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 <pre...@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijls...@chello.nl>
Cc: Ingo Molnar <mi...@elte.hu>
Cc: Xiao Guangrong <xiaoguangr...@cn.fujitsu.com>
Cc: srivatsa.b...@linux.vnet.ibm.com
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Rusty Russell <ru...@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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