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kernel/locking/qspinlock.c
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// SPDX-License-Identifier: GPL-2.0-or-later |
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/* * Queued spinlock * |
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* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P. |
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* (C) Copyright 2013-2014,2018 Red Hat, Inc. |
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* (C) Copyright 2015 Intel Corp. |
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* (C) Copyright 2015 Hewlett-Packard Enterprise Development LP |
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* |
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* Authors: Waiman Long <longman@redhat.com> |
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* Peter Zijlstra <peterz@infradead.org> */ |
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#ifndef _GEN_PV_LOCK_SLOWPATH |
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#include <linux/smp.h> #include <linux/bug.h> #include <linux/cpumask.h> #include <linux/percpu.h> #include <linux/hardirq.h> #include <linux/mutex.h> |
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#include <linux/prefetch.h> |
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#include <asm/byteorder.h> |
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#include <asm/qspinlock.h> /* |
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* Include queued spinlock statistics code */ #include "qspinlock_stat.h" /* |
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* The basic principle of a queue-based spinlock can best be understood * by studying a classic queue-based spinlock implementation called the * MCS lock. The paper below provides a good description for this kind * of lock. * * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf * * This queued spinlock implementation is based on the MCS lock, however to make * it fit the 4 bytes we assume spinlock_t to be, and preserve its existing * API, we must modify it somehow. * * In particular; where the traditional MCS lock consists of a tail pointer * (8 bytes) and needs the next pointer (another 8 bytes) of its own node to * unlock the next pending (next->locked), we compress both these: {tail, * next->locked} into a single u32 value. * * Since a spinlock disables recursion of its own context and there is a limit * to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there * are at most 4 nesting levels, it can be encoded by a 2-bit number. Now * we can encode the tail by combining the 2-bit nesting level with the cpu * number. With one byte for the lock value and 3 bytes for the tail, only a * 32-bit word is now needed. Even though we only need 1 bit for the lock, * we extend it to a full byte to achieve better performance for architectures * that support atomic byte write. * * We also change the first spinner to spin on the lock bit instead of its * node; whereby avoiding the need to carry a node from lock to unlock, and * preserving existing lock API. This also makes the unlock code simpler and * faster. |
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* * N.B. The current implementation only supports architectures that allow * atomic operations on smaller 8-bit and 16-bit data types. * |
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*/ #include "mcs_spinlock.h" |
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#define MAX_NODES 4 |
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|
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/* * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in * size and four of them will fit nicely in one 64-byte cacheline. For * pvqspinlock, however, we need more space for extra data. To accommodate * that, we insert two more long words to pad it up to 32 bytes. IOW, only * two of them can fit in a cacheline in this case. That is OK as it is rare * to have more than 2 levels of slowpath nesting in actual use. We don't * want to penalize pvqspinlocks to optimize for a rare case in native * qspinlocks. */ struct qnode { struct mcs_spinlock mcs; |
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#ifdef CONFIG_PARAVIRT_SPINLOCKS |
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long reserved[2]; |
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#endif |
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}; |
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|
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/* |
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* The pending bit spinning loop count. * This heuristic is used to limit the number of lockword accesses * made by atomic_cond_read_relaxed when waiting for the lock to * transition out of the "== _Q_PENDING_VAL" state. We don't spin * indefinitely because there's no guarantee that we'll make forward * progress. */ #ifndef _Q_PENDING_LOOPS #define _Q_PENDING_LOOPS 1 #endif /* |
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* Per-CPU queue node structures; we can never have more than 4 nested * contexts: task, softirq, hardirq, nmi. * * Exactly fits one 64-byte cacheline on a 64-bit architecture. |
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* * PV doubles the storage and uses the second cacheline for PV state. |
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*/ |
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static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]); |
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/* * We must be able to distinguish between no-tail and the tail at 0:0, * therefore increment the cpu number by one. */ |
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static inline __pure u32 encode_tail(int cpu, int idx) |
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{ u32 tail; |
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tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET; tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */ return tail; } |
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static inline __pure struct mcs_spinlock *decode_tail(u32 tail) |
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{ int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1; int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET; |
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return per_cpu_ptr(&qnodes[idx].mcs, cpu); } static inline __pure struct mcs_spinlock *grab_mcs_node(struct mcs_spinlock *base, int idx) { return &((struct qnode *)base + idx)->mcs; |
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} |
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#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK) |
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#if _Q_PENDING_BITS == 8 |
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/** |
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* clear_pending - clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,* -> *,0,* */ static __always_inline void clear_pending(struct qspinlock *lock) { WRITE_ONCE(lock->pending, 0); } /** |
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* clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,0 -> *,0,1 * * Lock stealing is not allowed if this function is used. */ static __always_inline void clear_pending_set_locked(struct qspinlock *lock) { |
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WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL); |
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} /* * xchg_tail - Put in the new queue tail code word & retrieve previous one * @lock : Pointer to queued spinlock structure * @tail : The new queue tail code word * Return: The previous queue tail code word * |
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* xchg(lock, tail), which heads an address dependency |
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* * p,*,* -> n,*,* ; prev = xchg(lock, node) */ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) { |
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/* |
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* We can use relaxed semantics since the caller ensures that the * MCS node is properly initialized before updating the tail. |
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*/ |
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return (u32)xchg_relaxed(&lock->tail, |
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tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET; |
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} #else /* _Q_PENDING_BITS == 8 */ |
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/** |
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* clear_pending - clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,* -> *,0,* */ static __always_inline void clear_pending(struct qspinlock *lock) { atomic_andnot(_Q_PENDING_VAL, &lock->val); } /** |
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* clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure * * *,1,0 -> *,0,1 */ static __always_inline void clear_pending_set_locked(struct qspinlock *lock) { atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val); } /** * xchg_tail - Put in the new queue tail code word & retrieve previous one * @lock : Pointer to queued spinlock structure * @tail : The new queue tail code word * Return: The previous queue tail code word * * xchg(lock, tail) * * p,*,* -> n,*,* ; prev = xchg(lock, node) */ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) { u32 old, new, val = atomic_read(&lock->val); for (;;) { new = (val & _Q_LOCKED_PENDING_MASK) | tail; |
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/* |
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* We can use relaxed semantics since the caller ensures that * the MCS node is properly initialized before updating the * tail. |
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*/ |
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old = atomic_cmpxchg_relaxed(&lock->val, val, new); |
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if (old == val) break; val = old; } return old; } |
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#endif /* _Q_PENDING_BITS == 8 */ |
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/** |
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* queued_fetch_set_pending_acquire - fetch the whole lock value and set pending * @lock : Pointer to queued spinlock structure * Return: The previous lock value * * *,*,* -> *,1,* */ #ifndef queued_fetch_set_pending_acquire static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock) { return atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val); } #endif /** |
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* set_locked - Set the lock bit and own the lock * @lock: Pointer to queued spinlock structure * * *,*,0 -> *,0,1 */ static __always_inline void set_locked(struct qspinlock *lock) { |
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WRITE_ONCE(lock->locked, _Q_LOCKED_VAL); |
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} |
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/* * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for * all the PV callbacks. */ static __always_inline void __pv_init_node(struct mcs_spinlock *node) { } |
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static __always_inline void __pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev) { } |
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static __always_inline void __pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node) { } |
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static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node) { return 0; } |
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#define pv_enabled() false #define pv_init_node __pv_init_node #define pv_wait_node __pv_wait_node #define pv_kick_node __pv_kick_node |
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#define pv_wait_head_or_lock __pv_wait_head_or_lock |
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#ifdef CONFIG_PARAVIRT_SPINLOCKS #define queued_spin_lock_slowpath native_queued_spin_lock_slowpath #endif #endif /* _GEN_PV_LOCK_SLOWPATH */ |
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/** |
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* queued_spin_lock_slowpath - acquire the queued spinlock * @lock: Pointer to queued spinlock structure * @val: Current value of the queued spinlock 32-bit word * |
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* (queue tail, pending bit, lock value) |
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* |
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* fast : slow : unlock * : : * uncontended (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0) * : | ^--------.------. / : * : v \ \ | : * pending : (0,1,1) +--> (0,1,0) \ | : * : | ^--' | | : * : v | | : * uncontended : (n,x,y) +--> (n,0,0) --' | : * queue : | ^--' | : * : v | : * contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' : * queue : ^--' : |
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*/ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) { struct mcs_spinlock *prev, *next, *node; |
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u32 old, tail; |
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int idx; BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS)); |
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if (pv_enabled()) |
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goto pv_queue; |
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|
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if (virt_spin_lock(lock)) |
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return; |
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/* |
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* Wait for in-progress pending->locked hand-overs with a bounded * number of spins so that we guarantee forward progress. |
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* * 0,1,0 -> 0,0,1 */ if (val == _Q_PENDING_VAL) { |
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int cnt = _Q_PENDING_LOOPS; val = atomic_cond_read_relaxed(&lock->val, (VAL != _Q_PENDING_VAL) || !cnt--); |
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} /* |
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* If we observe any contention; queue. */ if (val & ~_Q_LOCKED_MASK) goto queue; /* |
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* trylock || pending * |
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* 0,0,* -> 0,1,* -> 0,0,1 pending, trylock |
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*/ |
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val = queued_fetch_set_pending_acquire(lock); |
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|
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/* |
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* If we observe contention, there is a concurrent locker. * * Undo and queue; our setting of PENDING might have made the * n,0,0 -> 0,0,0 transition fail and it will now be waiting * on @next to become !NULL. |
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*/ if (unlikely(val & ~_Q_LOCKED_MASK)) { |
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/* Undo PENDING if we set it. */ |
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if (!(val & _Q_PENDING_MASK)) clear_pending(lock); |
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|
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goto queue; |
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} |
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/* |
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* We're pending, wait for the owner to go away. * * 0,1,1 -> 0,1,0 * * this wait loop must be a load-acquire such that we match the * store-release that clears the locked bit and create lock * sequentiality; this is because not all * clear_pending_set_locked() implementations imply full * barriers. */ if (val & _Q_LOCKED_MASK) atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK)); /* * take ownership and clear the pending bit. * * 0,1,0 -> 0,0,1 |
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*/ |
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clear_pending_set_locked(lock); |
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lockevent_inc(lock_pending); |
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return; |
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/* * End of pending bit optimistic spinning and beginning of MCS * queuing. */ queue: |
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lockevent_inc(lock_slowpath); |
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pv_queue: |
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node = this_cpu_ptr(&qnodes[0].mcs); |
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idx = node->count++; tail = encode_tail(smp_processor_id(), idx); |
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/* * 4 nodes are allocated based on the assumption that there will * not be nested NMIs taking spinlocks. That may not be true in * some architectures even though the chance of needing more than * 4 nodes will still be extremely unlikely. When that happens, * we fall back to spinning on the lock directly without using * any MCS node. This is not the most elegant solution, but is * simple enough. */ if (unlikely(idx >= MAX_NODES)) { |
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lockevent_inc(lock_no_node); |
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while (!queued_spin_trylock(lock)) cpu_relax(); goto release; } |
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node = grab_mcs_node(node, idx); |
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/* |
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* Keep counts of non-zero index values: */ |
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lockevent_cond_inc(lock_use_node2 + idx - 1, idx); |
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/* |
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* Ensure that we increment the head node->count before initialising * the actual node. If the compiler is kind enough to reorder these * stores, then an IRQ could overwrite our assignments. */ barrier(); |
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node->locked = 0; node->next = NULL; |
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pv_init_node(node); |
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/* |
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* 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. |
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*/ |
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if (queued_spin_trylock(lock)) goto release; |
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/* |
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* Ensure that the initialisation of @node is complete before we * publish the updated tail via xchg_tail() and potentially link * @node into the waitqueue via WRITE_ONCE(prev->next, node) below. */ smp_wmb(); /* * Publish the updated tail. |
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* We have already touched the queueing cacheline; don't bother with * pending stuff. * * p,*,* -> n,*,* |
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*/ |
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old = xchg_tail(lock, tail); |
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next = NULL; |
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/* * if there was a previous node; link it and wait until reaching the * head of the waitqueue. */ |
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if (old & _Q_TAIL_MASK) { |
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prev = decode_tail(old); |
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|
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/* Link @node into the waitqueue. */ WRITE_ONCE(prev->next, node); |
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|
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pv_wait_node(node, prev); |
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arch_mcs_spin_lock_contended(&node->locked); |
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/* * While waiting for the MCS lock, the next pointer may have * been set by another lock waiter. We optimistically load * the next pointer & prefetch the cacheline for writing * to reduce latency in the upcoming MCS unlock operation. */ next = READ_ONCE(node->next); if (next) prefetchw(next); |
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} /* |
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* we're at the head of the waitqueue, wait for the owner & pending to * go away. |
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* |
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* *,x,y -> *,0,0 |
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* * this wait loop must use a load-acquire such that we match the * store-release that clears the locked bit and create lock * sequentiality; this is because the set_locked() function below * does not imply a full barrier. * |
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* The PV pv_wait_head_or_lock function, if active, will acquire * the lock and return a non-zero value. So we have to skip the |
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* atomic_cond_read_acquire() call. As the next PV queue head hasn't * been designated yet, there is no way for the locked value to become |
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* _Q_SLOW_VAL. So both the set_locked() and the * atomic_cmpxchg_relaxed() calls will be safe. * * If PV isn't active, 0 will be returned instead. * |
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*/ |
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if ((val = pv_wait_head_or_lock(lock, node))) goto locked; |
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val = atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK)); |
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|
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locked: |
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/* * claim the lock: * |
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* n,0,0 -> 0,0,1 : lock, uncontended |
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* *,*,0 -> *,*,1 : lock, contended |
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502 |
* |
59fb586b4 locking/qspinlock... |
503 504 505 |
* If the queue head is the only one in the queue (lock value == tail) * and nobody is pending, clear the tail code and grab the lock. * Otherwise, we only need to grab the lock. |
a33fda35e locking/qspinlock... |
506 |
*/ |
c61da58d8 locking/qspinlock... |
507 |
|
ae75d9089 locking/qspinlock... |
508 |
/* |
756b1df4c locking/qspinlock... |
509 510 |
* In the PV case we might already have _Q_LOCKED_VAL set, because * of lock stealing; therefore we must also allow: |
ae75d9089 locking/qspinlock... |
511 |
* |
756b1df4c locking/qspinlock... |
512 513 514 515 516 |
* n,0,1 -> 0,0,1 * * Note: at this point: (val & _Q_PENDING_MASK) == 0, because of the * above wait condition, therefore any concurrent setting of * PENDING will make the uncontended transition fail. |
ae75d9089 locking/qspinlock... |
517 |
*/ |
756b1df4c locking/qspinlock... |
518 519 520 521 |
if ((val & _Q_TAIL_MASK) == tail) { if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL)) goto release; /* No contention */ } |
a33fda35e locking/qspinlock... |
522 |
|
756b1df4c locking/qspinlock... |
523 524 525 526 527 |
/* * Either somebody is queued behind us or _Q_PENDING_VAL got set * which will then detect the remaining tail and queue behind us * ensuring we'll see a @next. */ |
c61da58d8 locking/qspinlock... |
528 |
set_locked(lock); |
a33fda35e locking/qspinlock... |
529 |
/* |
aa68744f8 locking/qspinlock... |
530 |
* contended path; wait for next if not observed yet, release. |
a33fda35e locking/qspinlock... |
531 |
*/ |
c131a198c locking/qspinlock... |
532 533 |
if (!next) next = smp_cond_load_relaxed(&node->next, (VAL)); |
a33fda35e locking/qspinlock... |
534 |
|
2c83e8e94 locking/qspinlock... |
535 |
arch_mcs_spin_unlock_contended(&next->locked); |
75d227028 locking/pvqspinlo... |
536 |
pv_kick_node(lock, next); |
a33fda35e locking/qspinlock... |
537 538 539 540 541 |
release: /* * release the node */ |
0fa809ca7 locking/pvqspinlo... |
542 |
__this_cpu_dec(qnodes[0].mcs.count); |
a33fda35e locking/qspinlock... |
543 544 |
} EXPORT_SYMBOL(queued_spin_lock_slowpath); |
a23db284f locking/pvqspinlo... |
545 546 547 548 549 550 551 552 553 554 555 556 557 |
/* * Generate the paravirt code for queued_spin_unlock_slowpath(). */ #if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS) #define _GEN_PV_LOCK_SLOWPATH #undef pv_enabled #define pv_enabled() true #undef pv_init_node #undef pv_wait_node #undef pv_kick_node |
1c4941fd5 locking/pvqspinlo... |
558 |
#undef pv_wait_head_or_lock |
a23db284f locking/pvqspinlo... |
559 560 561 562 563 564 565 566 |
#undef queued_spin_lock_slowpath #define queued_spin_lock_slowpath __pv_queued_spin_lock_slowpath #include "qspinlock_paravirt.h" #include "qspinlock.c" #endif |