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sched: Replace rq::wake_list

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The recent commit: 90b5363acd ("sched: Clean up scheduler_ipi()")
got smp_call_function_single_async() subtly wrong. Even though it will
return -EBUSY when trying to re-use a csd, that condition is not
atomic and still requires external serialization.

The change in ttwu_queue_remote() got this wrong.

While on first reading ttwu_queue_remote() has an atomic test-and-set
that appears to serialize the use, the matching 'release' is not in
the right place to actually guarantee this serialization.

The actual race is vs the sched_ttwu_pending() call in the idle loop;
that can run the wakeup-list without consuming the CSD.

Instead of trying to chain the lists, merge them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161908.129371594@infradead.org
This commit is contained in:
Peter Zijlstra
2020-05-26 18:11:04 +02:00
committed by Ingo Molnar
parent 126c2092e5
commit a148866489
6 changed files with 49 additions and 34 deletions
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include/linux/sched.h
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@@ -654,6 +654,7 @@ struct task_struct {
#ifdef CONFIG_SMP
struct llist_node wake_entry;
unsigned int wake_entry_type;
int on_cpu;
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* Current CPU: */