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Date: Tue, 23 Sep 2014 17:06:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kirill Tkhai <tkhai@...dex.ru>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Kirill Tkhai <ktkhai@...allels.com>, oleg@...hat.com,
linux@...ck-us.net, ralf@...ux-mips.org, tony.luck@...el.com,
linux-mips@...ux-mips.org, linux-ia64@...r.kernel.org
Subject: [RFC][PATCH] sched,mips,ia64: Remove __ARCH_WANT_UNLOCKED_CTXSW
On Mon, Sep 22, 2014 at 10:36:18PM +0400, Kirill Tkhai wrote:
> From: Kirill Tkhai <ktkhai@...allels.com>
>
> Architectures, which define __ARCH_WANT_UNLOCKED_CTXSW,
> may pull a task when it's in the middle of schedule().
>
> CPU1(task1 calls schedule) CPU2
> ... schedule()
> ... idle_balance()
> ... load_balance()
> ... ...
> schedule() ...
> prepare_lock_switch() ...
> raw_spin_unlock(&rq1->lock) ...
> ... raw_spin_lock(&rq1->lock)
> ... detach_tasks();
> ... can_migrate_task(task1)
> ... attach_tasks(); <--- move task1 to rq2
> ... raw_spin_unlock(&rq1->lock)
> ... context_switch() <--- switch to task1's stack
> ... ...
> (using task1's stack) (using task1's stack)
> ... ...
> context_switch() ...
>
>
> Parallel use of a single stack is not a good idea.
Indeed it is, but how about we do this instead?
---
Subject: sched,mips,ia64: Remove __ARCH_WANT_UNLOCKED_CTXSW
Kirill found that there's a subtle race in the
__ARCH_WANT_UNLOCKED_CTXSW code, and instead of fixing it, remove the
entire exception because neither arch that uses it seems to actually
still require it.
Boot tested on mips64el (qemu) only.
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Guenter Roeck <linux@...ck-us.net>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: Tony Luck <tony.luck@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/ia64/include/asm/processor.h | 1 -
arch/mips/include/asm/processor.h | 6 ------
kernel/sched/core.c | 6 ------
kernel/sched/sched.h | 30 ------------------------------
4 files changed, 43 deletions(-)
diff --git a/arch/ia64/include/asm/processor.h b/arch/ia64/include/asm/processor.h
index c736713..ce53c50 100644
--- a/arch/ia64/include/asm/processor.h
+++ b/arch/ia64/include/asm/processor.h
@@ -19,7 +19,6 @@
#include <asm/ptrace.h>
#include <asm/ustack.h>
-#define __ARCH_WANT_UNLOCKED_CTXSW
#define ARCH_HAS_PREFETCH_SWITCH_STACK
#define IA64_NUM_PHYS_STACK_REG 96
diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
index 05f0843..f1df4cb 100644
--- a/arch/mips/include/asm/processor.h
+++ b/arch/mips/include/asm/processor.h
@@ -397,12 +397,6 @@ unsigned long get_wchan(struct task_struct *p);
#define ARCH_HAS_PREFETCHW
#define prefetchw(x) __builtin_prefetch((x), 1, 1)
-/*
- * See Documentation/scheduler/sched-arch.txt; prevents deadlock on SMP
- * systems.
- */
-#define __ARCH_WANT_UNLOCKED_CTXSW
-
#endif
#endif /* _ASM_PROCESSOR_H */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2a93b87..ccbafb0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2304,10 +2304,6 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
*/
post_schedule(rq);
-#ifdef __ARCH_WANT_UNLOCKED_CTXSW
- /* In this case, finish_task_switch does not reenable preemption */
- preempt_enable();
-#endif
if (current->set_child_tid)
put_user(task_pid_vnr(current), current->set_child_tid);
}
@@ -2350,9 +2346,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
* of the scheduler it's an obvious special-case), so we
* do an early lockdep release here:
*/
-#ifndef __ARCH_WANT_UNLOCKED_CTXSW
spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
-#endif
context_tracking_task_switch(prev, next);
/* Here we just switch the register state and the stack. */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1bc6aad..d87f122 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -966,7 +966,6 @@ static inline int task_on_rq_migrating(struct task_struct *p)
# define finish_arch_post_lock_switch() do { } while (0)
#endif
-#ifndef __ARCH_WANT_UNLOCKED_CTXSW
static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
{
#ifdef CONFIG_SMP
@@ -1004,35 +1003,6 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
raw_spin_unlock_irq(&rq->lock);
}
-#else /* __ARCH_WANT_UNLOCKED_CTXSW */
-static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
-{
-#ifdef CONFIG_SMP
- /*
- * We can optimise this out completely for !SMP, because the
- * SMP rebalancing from interrupt is the only thing that cares
- * here.
- */
- next->on_cpu = 1;
-#endif
- raw_spin_unlock(&rq->lock);
-}
-
-static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
-{
-#ifdef CONFIG_SMP
- /*
- * After ->on_cpu is cleared, the task can be moved to a different CPU.
- * We must ensure this doesn't happen until the switch is completely
- * finished.
- */
- smp_wmb();
- prev->on_cpu = 0;
-#endif
- local_irq_enable();
-}
-#endif /* __ARCH_WANT_UNLOCKED_CTXSW */
-
/*
* wake flags
*/
--
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