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Message-ID: <20130705162711.16888.30274.stgit@preeti.in.ibm.com> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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