cregit-Linux how code gets into the kernel

Release 4.15 kernel/locking/osq_lock.c

Directory: kernel/locking
// SPDX-License-Identifier: GPL-2.0
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/osq_lock.h>

 * An MCS like lock especially tailored for optimistic spinning for sleeping
 * lock implementations (mutex, rwsem, etc).
 * Using a single mcs node per CPU is safe because sleeping locks should not be
 * called from interrupt context and we have preemption disabled while
 * spinning.
static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);

 * We use the value 0 to represent "no CPU", thus the encoded value
 * will be the CPU number incremented by 1.

static inline int encode_cpu(int cpu_nr) { return cpu_nr + 1; }


Jason Low15100.00%1100.00%

static inline int node_cpu(struct optimistic_spin_node *node) { return node->cpu - 1; }


Pan Xinhui19100.00%1100.00%

static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) { int cpu_nr = encoded_cpu_val - 1; return per_cpu_ptr(&osq_node, cpu_nr); }


Jason Low28100.00%1100.00%

/* * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. * Can return NULL in case we were the last queued and we updated @lock instead. */
static inline struct optimistic_spin_node * osq_wait_next(struct optimistic_spin_queue *lock, struct optimistic_spin_node *node, struct optimistic_spin_node *prev) { struct optimistic_spin_node *next = NULL; int curr = encode_cpu(smp_processor_id()); int old; /* * If there is a prev node in queue, then the 'old' value will be * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if * we're currently last in queue, then the queue will then become empty. */ old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; for (;;) { if (atomic_read(&lock->tail) == curr && atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) { /* * We were the last queued, we moved @lock back. @prev * will now observe @lock and will complete its * unlock()/unqueue(). */ break; } /* * We must xchg() the @node->next value, because if we were to * leave it in, a concurrent unlock()/unqueue() from * @node->next might complete Step-A and think its @prev is * still valid. * * If the concurrent unlock()/unqueue() wins the race, we'll * wait for either @lock to point to us, through its Step-B, or * wait for a new @node->next from its Step-C. */ if (node->next) { next = xchg(&node->next, NULL); if (next) break; } cpu_relax(); } return next; }


Peter Zijlstra8065.04%120.00%
Jason Low4133.33%240.00%
Davidlohr Bueso A10.81%120.00%
Christian Bornträger10.81%120.00%

bool osq_lock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); struct optimistic_spin_node *prev, *next; int curr = encode_cpu(smp_processor_id()); int old; node->locked = 0; node->next = NULL; node->cpu = curr; /* * We need both ACQUIRE (pairs with corresponding RELEASE in * unlock() uncontended, or fastpath) and RELEASE (to publish * the node fields we just initialised) semantics when updating * the lock tail. */ old = atomic_xchg(&lock->tail, curr); if (old == OSQ_UNLOCKED_VAL) return true; prev = decode_cpu(old); node->prev = prev; /* * osq_lock() unqueue * * node->prev = prev osq_wait_next() * WMB MB * prev->next = node next->prev = prev // unqueue-C * * Here 'node->prev' and 'next->prev' are the same variable and we need * to ensure these stores happen in-order to avoid corrupting the list. */ smp_wmb(); WRITE_ONCE(prev->next, node); /* * Normally @prev is untouchable after the above store; because at that * moment unlock can proceed and wipe the node element from stack. * * However, since our nodes are static per-cpu storage, we're * guaranteed their existence -- this allows us to apply * cmpxchg in an attempt to undo our queueing. */ while (!READ_ONCE(node->locked)) { /* * If we need to reschedule bail... so we can block. * Use vcpu_is_preempted() to avoid waiting for a preempted * lock holder: */ if (need_resched() || vcpu_is_preempted(node_cpu(node->prev))) goto unqueue; cpu_relax(); } return true; unqueue: /* * Step - A -- stabilize @prev * * Undo our @prev->next assignment; this will make @prev's * unlock()/unqueue() wait for a next pointer since @lock points to us * (or later). */ for (;;) { if (prev->next == node && cmpxchg(&prev->next, node, NULL) == node) break; /* * We can only fail the cmpxchg() racing against an unlock(), * in which case we should observe @node->locked becomming * true. */ if (smp_load_acquire(&node->locked)) return true; cpu_relax(); /* * Or we race against a concurrent unqueue()'s step-B, in which * case its step-C will write us a new @node->prev pointer. */ prev = READ_ONCE(node->prev); } /* * Step - B -- stabilize @next * * Similar to unlock(), wait for @node->next or move @lock from @node * back to @prev. */ next = osq_wait_next(lock, node, prev); if (!next) return false; /* * Step - C -- unlink * * @prev is stable because its still waiting for a new @prev->next * pointer, @next is stable because our @node->next pointer is NULL and * it will wait in Step-A. */ WRITE_ONCE(next->prev, prev); WRITE_ONCE(prev->next, next); return false; }


Peter Zijlstra17972.47%112.50%
Jason Low3815.38%225.00%
Davidlohr Bueso A114.45%112.50%
Pan Xinhui114.45%112.50%
Prateek Sood41.62%112.50%
Will Deacon20.81%112.50%
Christian Bornträger20.81%112.50%

void osq_unlock(struct optimistic_spin_queue *lock) { struct optimistic_spin_node *node, *next; int curr = encode_cpu(smp_processor_id()); /* * Fast path for the uncontended case. */ if (likely(atomic_cmpxchg_release(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) return; /* * Second most likely case. */ node = this_cpu_ptr(&osq_node); next = xchg(&node->next, NULL); if (next) { WRITE_ONCE(next->locked, 1); return; } next = osq_wait_next(lock, node, NULL); if (next) WRITE_ONCE(next->locked, 1); }


Peter Zijlstra7669.72%116.67%
Jason Low2623.85%350.00%
Davidlohr Bueso A76.42%233.33%

Overall Contributors

Peter Zijlstra35362.70%18.33%
Jason Low15026.64%325.00%
Pan Xinhui305.33%18.33%
Davidlohr Bueso A203.55%325.00%
Prateek Sood40.71%18.33%
Christian Bornträger30.53%18.33%
Will Deacon20.36%18.33%
Greg Kroah-Hartman10.18%18.33%
Directory: kernel/locking
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