Messages in this thread Patch in this message | | | Date | Tue, 5 Jul 2022 19:59:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/13] locking/qspinlock: simplify code generation |
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On Tue, Jul 05, 2022 at 12:38:07AM +1000, Nicholas Piggin wrote: > Hi, > > Been recently looking a bit closer at queued spinlock code, and > found it's a little tricky to follow especially the pv generation. > This series tries to improve the situation. It's not well tested > outside powerpc, but it's really the x86 pv code that is the > other major complexity that should need some review and testing. > Opinions?
perhaps something like so on top/instead? This would still allow slotting in other implementations with relative ease and the compilers should constant fold all this.
--- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -609,7 +609,7 @@ static void pv_kick_node(struct qspinloc * * The current value of the lock will be returned for additional processing. */ -static void pv_wait_head_or_lock(struct qspinlock *lock, struct qnode *node) +static u32 pv_wait_head_or_lock(struct qspinlock *lock, struct qnode *node) { struct qspinlock **lp = NULL; int waitcnt = 0; @@ -641,7 +641,7 @@ static void pv_wait_head_or_lock(struct set_pending(lock); for (loop = SPIN_THRESHOLD; loop; loop--) { if (trylock_clear_pending(lock)) - return; /* got lock */ + goto out; /* got lock */ cpu_relax(); } clear_pending(lock); @@ -669,7 +669,7 @@ static void pv_wait_head_or_lock(struct */ WRITE_ONCE(lock->locked, _Q_LOCKED_VAL); WRITE_ONCE(*lp, NULL); - return; /* got lock */ + goto out; /* got lock */ } } WRITE_ONCE(node->state, vcpu_hashed); @@ -683,12 +683,22 @@ static void pv_wait_head_or_lock(struct */ } +out: /* * The cmpxchg() or xchg() call before coming here provides the * acquire semantics for locking. */ + return atomic_read(&lock->val); } +static const struct queue_ops pv_ops = { + .init_node = pv_init_node, + .trylock = pv_hybrid_queued_unfair_trylock, + .wait_node = pv_wait_node, + .wait_head_or_lock = pv_wait_head_or_lock, + .kick_node = pv_kick_node, +}; + /* * PV versions of the unlock fastpath and slowpath functions to be used * instead of queued_spin_unlock(). @@ -756,18 +766,18 @@ __visible void __pv_queued_spin_unlock(s EXPORT_SYMBOL(__pv_queued_spin_unlock); #endif -#else /* CONFIG_PARAVIRT_SPINLOCKS */ -static __always_inline void pv_init_node(struct qnode *node) { } -static __always_inline void pv_wait_node(struct qnode *node, - struct qnode *prev) { } -static __always_inline void pv_kick_node(struct qspinlock *lock, - struct qnode *node) { } -static __always_inline void pv_wait_head_or_lock(struct qspinlock *lock, - struct qnode *node) { } -static __always_inline bool pv_hybrid_queued_unfair_trylock(struct qspinlock *lock) { BUILD_BUG(); } #endif /* CONFIG_PARAVIRT_SPINLOCKS */ -static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, bool paravirt) +struct queue_ops { + void (*init_node)(struct qnode *node); + bool (*trylock)(struct qspinlock *lock); + void (*wait_node)(struct qnode *node, struct qnode *prev); + u32 (*wait_head_or_lock)(struct qspinlock *lock, struct qnode *node); + void (*kick_node)(struct qspinlock *lock, struct qnode *node); +}; + +static __always_inline +void queued_spin_lock_mcs_queue(struct qspinlock *lock, const struct queue_ops *ops) { struct qnode *prev, *next, *node; u32 val, old, tail; @@ -813,16 +823,16 @@ static inline void queued_spin_lock_mcs_ node->locked = 0; node->next = NULL; - if (paravirt) - pv_init_node(node); + if (ops && ops->init_node) + ops->init_node(node); /* * We touched a (possibly) cold cacheline in the per-cpu queue node; * attempt the trylock once more in the hope someone let go while we * weren't watching. */ - if (paravirt) { - if (pv_hybrid_queued_unfair_trylock(lock)) + if (ops && ops->trylock) { + if (ops->trylock(lock)) goto release; } else { if (queued_spin_trylock(lock)) @@ -857,8 +867,8 @@ static inline void queued_spin_lock_mcs_ WRITE_ONCE(prev->next, node); /* Wait for mcs node lock to be released */ - if (paravirt) - pv_wait_node(node, prev); + if (ops && ops->wait_node) + ops->wait_node(node, prev); else smp_cond_load_acquire(&node->locked, VAL); @@ -893,12 +903,11 @@ static inline void queued_spin_lock_mcs_ * If PV isn't active, 0 will be returned instead. * */ - if (paravirt) { - pv_wait_head_or_lock(lock, node); - val = atomic_read(&lock->val); + if (ops && ops->wait_head_or_lock) { + val = ops->wait_head_or_lock(lock, node); } else { val = atomic_cond_read_acquire(&lock->val, - !(VAL & _Q_LOCKED_PENDING_MASK)); + !(VAL & _Q_LOCKED_PENDING_MASK)); } /* @@ -1049,14 +1058,14 @@ void queued_spin_lock_slowpath(struct qs */ queue: lockevent_inc(lock_slowpath); - queued_spin_lock_mcs_queue(lock, false); + queued_spin_lock_mcs_queue(lock, NULL); } EXPORT_SYMBOL(queued_spin_lock_slowpath); #ifdef CONFIG_PARAVIRT_SPINLOCKS void __pv_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) { - queued_spin_lock_mcs_queue(lock, true); + queued_spin_lock_mcs_queue(lock, &pv_ops); } EXPORT_SYMBOL(__pv_queued_spin_lock_slowpath);
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