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 <ktk...@parallels.com>
CC: Peter Zijlstra <pet...@infradead.org>
CC: Ingo Molnar <mi...@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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