Messages in this thread Patch in this message | | | Subject | [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlock scenarios in smp_call_function variants. | From | Preeti U Murthy <> | Date | Fri, 05 Jul 2013 21:57:11 +0530 |
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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@linux.vnet.ibm.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Ingo Molnar <mingo@elte.hu> Cc: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> Cc: srivatsa.bhat@linux.vnet.ibm.com Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Rusty Russell <rusty@rustcorp.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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