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Vikram reported that his ARM64 compiler managed to 'optimize' away the
preempt_count manipulations in code like:
preempt_enable_no_resched();
put_user();
preempt_disable();
Irrespective of that fact that that is horrible code that should be
fixed for many reasons, it does highlight a deficiency in the generic
preempt_count manipulators. As it is never right to combine/elide
preempt_count manipulations like this.
Therefore sprinkle some volatile in the two generic accessors to
ensure the compiler is aware of the fact that the preempt_count is
observed outside of the regular program-order view and thus cannot be
optimized away like this.
x86; the only arch not using the generic code is not affected as we
do all this in asm in order to use the segment base per-cpu stuff.
Reported-by: Vikram Mulukutla <markivx@codeaurora.org>
Tested-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: a787870924 ("sched, arch: Create asm/preempt.h")
Link: http://lkml.kernel.org/r/20160516131751.GH3205@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
88 lines
1.9 KiB
C
88 lines
1.9 KiB
C
#ifndef __ASM_PREEMPT_H
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#define __ASM_PREEMPT_H
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#include <linux/thread_info.h>
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#define PREEMPT_ENABLED (0)
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static __always_inline int preempt_count(void)
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{
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return READ_ONCE(current_thread_info()->preempt_count);
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}
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static __always_inline volatile int *preempt_count_ptr(void)
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{
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return ¤t_thread_info()->preempt_count;
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}
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static __always_inline void preempt_count_set(int pc)
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{
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*preempt_count_ptr() = pc;
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}
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/*
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* must be macros to avoid header recursion hell
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*/
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#define init_task_preempt_count(p) do { \
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task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \
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} while (0)
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#define init_idle_preempt_count(p, cpu) do { \
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task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
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} while (0)
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static __always_inline void set_preempt_need_resched(void)
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{
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}
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static __always_inline void clear_preempt_need_resched(void)
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{
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}
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static __always_inline bool test_preempt_need_resched(void)
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{
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return false;
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}
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/*
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* The various preempt_count add/sub methods
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*/
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static __always_inline void __preempt_count_add(int val)
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{
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*preempt_count_ptr() += val;
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}
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static __always_inline void __preempt_count_sub(int val)
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{
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*preempt_count_ptr() -= val;
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}
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static __always_inline bool __preempt_count_dec_and_test(void)
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{
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/*
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* Because of load-store architectures cannot do per-cpu atomic
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* operations; we cannot use PREEMPT_NEED_RESCHED because it might get
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* lost.
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*/
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return !--*preempt_count_ptr() && tif_need_resched();
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}
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/*
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* Returns true when we need to resched and can (barring IRQ state).
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*/
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static __always_inline bool should_resched(int preempt_offset)
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{
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return unlikely(preempt_count() == preempt_offset &&
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tif_need_resched());
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}
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#ifdef CONFIG_PREEMPT
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extern asmlinkage void preempt_schedule(void);
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#define __preempt_schedule() preempt_schedule()
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extern asmlinkage void preempt_schedule_notrace(void);
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#define __preempt_schedule_notrace() preempt_schedule_notrace()
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#endif /* CONFIG_PREEMPT */
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#endif /* __ASM_PREEMPT_H */
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