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McKenney" , Tejun Heo , Barret Rhoden , Josh Don , Dohyun Kim , linux-arm-kernel@lists.infradead.org, kkd@meta.com, kernel-team@meta.com Subject: [PATCH bpf-next v4 13/25] rqspinlock: Add a test-and-set fallback Date: Sat, 15 Mar 2025 21:05:29 -0700 Message-ID: <20250316040541.108729-14-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=4142; h=from:subject; bh=6EUILfgiZohkAPsyvtNzcRk40CzXPZuYHAK+ukHSlzU=; b=owEBbQKS/ZANAwAIAUzgyIZIvxHKAcsmYgBn1k3dWZVH4cGT4jygyYaRMgYBKoSMntnk4oFzqRKP +TJ3LmiJAjMEAAEIAB0WIQRLvip+Buz51YI8YRFM4MiGSL8RygUCZ9ZN3QAKCRBM4MiGSL8RyhmDEA C1zcUyxSUagfAmjyxuovyTwYJTtWqoL5BpkcoMeatgpoXvSFS4tL7BP+3f57t/dG51OVYvBhT2RXN3 nhHuP1IuXSpAdZG+q3swIHEVfIED9SS3PCn0geldDUhkAzxMkpbZqWPwJhh4IcqZB+IWBsN7BZYtW+ eiq/JKntCpyChduNnwrkkDRMWiws3F5N+JllmYp8XC54/J48uYmiFyJaYYEUR7ONy+IL/f21dHKQrT 8xCYRQblvr2tXiLe/rj9uJrq5h9J1j+Qw16WsNZ8WDBUWx52xoX1xV1UdOS/8vQX9HCEOJ8KDbo/db spk5jmSQFHBWJjYK39tihe0CXejyi/J+FpiRzMRNopnL44+efFbw10U5IgaVjMEP9LkozIVmnowmrI jCZ5a0OfO+eZf6xAeHlknuB9N5Zl0SNEZhWh3ayRtPeF/OpHs9sZrE+CaTsjnHnh2g8vIQt/VzzuU9 bJyTo/zGSOiYSFmfYQPN3JfpgR58GeJTqb15WlpNy21ivZWR9dptXHGTECX7xeEoJ7ns+MuNnYAC9e miR4qDevVacWqmiWfTKfc8CbfAmZI//+1NKlJgfhOomrfHwGNvSEeKxTnMs0pANCDx/wKI/kHLuWqC rR72s0G1l2FIBNETNQNhbftHp/sZqx1oEyYMlCsrRgBNyzGuhAHxNLnCg7Ww== X-Developer-Key: i=memxor@gmail.com; a=openpgp; fpr=4BBE2A7E06ECF9D5823C61114CE0C88648BF11CA X-Patchwork-Delegate: bpf@iogearbox.net Include a test-and-set fallback when queued spinlock support is not available. Introduce a rqspinlock type to act as a fallback when qspinlock support is absent. Include ifdef guards to ensure the slow path in this file is only compiled when CONFIG_QUEUED_SPINLOCKS=y. Subsequent patches will add further logic to ensure fallback to the test-and-set implementation when queued spinlock support is unavailable on an architecture. Unlike other waiting loops in rqspinlock code, the one for test-and-set has no theoretical upper bound under contention, therefore we need a longer timeout than usual. Bump it up to a second in this case. Signed-off-by: Kumar Kartikeya Dwivedi --- include/asm-generic/rqspinlock.h | 17 ++++++++++++ kernel/bpf/rqspinlock.c | 46 ++++++++++++++++++++++++++++++-- 2 files changed, 61 insertions(+), 2 deletions(-) diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h index 34c3dcb4299e..12f72c4a97cd 100644 --- a/include/asm-generic/rqspinlock.h +++ b/include/asm-generic/rqspinlock.h @@ -12,11 +12,28 @@ #include #include #include +#ifdef CONFIG_QUEUED_SPINLOCKS +#include +#endif + +struct rqspinlock { + union { + atomic_t val; + u32 locked; + }; +}; struct qspinlock; +#ifdef CONFIG_QUEUED_SPINLOCKS typedef struct qspinlock rqspinlock_t; +#else +typedef struct rqspinlock rqspinlock_t; +#endif +extern int resilient_tas_spin_lock(rqspinlock_t *lock); +#ifdef CONFIG_QUEUED_SPINLOCKS extern int resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val); +#endif /* * Default timeout for waiting loops is 0.25 seconds diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c index bddbcc47d38f..714dfab5caa8 100644 --- a/kernel/bpf/rqspinlock.c +++ b/kernel/bpf/rqspinlock.c @@ -21,7 +21,9 @@ #include #include #include +#ifdef CONFIG_QUEUED_SPINLOCKS #include +#endif #include #include #include @@ -29,9 +31,12 @@ /* * Include queued spinlock definitions and statistics code */ +#ifdef CONFIG_QUEUED_SPINLOCKS #include "../locking/qspinlock.h" #include "../locking/lock_events.h" #include "rqspinlock.h" +#include "../locking/mcs_spinlock.h" +#endif /* * The basic principle of a queue-based spinlock can best be understood @@ -70,8 +75,6 @@ * */ -#include "../locking/mcs_spinlock.h" - struct rqspinlock_timeout { u64 timeout_end; u64 duration; @@ -263,6 +266,43 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask, */ #define RES_RESET_TIMEOUT(ts, _duration) ({ (ts).timeout_end = 0; (ts).duration = _duration; }) +/* + * Provide a test-and-set fallback for cases when queued spin lock support is + * absent from the architecture. + */ +int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock) +{ + struct rqspinlock_timeout ts; + int val, ret = 0; + + RES_INIT_TIMEOUT(ts); + grab_held_lock_entry(lock); + + /* + * Since the waiting loop's time is dependent on the amount of + * contention, a short timeout unlike rqspinlock waiting loops + * isn't enough. Choose a second as the timeout value. + */ + RES_RESET_TIMEOUT(ts, NSEC_PER_SEC); +retry: + val = atomic_read(&lock->val); + + if (val || !atomic_try_cmpxchg(&lock->val, &val, 1)) { + if (RES_CHECK_TIMEOUT(ts, ret, ~0u)) + goto out; + cpu_relax(); + goto retry; + } + + return 0; +out: + release_held_lock_entry(); + return ret; +} +EXPORT_SYMBOL_GPL(resilient_tas_spin_lock); + +#ifdef CONFIG_QUEUED_SPINLOCKS + /* * Per-CPU queue node structures; we can never have more than 4 nested * contexts: task, softirq, hardirq, nmi. @@ -616,3 +656,5 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val) return ret; } EXPORT_SYMBOL_GPL(resilient_queued_spin_lock_slowpath); + +#endif /* CONFIG_QUEUED_SPINLOCKS */