Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state()
From: Waiman Long
Date: Fri Sep 10 2021 - 12:09:50 EST
On 9/10/21 10:01 AM, Peter Zijlstra wrote:
On Fri, Sep 10, 2021 at 03:17:04PM +0200, Peter Zijlstra wrote:
On Fri, Sep 10, 2021 at 01:57:26PM +0100, Will Deacon wrote:
On Thu, Sep 09, 2021 at 04:27:46PM +0200, Peter Zijlstra wrote:
Moo yes, so the earlier changelog I wrote was something like:
current_save_and_set_rtlock_wait_state();
for (;;) {
if (try_lock())
break;
raw_spin_unlock_irq(&lock->wait_lock);
if (!cond)
schedule();
raw_spin_lock_irq(&lock->wait_lock);
set_current_state(TASK_RTLOCK_WAIT);
}
current_restore_rtlock_saved_state();
which is more what the code looks like before these patches, and in that
case the @cond load can be lifted before __state.
Ah, so that makes more sense, thanks. I can't see how the try_lock() could
be reordered though, as it's going to have to do an atomic rmw.
OK, lemme go update the Changelog and make it __flags for bigeasy :-)
The patch now reads:
---
Subject: sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Thu, 09 Sep 2021 12:59:16 +0200
While looking at current_save_and_set_rtlock_wait_state() I'm thinking
it really ought to use smp_store_mb(), because using it for a more
traditional wait loop like:
current_save_and_set_rtlock_wait_state();
for (;;) {
if (cond)
schedule();
set_current_state(TASK_RTLOCK_WAIT);
}
current_restore_rtlock_saved_state();
is actually broken, since the cond load could be re-ordered against
the state store, which could lead to a missed wakeup -> BAD (tm).
While there, make them consistent with the IRQ usage in
set_special_state().
Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210909110203.767330253@xxxxxxxxxxxxx
---
include/linux/sched.h | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -218,7 +218,7 @@ struct task_group;
*/
#define set_special_state(state_value) \
do { \
- unsigned long flags; /* may shadow */ \
+ unsigned long __flags; /* may shadow */ \
Do you still need the "may shadow" comment?
Cheers,
Longman