linux-kernel - [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call

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Date:	Fri, 05 Jul 2013 21:57:11 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	xiaoguangrong@...fujitsu.com, mingo@...e.hu,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl
Cc:	npiggin@...e.de, deepthi@...ux.vnet.ibm.com, peterz@...radead.org,
	rusty@...tcorp.com.au, heiko.carstens@...ibm.com,
	udknight@...il.com, rostedt@...dmis.org, miltonm@....com,
	srivatsa.bhat@...ux.vnet.ibm.com, jens.axboe@...cle.com,
	tj@...nel.org, akpm@...ux-foundation.org,
	svaidy@...ux.vnet.ibm.com, shli@...nel.org, tglx@...utronix.de,
	lig.fnst@...fujitsu.com, anton@...ba.org
Subject: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock
 scenarios in smp_call_function variants.

Elaborate on when deadlocks can occur when a call is made to
smp_call_function_single() and its friends. This avoids ambiguity about
when to use these calls.

Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc: srivatsa.bhat@...ux.vnet.ibm.com
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Rusty Russell <rusty@...tcorp.com.au
---

 kernel/smp.c |   46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 89be6e6..b6981ae 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -230,7 +230,23 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 	this_cpu = get_cpu();
 
 	/*
-	 * Can deadlock when called with interrupts disabled.
+	 * Can deadlock when called with interrupts disabled under two
+	 * different circumstances depending on the wait parameter.
+	 *
+	 * 1. wait = 1: Two CPUs execute smp_call_function_single(), send an
+	 * IPI to each other, and wait for func to finish on each other.
+	 * Since they are interrupt disabled, neither receives this IPI,
+	 * nor do they proceed forward,as they wait for each other to complete
+	 * execution of func.
+	 *
+	 * 2. wait = 0: This function could be called from an interrupt
+	 * context, and can get blocked on the csd_lock(csd) below in
+	 * "non wait cases".
+	 * This is because the percpu copy of csd of this_cpu is used
+	 * in non wait cases. Under such circumstances, if the previous caller
+	 * of this function who got preempted by this interrupt has already taken
+	 * the lock under non wait condition, it will result in deadlock.
+	 *
 	 * We allow cpu's that are not yet online though, as no one else can
 	 * send smp call function interrupt to this cpu and as such deadlocks
 	 * can't happen.
@@ -329,6 +345,16 @@ void __smp_call_function_single(int cpu, struct call_single_data *csd,
 	this_cpu = get_cpu();
 	/*
 	 * Can deadlock when called with interrupts disabled.
+	 * 1. wait = 1: Two CPUs execute smp_call_function_single(), send an
+	 * IPI to each other, and wait for func to finish on each other.
+	 * Since they are interrupt disabled, neither receives this IPI,
+	 * nor do they proceed forward,as they wait for each other to complete
+	 * execution of func.
+	 *
+	 * 2. wait = 0:  A scenario similar to smp_call_function_single()
+	 * does not happen here, because each caller of
+	 * __smp_call_function_single() passes unique copies of csd.
+	 *
 	 * We allow cpu's that are not yet online though, as no one else can
 	 * send smp call function interrupt to this cpu and as such deadlocks
 	 * can't happen.
@@ -368,7 +394,23 @@ void smp_call_function_many(const struct cpumask *mask,
 	int cpu, next_cpu, this_cpu = smp_processor_id();
 
 	/*
-	 * Can deadlock when called with interrupts disabled.
+	 * Can deadlock when called with interrupts disabled under two
+	 * different circumstances depending on the wait parameter.
+	 *
+	 * 1. wait = 1: Two CPUs execute smp_call_function_single(), send an
+	 * IPI to each other, and wait for func to finish on each other.
+	 * Since they are interrupt disabled, neither receives this IPI,
+	 * nor do they proceed forward,as they wait for each other to complete
+	 * execution of func.
+	 *
+	 * 2. wait = 0: This function could be called from an interrupt
+	 * context, and can get blocked on the csd_lock(csd) below in
+	 * "non wait cases".
+	 * This is because the percpu copy of csd of this_cpu is used
+	 * in non wait cases. Under such circumstances, if the previous caller
+	 * of this function who got preempted by this interrupt has already taken
+	 * the lock under non wait condition, it will result in deadlock.
+	 *
 	 * We allow cpu's that are not yet online though, as no one else can
 	 * send smp call function interrupt to this cpu and as such deadlocks
 	 * can't happen.

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