Commit be690035 authored Jun 04, 2015 by Frederic Weisbecker Committed by Ingo Molnar Jun 07, 2015

sched: Make preempt_schedule_context() function-tracing safe



Since function tracing disables preemption, it needs a safe preemption
point to use when preemption is re-enabled without worrying about tracing
recursion. Ie: to avoid tracing recursion, that preemption point can't
be traced (use of notrace qualifier) and it can't call any traceable
function before that preemption point disables preemption itself, which
disarms the recursion.

preempt_schedule() was fine until commit:

  b30f0e3f ("sched/preempt: Optimize preemption operations on __schedule() callers")

because PREEMPT_ACTIVE (which has the property to disable preemption
and this disarm tracing preemption recursion) was set before calling
any further function.

But that commit introduced the use of preempt_count_add/sub() functions
to set PREEMPT_ACTIVE and because these functions are called before
preemption gets a chance to be disabled, we have a tracing recursion.

preempt_schedule_context() is one of the possible preemption functions
used by tracing. Its special purpose is to avoid tracing recursion
against context tracking. Lets enhance this function to become more
generally tracing safe by disabling preemption with raw accessors, such
that no function is called before preemption gets disabled and disarm
the tracing recursion.

This function is going to become the specific tracing-safe preemption
point in further commit.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1433432349-1021-2-git-send-email-fweisbec@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent f407a825

kernel/sched/core.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -2960,7 +2960,13 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void)
		return;

		do {
		preempt_active_enter();
		/*
		* Use raw __prempt_count() ops that don't call function.
		* We can't call functions before disabling preemption which
		* disarm preemption tracing recursions.
		*/
		__preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET);
		barrier();
		/*
		* Needs preempt disabled in case user_exit() is traced
		* and the tracer calls preempt_enable_notrace() causing
		@@ -2970,7 +2976,8 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void)
		__schedule();
		exception_exit(prev_ctx);

		preempt_active_exit();
		barrier();
		__preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET);
		} while (need_resched());
		}
		EXPORT_SYMBOL_GPL(preempt_schedule_context);

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