62 lines
2.3 KiB
Diff
62 lines
2.3 KiB
Diff
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From d4569da882ca665e520733527471e3f845210bc9 Mon Sep 17 00:00:00 2001
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From: Steven Rostedt <rostedt@goodmis.org>
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Date: Thu, 5 Dec 2013 09:16:52 -0500
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Subject: [PATCH 279/365] cpu hotplug: Document why PREEMPT_RT uses a spinlock
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The patch:
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cpu: Make hotplug.lock a "sleeping" spinlock on RT
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Tasks can block on hotplug.lock in pin_current_cpu(), but their
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state might be != RUNNING. So the mutex wakeup will set the state
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unconditionally to RUNNING. That might cause spurious unexpected
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wakeups. We could provide a state preserving mutex_lock() function,
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but this is semantically backwards. So instead we convert the
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hotplug.lock() to a spinlock for RT, which has the state preserving
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semantics already.
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Fixed a bug where the hotplug lock on PREEMPT_RT can be called after a
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task set its state to TASK_UNINTERRUPTIBLE and before it called
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schedule. If the hotplug_lock used a mutex, and there was contention,
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the current task's state would be turned to TASK_RUNNABLE and the
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schedule call will not sleep. This caused unexpected results.
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Although the patch had a description of the change, the code had no
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comments about it. This causes confusion to those that review the code,
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and as PREEMPT_RT is held in a quilt queue and not git, it's not as easy
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to see why a change was made. Even if it was in git, the code should
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still have a comment for something as subtle as this.
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Document the rational for using a spinlock on PREEMPT_RT in the hotplug
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lock code.
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Reported-by: Nicholas Mc Guire <der.herr@hofr.at>
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Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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kernel/cpu.c | 8 ++++++++
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1 file changed, 8 insertions(+)
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diff --git a/kernel/cpu.c b/kernel/cpu.c
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index a923a942b684..06d5e5dc3448 100644
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--- a/kernel/cpu.c
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+++ b/kernel/cpu.c
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@@ -268,6 +268,14 @@ struct hotplug_pcp {
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int grab_lock;
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struct completion synced;
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#ifdef CONFIG_PREEMPT_RT_FULL
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+ /*
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+ * Note, on PREEMPT_RT, the hotplug lock must save the state of
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+ * the task, otherwise the mutex will cause the task to fail
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+ * to sleep when required. (Because it's called from migrate_disable())
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+ *
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+ * The spinlock_t on PREEMPT_RT is a mutex that saves the task's
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+ * state.
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+ */
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spinlock_t lock;
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#else
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struct mutex mutex;
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--
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2.28.0
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