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McKenney" , Tejun Heo , Josh Don , Dohyun Kim , linux-arm-kernel@lists.infradead.org, kkd@meta.com, kernel-team@meta.com Subject: [PATCH bpf-next v3 09/25] rqspinlock: Protect pending bit owners from stalls Date: Mon, 3 Mar 2025 07:22:49 -0800 Message-ID: <20250303152305.3195648-10-memxor@gmail.com> X-Mailer: git-send-email 2.43.5 In-Reply-To: <20250303152305.3195648-1-memxor@gmail.com> References: <20250303152305.3195648-1-memxor@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=4562; h=from:subject; bh=1XvWGmZCOVeyUxqvFf8Y0s6SXxKLnF+WaHRbErsWH/c=; b=owEBbQKS/ZANAwAIAUzgyIZIvxHKAcsmYgBnxcWW53FYHDbQPTx2MaF10QE+C9S9CqxKdZN1xCD5 5EH85qGJAjMEAAEIAB0WIQRLvip+Buz51YI8YRFM4MiGSL8RygUCZ8XFlgAKCRBM4MiGSL8RygSbD/ 9QtT69Fbx2ynZFLU/vCOAn9155npdoPHm32pPMjxG7qV6+Th71XhvC5I9p08WQrwACm1JzGxkepYjm pDnmwck8jRMUBZWjdXi29dJEj7At072EjdZOXCp/Pzn/DxG1hBXTY/ZH9GJo3q8nxLqI/8mzOGpvR2 b9bpAUy1mnqdSz0AULDNnLik1YTh5mj/YEE1p2drByM6bXTGqqEfUlBQDrhOqf8KdBxa4dxNRcWKlB 0tNMmc0lPHuNsFGhIa98RSTjnkXRl2GzOutIcnXs93hpXKn8yL6/Un1t4hb0mHzlLTyxwVwY/g2PPn 0AA/IDGt1/cVQydezHH65YVA5Teh+546Vhq2IbW4nmbmef5EAMQ84COS1A81yPDyV5q0JTKBMjPvx1 p9XYSN4yy8hzTmwMquOoMa64dIFJQHMW0aXviAnk5fYkdWxSk5Gv34nqs96z8x2Eo7/RrZBWNCXpxA iRgBViTStzOx2G/rqo/jRde/IWmULIVwPeiAlMApDAYkehjUopupyV601mAfuY2y35Aiv6QNqYtJWc T6+pG9F4lzVDFo4lc2g8miE5ZrRBaoNA8GI9PRcWFMSNQAd8K0mEFPFYcYwWm/d8UaoZIqpQnY8Z2P uxJ+hkyt/kL3Vl61gkgHMJVOdMx+RocUAFG+5xXN2fv9S6IKl87+y7Q5xRlQ== X-Developer-Key: i=memxor@gmail.com; a=openpgp; fpr=4BBE2A7E06ECF9D5823C61114CE0C88648BF11CA X-Patchwork-Delegate: bpf@iogearbox.net The pending bit is used to avoid queueing in case the lock is uncontended, and has demonstrated benefits for the 2 contender scenario, esp. on x86. In case the pending bit is acquired and we wait for the locked bit to disappear, we may get stuck due to the lock owner not making progress. Hence, this waiting loop must be protected with a timeout check. To perform a graceful recovery once we decide to abort our lock acquisition attempt in this case, we must unset the pending bit since we own it. All waiters undoing their changes and exiting gracefully allows the lock word to be restored to the unlocked state once all participants (owner, waiters) have been recovered, and the lock remains usable. Hence, set the pending bit back to zero before returning to the caller. Introduce a lockevent (rqspinlock_lock_timeout) to capture timeout event statistics. Reviewed-by: Barret Rhoden Signed-off-by: Kumar Kartikeya Dwivedi --- include/asm-generic/rqspinlock.h | 2 +- kernel/locking/lock_events_list.h | 5 +++++ kernel/locking/rqspinlock.c | 28 +++++++++++++++++++++++----- 3 files changed, 29 insertions(+), 6 deletions(-) diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h index 96cea871fdd2..d23793d8e64d 100644 --- a/include/asm-generic/rqspinlock.h +++ b/include/asm-generic/rqspinlock.h @@ -15,7 +15,7 @@ struct qspinlock; typedef struct qspinlock rqspinlock_t; -extern void resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val); +extern int resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val); /* * Default timeout for waiting loops is 0.25 seconds diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h index 97fb6f3f840a..c5286249994d 100644 --- a/kernel/locking/lock_events_list.h +++ b/kernel/locking/lock_events_list.h @@ -49,6 +49,11 @@ LOCK_EVENT(lock_use_node4) /* # of locking ops that use 4th percpu node */ LOCK_EVENT(lock_no_node) /* # of locking ops w/o using percpu node */ #endif /* CONFIG_QUEUED_SPINLOCKS */ +/* + * Locking events for Resilient Queued Spin Lock + */ +LOCK_EVENT(rqspinlock_lock_timeout) /* # of locking ops that timeout */ + /* * Locking events for rwsem */ diff --git a/kernel/locking/rqspinlock.c b/kernel/locking/rqspinlock.c index efa937ea80d9..6be36798ded9 100644 --- a/kernel/locking/rqspinlock.c +++ b/kernel/locking/rqspinlock.c @@ -154,12 +154,12 @@ static DEFINE_PER_CPU_ALIGNED(struct qnode, rqnodes[_Q_MAX_NODES]); * contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' : * queue : ^--' : */ -void __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) +int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) { struct mcs_spinlock *prev, *next, *node; struct rqspinlock_timeout ts; + int idx, ret = 0; u32 old, tail; - int idx; BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS)); @@ -217,8 +217,25 @@ void __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) * clear_pending_set_locked() implementations imply full * barriers. */ - if (val & _Q_LOCKED_MASK) - smp_cond_load_acquire(&lock->locked, !VAL); + if (val & _Q_LOCKED_MASK) { + RES_RESET_TIMEOUT(ts, RES_DEF_TIMEOUT); + res_smp_cond_load_acquire(&lock->locked, !VAL || RES_CHECK_TIMEOUT(ts, ret)); + } + + if (ret) { + /* + * We waited for the locked bit to go back to 0, as the pending + * waiter, but timed out. We need to clear the pending bit since + * we own it. Once a stuck owner has been recovered, the lock + * must be restored to a valid state, hence removing the pending + * bit is necessary. + * + * *,1,* -> *,0,* + */ + clear_pending(lock); + lockevent_inc(rqspinlock_lock_timeout); + return ret; + } /* * take ownership and clear the pending bit. @@ -227,7 +244,7 @@ void __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) */ clear_pending_set_locked(lock); lockevent_inc(lock_pending); - return; + return 0; /* * End of pending bit optimistic spinning and beginning of MCS @@ -378,5 +395,6 @@ void __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) * release the node */ __this_cpu_dec(rqnodes[0].mcs.count); + return 0; } EXPORT_SYMBOL(resilient_queued_spin_lock_slowpath);