@@ -450,7 +450,7 @@ void smp_call_function_many(const struct
 {
 	struct call_function_data *data;
 	unsigned long flags;
-	int cpu, next_cpu, this_cpu = smp_processor_id();
+	int refs, cpu, next_cpu, this_cpu = smp_processor_id();
 
 	/*
 	 * Can deadlock when called with interrupts disabled.
@@ -461,7 +461,7 @@ void smp_call_function_many(const struct
 	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
 		     && !oops_in_progress && !early_boot_irqs_disabled);
 
-	/* So, what's a CPU they want? Ignoring this one. */
+	/* Try to fastpath.  So, what's a CPU they want? Ignoring this one. */
 	cpu = cpumask_first_and(mask, cpu_online_mask);
 	if (cpu == this_cpu)
 		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
@@ -519,6 +519,13 @@ void smp_call_function_many(const struct
 	/* We rely on the "and" being processed before the store */
 	cpumask_and(data->cpumask, mask, cpu_online_mask);
 	cpumask_clear_cpu(this_cpu, data->cpumask);
+	refs = cpumask_weight(data->cpumask);
+
+	/* Some callers race with other cpus changing the passed mask */
+	if (unlikely(!refs)) {
+		csd_unlock(&data->csd);
+		return;
+	}
 
 	raw_spin_lock_irqsave(&call_function.lock, flags);
 	/*
@@ -532,7 +539,7 @@ void smp_call_function_many(const struct
 	 * to the cpumask before this write to refs, which indicates
 	 * data is on the list and is ready to be processed.
 	 */
-	atomic_set(&data->refs, cpumask_weight(data->cpumask));
+	atomic_set(&data->refs, refs);
 	raw_spin_unlock_irqrestore(&call_function.lock, flags);
 
 	/*
--
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