On Tue, Jul 15, 2014 at 04:25:25PM +0200, Oleg Nesterov wrote:
> And probably I missed something again, but it seems that this logic is broken
> with __ARCH_WANT_UNLOCKED_CTXSW.
>
> Of course, even if I am right this is pure theoretical, but smp_wmb() before
> "->on_cpu = 0" is not enough and we need a full barrier ?
(long delay there, forgot about this thread, sorry)
Yes, I think I see that.. but now I think the comment is further wrong.
Its not rq->lock that is important, remember, a concurrent wakeup onto
another CPU does not require our rq->lock at all.
It is the ->on_cpu = 0 store that is important (for both the
UNLOCKED_CTXSW cases). As soon as that store comes through the task can
start running on the remote cpu.
Now the below patch 'fixes' this but at the cost of adding a full
barrier which is somewhat unfortunate to say the least.
wmb's are free on x86 and generally cheaper than mbs, so it would to
find another solution to this problem...
---
kernel/sched/core.c | 10 +++++-----
kernel/sched/sched.h | 10 ++++++++--
2 files changed, 13 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2676866b4394..950264381644 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2214,11 +2214,11 @@ static void finish_task_switch(struct rq *rq, struct
task_struct *prev)
* If a task dies, then it sets TASK_DEAD in tsk->state and calls
* schedule one last time. The schedule call will never return, and
* the scheduled task must drop that reference.
- * The test for TASK_DEAD must occur while the runqueue locks are
- * still held, otherwise prev could be scheduled on another cpu, die
- * there before we look at prev->state, and then the reference would
- * be dropped twice.
- * Manfred Spraul <[email protected]>
+ *
+ * We must observe prev->state before clearing prev->on_cpu (in
+ * finish_lock_switch), otherwise a concurrent wakeup can get prev
+ * running on another CPU and we could race with its RUNNING -> DEAD
+ * transition, and then the reference would be dropped twice.
*/
prev_state = prev->state;
vtime_task_switch(prev);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 579712f4e9d5..259632c09c98 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -973,8 +973,11 @@ static inline void finish_lock_switch(struct rq *rq,
struct task_struct *prev)
* 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.
+ *
+ * We must furthermore ensure the prev->state read in
+ * finish_task_switch() is complete before allowing this store.
*/
- smp_wmb();
+ smp_mb();
prev->on_cpu = 0;
#endif
#ifdef CONFIG_DEBUG_SPINLOCK
@@ -1012,8 +1015,11 @@ static inline void finish_lock_switch(struct rq *rq,
struct task_struct *prev)
* 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.
+ *
+ * We must furthermore ensure the prev->state read in
+ * finish_task_switch() is complete before allowing this store.
*/
- smp_wmb();
+ smp_mb();
prev->on_cpu = 0;
#endif
local_irq_enable();
--
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