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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 v4 09/25] rqspinlock: Protect pending bit owners from stalls Date: Sat, 15 Mar 2025 21:05:25 -0700 Message-ID: <20250316040541.108729-10-memxor@gmail.com> X-Mailer: git-send-email 2.47.1 In-Reply-To: <20250316040541.108729-1-memxor@gmail.com> References: <20250316040541.108729-1-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=5013; h=from:subject; bh=akh5yhXfrEG7tkbyObFzFj7wBccI2uw5HAmi9Ua2XD8=; b=owEBbQKS/ZANAwAIAUzgyIZIvxHKAcsmYgBn1k3c4jSnmA5ONpoVoAIOIOlsxwJqGlCaF71ZqrDS 9f300ICJAjMEAAEIAB0WIQRLvip+Buz51YI8YRFM4MiGSL8RygUCZ9ZN3AAKCRBM4MiGSL8Ryj+AD/ 9QOKlJQsL0hfRsxlnp+LcUhoinUUhuf5PSUgCRTOdkG5asYnxKe4RE+k6QEZqq50n7HbKHjFsdfrSv 5px/fRWV280XEsplziWpBm3nu9/uGIn06D/6DArjzNdiOULmNoKhIVTCdpvva8L3HI7GSTxZani9n6 MMd1j6C8TM9+oKJsg31rq03joUCbrTzttht7zSk0WcHYwGmbbQNFYepfby2aIG6QR4whRiRfFigmjD 1nOdu68WNCp+zFeB5UwD1SKNXHXAiI47mTVfHATyZW2gO+qoVjxj0jcjEQISr2U7ilsZqhtUV4qu79 0rmI1TdWYDffcf35yEPkekzca8OUaen5puFYbpAn2C5IQghtsvhiDeLpI+qrW+Dh9VrVLfJeZb4PA/ CmKe2nkK73+vP66V/atyI3Nop6a65yKO4dVVKbSm3r6dXZdycqN3CO6uWPWXasVJOnEwagR0GKKcHV 5lNkIep13oO1YQkgmpMqkC4RCf6IhbIwCFAm6j9Mt+0TJyAB8WHcAIwetF6IDcyMBK5R+DiMlKafao b9h8GkivtSiN4L7TcMtqek4PinmZ+hoaj24dkKmwfoF1g7gqEI1D1SHHR9nGA/BtBKsGojUhnVbY1O sUkCihUbg8MADno6l9S45vaZux7YwSBu0+kqCFmosGJEUxcqTgLWNTqfWjaA== 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/bpf/rqspinlock.c | 32 ++++++++++++++++++++++++++----- kernel/locking/lock_events_list.h | 5 +++++ 3 files changed, 33 insertions(+), 6 deletions(-) diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h index 5dd4dd8aee69..9bd11cb7acd6 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/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c index d429b923b58f..262294cfd36f 100644 --- a/kernel/bpf/rqspinlock.c +++ b/kernel/bpf/rqspinlock.c @@ -138,6 +138,10 @@ static DEFINE_PER_CPU_ALIGNED(struct qnode, rqnodes[_Q_MAX_NODES]); * @lock: Pointer to queued spinlock structure * @val: Current value of the queued spinlock 32-bit word * + * Return: + * * 0 - Lock was acquired successfully. + * * -ETIMEDOUT - Lock acquisition failed because of timeout. + * * (queue tail, pending bit, lock value) * * fast : slow : unlock @@ -154,12 +158,12 @@ 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 +221,25 @@ 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 +248,7 @@ 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 +399,6 @@ 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_GPL(resilient_queued_spin_lock_slowpath); 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 */