Messages in this thread Patch in this message |  | | Subject | [PATCH] sched/core: Create new task with twice disabled preemption | From | Kirill Tkhai <> | Date | Thu, 13 Feb 2014 19:51:56 +0400 |
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Preemption state on enter in finish_task_switch() is different in cases of context_switch() and schedule_tail().
In the first case we have it twice disabled: at the start of schedule() and during spin locking. In the second it is only once: the value which was set in init_task_preempt_count().
For archs without __ARCH_WANT_UNLOCKED_CTXSW set this means that all newly created tasks execute finish_arch_post_lock_switch() and post_schedule() with preemption enabled.
It seems there is possible a problem in rare situations on arm64, when one freshly created thread preempts another before finish_arch_post_lock_switch() has finished. If mm is the same, then TIF_SWITCH_MM on the second won't be set.
The second rare but possible issue is zeroing of post_schedule() on a wrong cpu.
So, lets fix this and unify preempt_count state.
Signed-off-by: Kirill Tkhai <ktkhai@parallels.com> CC: Peter Zijlstra <peterz@infradead.org> CC: Ingo Molnar <mingo@redhat.com> --- arch/x86/include/asm/preempt.h | 6 ++++-- include/asm-generic/preempt.h | 6 ++++-- include/linux/sched.h | 2 ++ kernel/sched/core.c | 6 ++---- 4 files changed, 12 insertions(+), 8 deletions(-) diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h index c8b0519..07fdf52 100644 --- a/arch/x86/include/asm/preempt.h +++ b/arch/x86/include/asm/preempt.h @@ -32,9 +32,11 @@ static __always_inline void preempt_count_set(int pc) */ #define task_preempt_count(p) \ (task_thread_info(p)->saved_preempt_count & ~PREEMPT_NEED_RESCHED) - +/* + * Disable it twice to enter schedule_tail() with preemption disabled. + */ #define init_task_preempt_count(p) do { \ - task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \ + task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED_TWICE; \ } while (0) #define init_idle_preempt_count(p, cpu) do { \ diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h index 1cd3f5d..0f67846 100644 --- a/include/asm-generic/preempt.h +++ b/include/asm-generic/preempt.h @@ -25,9 +25,11 @@ static __always_inline void preempt_count_set(int pc) */ #define task_preempt_count(p) \ (task_thread_info(p)->preempt_count & ~PREEMPT_NEED_RESCHED) - +/* + * Disable it twice to enter schedule_tail() with preemption disabled. + */ #define init_task_preempt_count(p) do { \ - task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \ + task_thread_info(p)->preempt_count = PREEMPT_DISABLED_TWICE; \ } while (0) #define init_idle_preempt_count(p, cpu) do { \ diff --git a/include/linux/sched.h b/include/linux/sched.h index c49a258..f6a6c1e 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -520,8 +520,10 @@ struct task_cputime { #ifdef CONFIG_PREEMPT_COUNT #define PREEMPT_DISABLED (1 + PREEMPT_ENABLED) +#define PREEMPT_DISABLED_TWICE (2 + PREEMPT_ENABLED) #else #define PREEMPT_DISABLED PREEMPT_ENABLED +#define PREEMPT_DISABLED_TWICE PREEMPT_ENABLED #endif /* diff --git a/kernel/sched/core.c b/kernel/sched/core.c index fb9764f..18aa7f2 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2203,12 +2203,10 @@ asmlinkage void schedule_tail(struct task_struct *prev) finish_task_switch(rq, prev); - /* - * FIXME: do we need to worry about rq being invalidated by the - * task_switch? - */ post_schedule(rq); + preempt_enable(); + #ifdef __ARCH_WANT_UNLOCKED_CTXSW /* In this case, finish_task_switch does not reenable preemption */ preempt_enable();
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