5.4-stable review patch. If anyone has any objections, please let me know.
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From: Thomas Gleixner tglx@linutronix.de
[ Upstream commit 66630058e56b26b3a9cf2625e250a8c592dd0207 ]
Code which solely needs to prevent migration of a task uses preempt_disable()/enable() pairs. This is the only reliable way to do so as setting the task affinity to a single CPU can be undone by a setaffinity operation from a different task/process.
RT provides a seperate migrate_disable/enable() mechanism which does not disable preemption to achieve the semantic requirements of a (almost) fully preemptible kernel.
As it is unclear from looking at a given code path whether the intention is to disable preemption or migration, introduce migrate_disable/enable() inline functions which can be used to annotate code which merely needs to disable migration. Map them to preempt_disable/enable() for now. The RT substitution will be provided later.
Code which is annotated that way documents that it has no requirement to protect against reentrancy of a preempting task. Either this is not required at all or the call sites are already serialized by other means.
Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: Juri Lelli juri.lelli@redhat.com Cc: Vincent Guittot vincent.guittot@linaro.org Cc: Dietmar Eggemann dietmar.eggemann@arm.com Cc: Steven Rostedt rostedt@goodmis.org Cc: Ben Segall bsegall@google.com Cc: Mel Gorman mgorman@suse.de Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/r/878slclv1u.fsf@nanos.tec.linutronix.de Stable-dep-of: 36c75ce3bd29 ("nd_btt: Make BTT lanes preemptible") Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/preempt.h | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+)
diff --git a/include/linux/preempt.h b/include/linux/preempt.h index bbb68dba37cc8..bc3f1aecaa194 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -322,4 +322,34 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
#endif
+/** + * migrate_disable - Prevent migration of the current task + * + * Maps to preempt_disable() which also disables preemption. Use + * migrate_disable() to annotate that the intent is to prevent migration, + * but not necessarily preemption. + * + * Can be invoked nested like preempt_disable() and needs the corresponding + * number of migrate_enable() invocations. + */ +static __always_inline void migrate_disable(void) +{ + preempt_disable(); +} + +/** + * migrate_enable - Allow migration of the current task + * + * Counterpart to migrate_disable(). + * + * As migrate_disable() can be invoked nested, only the outermost invocation + * reenables migration. + * + * Currently mapped to preempt_enable(). + */ +static __always_inline void migrate_enable(void) +{ + preempt_enable(); +} + #endif /* __LINUX_PREEMPT_H */