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Commit 009f60e2 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar
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sched: stop the unbound recursion in preempt_schedule_context() 



preempt_schedule_context() does preempt_enable_notrace() at the end
and this can call the same function again; exception_exit() is heavy
and it is quite possible that need-resched is true again.

1. Change this code to dec preempt_count() and check need_resched()
   by hand.

2. As Linus suggested, we can use the PREEMPT_ACTIVE bit and avoid
   the enable/disable dance around __schedule(). But in this case
   we need to move into sched/core.c.

3. Cosmetic, but x86 forgets to declare this function. This doesn't
   really matter because it is only called by asm helpers, still it
   make sense to add the declaration into asm/preempt.h to match
   preempt_schedule().

Reported-by: default avatarSasha Levin <sasha.levin@oracle.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Graf <agraf@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20141005202322.GB27962@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 64192658

arch/x86/include/asm/preempt.h

+1 −0
Original line number Diff line number Diff line
@@ -105,6 +105,7 @@ static __always_inline bool should_resched(void) 
# ifdef CONFIG_CONTEXT_TRACKING

    extern asmlinkage void ___preempt_schedule_context(void);

#   define __preempt_schedule_context() asm ("call ___preempt_schedule_context")

    extern asmlinkage void preempt_schedule_context(void);

# endif

#endif

kernel/context_tracking.c

+0 −40
Original line number Diff line number Diff line
@@ -107,46 +107,6 @@ void context_tracking_user_enter(void) 
}

NOKPROBE_SYMBOL(context_tracking_user_enter);



#ifdef CONFIG_PREEMPT

/**

 * preempt_schedule_context - preempt_schedule called by tracing

 *

 * The tracing infrastructure uses preempt_enable_notrace to prevent

 * recursion and tracing preempt enabling caused by the tracing

 * infrastructure itself. But as tracing can happen in areas coming

 * from userspace or just about to enter userspace, a preempt enable

 * can occur before user_exit() is called. This will cause the scheduler

 * to be called when the system is still in usermode.

 *

 * To prevent this, the preempt_enable_notrace will use this function

 * instead of preempt_schedule() to exit user context if needed before

 * calling the scheduler.

 */

asmlinkage __visible void __sched notrace preempt_schedule_context(void)

{

	enum ctx_state prev_ctx;



	if (likely(!preemptible()))

		return;



	/*

	 * Need to disable preemption in case user_exit() is traced

	 * and the tracer calls preempt_enable_notrace() causing

	 * an infinite recursion.

	 */

	preempt_disable_notrace();

	prev_ctx = exception_enter();

	preempt_enable_no_resched_notrace();



	preempt_schedule();



	preempt_disable_notrace();

	exception_exit(prev_ctx);

	preempt_enable_notrace();

}

EXPORT_SYMBOL_GPL(preempt_schedule_context);

#endif /* CONFIG_PREEMPT */



/**

 * context_tracking_user_exit - Inform the context tracking that the CPU is

 *                              exiting userspace mode and entering the kernel.

kernel/sched/core.c

+41 −0
Original line number Diff line number Diff line
@@ -2951,6 +2951,47 @@ asmlinkage __visible void __sched notrace preempt_schedule(void) 
}

NOKPROBE_SYMBOL(preempt_schedule);

EXPORT_SYMBOL(preempt_schedule);



#ifdef CONFIG_CONTEXT_TRACKING

/**

 * preempt_schedule_context - preempt_schedule called by tracing

 *

 * The tracing infrastructure uses preempt_enable_notrace to prevent

 * recursion and tracing preempt enabling caused by the tracing

 * infrastructure itself. But as tracing can happen in areas coming

 * from userspace or just about to enter userspace, a preempt enable

 * can occur before user_exit() is called. This will cause the scheduler

 * to be called when the system is still in usermode.

 *

 * To prevent this, the preempt_enable_notrace will use this function

 * instead of preempt_schedule() to exit user context if needed before

 * calling the scheduler.

 */

asmlinkage __visible void __sched notrace preempt_schedule_context(void)

{

	enum ctx_state prev_ctx;



	if (likely(!preemptible()))

		return;



	do {

		__preempt_count_add(PREEMPT_ACTIVE);

		/*

		 * Needs preempt disabled in case user_exit() is traced

		 * and the tracer calls preempt_enable_notrace() causing

		 * an infinite recursion.

		 */

		prev_ctx = exception_enter();

		__schedule();

		exception_exit(prev_ctx);



		__preempt_count_sub(PREEMPT_ACTIVE);

		barrier();

	} while (need_resched());

}

EXPORT_SYMBOL_GPL(preempt_schedule_context);

#endif /* CONFIG_CONTEXT_TRACKING */



#endif /* CONFIG_PREEMPT */



/*