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Date: Fri, 15 Jul 2022 00:42:27 +0800
From: kernel test robot <lkp@...el.com>
To: Nicholas Piggin <npiggin@...il.com>
Cc: kbuild-all@...ts.01.org, Nicholas Piggin <npiggin@...il.com>,
Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Waiman Long <longman@...hat.com>,
Boqun Feng <boqun.feng@...il.com>,
"linux-kernel @ vger . kernel . org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/12] locking/qspinlock: merge qspinlock_paravirt.h
into qspinlock.c
Hi Nicholas,
I love your patch! Perhaps something to improve:
[auto build test WARNING on tip/locking/core]
[also build test WARNING on tip/x86/core powerpc/next linus/master v5.19-rc6 next-20220714]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Nicholas-Piggin/locking-qspinlock-simplify-code-generation/20220713-151009
base: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git eae6d58d67d9739be5f7ae2dbead1d0ef6528243
config: x86_64-rhel-8.3-func (https://download.01.org/0day-ci/archive/20220715/202207150022.DkhjTAWE-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-3) 11.3.0
reproduce (this is a W=1 build):
# https://github.com/intel-lab-lkp/linux/commit/91668ee1ed703d7ea84e314136dc732da05ec9e7
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Nicholas-Piggin/locking-qspinlock-simplify-code-generation/20220713-151009
git checkout 91668ee1ed703d7ea84e314136dc732da05ec9e7
# save the config file
mkdir build_dir && cp config build_dir/.config
make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash drivers/nvme/target/ fs/xfs/ kernel/locking/
If you fix the issue, kindly add following tag where applicable
Reported-by: kernel test robot <lkp@...el.com>
All warnings (new ones prefixed by >>):
kernel/locking/qspinlock.c: In function 'pv_wait_node':
>> kernel/locking/qspinlock.c:513:14: warning: variable 'wait_early' set but not used [-Wunused-but-set-variable]
513 | bool wait_early;
| ^~~~~~~~~~
kernel/locking/qspinlock.c: At top level:
>> kernel/locking/qspinlock.c:705:1: warning: no previous prototype for '__pv_queued_spin_unlock_slowpath' [-Wmissing-prototypes]
705 | __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
vim +/wait_early +513 kernel/locking/qspinlock.c
504
505 /*
506 * Wait for node->locked to become true, halt the vcpu after a short spin.
507 * pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its
508 * behalf.
509 */
510 static void pv_wait_node(struct qnode *node, struct qnode *prev)
511 {
512 int loop;
> 513 bool wait_early;
514
515 for (;;) {
516 for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
517 if (READ_ONCE(node->locked))
518 return;
519 if (pv_wait_early(prev, loop)) {
520 wait_early = true;
521 break;
522 }
523 cpu_relax();
524 }
525
526 /*
527 * Order node->state vs node->locked thusly:
528 *
529 * [S] node->state = vcpu_halted [S] next->locked = 1
530 * MB MB
531 * [L] node->locked [RmW] node->state = vcpu_hashed
532 *
533 * Matches the cmpxchg() from pv_kick_node().
534 */
535 smp_store_mb(node->state, vcpu_halted);
536
537 if (!READ_ONCE(node->locked)) {
538 lockevent_inc(pv_wait_node);
539 lockevent_cond_inc(pv_wait_early, wait_early);
540 pv_wait(&node->state, vcpu_halted);
541 }
542
543 /*
544 * If pv_kick_node() changed us to vcpu_hashed, retain that
545 * value so that pv_wait_head_or_lock() knows to not also try
546 * to hash this lock.
547 */
548 cmpxchg(&node->state, vcpu_halted, vcpu_running);
549
550 /*
551 * If the locked flag is still not set after wakeup, it is a
552 * spurious wakeup and the vCPU should wait again. However,
553 * there is a pretty high overhead for CPU halting and kicking.
554 * So it is better to spin for a while in the hope that the
555 * MCS lock will be released soon.
556 */
557 lockevent_cond_inc(pv_spurious_wakeup,
558 !READ_ONCE(node->locked));
559 }
560
561 /*
562 * By now our node->locked should be 1 and our caller will not actually
563 * spin-wait for it. We do however rely on our caller to do a
564 * load-acquire for us.
565 */
566 }
567
568 /*
569 * Called after setting next->locked = 1 when we're the lock owner.
570 *
571 * Instead of waking the waiters stuck in pv_wait_node() advance their state
572 * such that they're waiting in pv_wait_head_or_lock(), this avoids a
573 * wake/sleep cycle.
574 */
575 static void pv_kick_node(struct qspinlock *lock, struct qnode *node)
576 {
577 /*
578 * If the vCPU is indeed halted, advance its state to match that of
579 * pv_wait_node(). If OTOH this fails, the vCPU was running and will
580 * observe its next->locked value and advance itself.
581 *
582 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
583 *
584 * The write to next->locked in arch_mcs_spin_unlock_contended()
585 * must be ordered before the read of node->state in the cmpxchg()
586 * below for the code to work correctly. To guarantee full ordering
587 * irrespective of the success or failure of the cmpxchg(),
588 * a relaxed version with explicit barrier is used. The control
589 * dependency will order the reading of node->state before any
590 * subsequent writes.
591 */
592 smp_mb__before_atomic();
593 if (cmpxchg_relaxed(&node->state, vcpu_halted, vcpu_hashed)
594 != vcpu_halted)
595 return;
596
597 /*
598 * Put the lock into the hash table and set the _Q_SLOW_VAL.
599 *
600 * As this is the same vCPU that will check the _Q_SLOW_VAL value and
601 * the hash table later on at unlock time, no atomic instruction is
602 * needed.
603 */
604 WRITE_ONCE(lock->locked, _Q_SLOW_VAL);
605 (void)pv_hash(lock, node);
606 }
607
608 /*
609 * Wait for l->locked to become clear and acquire the lock;
610 * halt the vcpu after a short spin.
611 * __pv_queued_spin_unlock() will wake us.
612 *
613 * The current value of the lock will be returned for additional processing.
614 */
615 static u32
616 pv_wait_head_or_lock(struct qspinlock *lock, struct qnode *node)
617 {
618 struct qspinlock **lp = NULL;
619 int waitcnt = 0;
620 int loop;
621
622 /*
623 * If pv_kick_node() already advanced our state, we don't need to
624 * insert ourselves into the hash table anymore.
625 */
626 if (READ_ONCE(node->state) == vcpu_hashed)
627 lp = (struct qspinlock **)1;
628
629 /*
630 * Tracking # of slowpath locking operations
631 */
632 lockevent_inc(lock_slowpath);
633
634 for (;; waitcnt++) {
635 /*
636 * Set correct vCPU state to be used by queue node wait-early
637 * mechanism.
638 */
639 WRITE_ONCE(node->state, vcpu_running);
640
641 /*
642 * Set the pending bit in the active lock spinning loop to
643 * disable lock stealing before attempting to acquire the lock.
644 */
645 set_pending(lock);
646 for (loop = SPIN_THRESHOLD; loop; loop--) {
647 if (trylock_clear_pending(lock))
648 goto gotlock;
649 cpu_relax();
650 }
651 clear_pending(lock);
652
653
654 if (!lp) { /* ONCE */
655 lp = pv_hash(lock, node);
656
657 /*
658 * We must hash before setting _Q_SLOW_VAL, such that
659 * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
660 * we'll be sure to be able to observe our hash entry.
661 *
662 * [S] <hash> [Rmw] l->locked == _Q_SLOW_VAL
663 * MB RMB
664 * [RmW] l->locked = _Q_SLOW_VAL [L] <unhash>
665 *
666 * Matches the smp_rmb() in __pv_queued_spin_unlock().
667 */
668 if (xchg(&lock->locked, _Q_SLOW_VAL) == 0) {
669 /*
670 * The lock was free and now we own the lock.
671 * Change the lock value back to _Q_LOCKED_VAL
672 * and unhash the table.
673 */
674 WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
675 WRITE_ONCE(*lp, NULL);
676 goto gotlock;
677 }
678 }
679 WRITE_ONCE(node->state, vcpu_hashed);
680 lockevent_inc(pv_wait_head);
681 lockevent_cond_inc(pv_wait_again, waitcnt);
682 pv_wait(&lock->locked, _Q_SLOW_VAL);
683
684 /*
685 * Because of lock stealing, the queue head vCPU may not be
686 * able to acquire the lock before it has to wait again.
687 */
688 }
689
690 /*
691 * The cmpxchg() or xchg() call before coming here provides the
692 * acquire semantics for locking. The dummy ORing of _Q_LOCKED_VAL
693 * here is to indicate to the compiler that the value will always
694 * be nozero to enable better code optimization.
695 */
696 gotlock:
697 return (u32)(atomic_read(&lock->val) | _Q_LOCKED_VAL);
698 }
699
700 /*
701 * PV versions of the unlock fastpath and slowpath functions to be used
702 * instead of queued_spin_unlock().
703 */
704 __visible void
> 705 __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
706 {
707 struct qnode *node;
708
709 if (unlikely(locked != _Q_SLOW_VAL)) {
710 WARN(!debug_locks_silent,
711 "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
712 (unsigned long)lock, atomic_read(&lock->val));
713 return;
714 }
715
716 /*
717 * A failed cmpxchg doesn't provide any memory-ordering guarantees,
718 * so we need a barrier to order the read of the node data in
719 * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
720 *
721 * Matches the cmpxchg() in pv_wait_head_or_lock() setting _Q_SLOW_VAL.
722 */
723 smp_rmb();
724
725 /*
726 * Since the above failed to release, this must be the SLOW path.
727 * Therefore start by looking up the blocked node and unhashing it.
728 */
729 node = pv_unhash(lock);
730
731 /*
732 * Now that we have a reference to the (likely) blocked qnode,
733 * release the lock.
734 */
735 smp_store_release(&lock->locked, 0);
736
737 /*
738 * At this point the memory pointed at by lock can be freed/reused,
739 * however we can still use the qnode to kick the CPU.
740 * The other vCPU may not really be halted, but kicking an active
741 * vCPU is harmless other than the additional latency in completing
742 * the unlock.
743 */
744 lockevent_inc(pv_kick_unlock);
745 pv_kick(node->cpu);
746 }
747
--
0-DAY CI Kernel Test Service
https://01.org/lkp
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