kernel/locking/qrwlock.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * Queued read/write locks
   4  *
   5  * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
   6  *
   7  * Authors: Waiman Long <waiman.long@hp.com>
   8  */
   9 #include <linux/smp.h>
  10 #include <linux/bug.h>
  11 #include <linux/cpumask.h>
  12 #include <linux/percpu.h>
  13 #include <linux/hardirq.h>
  14 #include <linux/spinlock.h>
  15 #include <trace/events/lock.h>
  16
  17 /**
  18  * queued_read_lock_slowpath - acquire read lock of a queued rwlock
  19  * @lock: Pointer to queued rwlock structure
  20  */
  21 void __lockfunc queued_read_lock_slowpath(struct qrwlock *lock)
  22 {
  23         /*
  24          * Readers come here when they cannot get the lock without waiting
  25          */
  26         if (unlikely(in_interrupt())) {
  27                 /*
  28                  * Readers in interrupt context will get the lock immediately
  29                  * if the writer is just waiting (not holding the lock yet),
  30                  * so spin with ACQUIRE semantics until the lock is available
  31                  * without waiting in the queue.
  32                  */
  33                 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  34                 return;
  35         }
  36         atomic_sub(_QR_BIAS, &lock->cnts);
  37
  38         trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
  39
  40         /*
  41          * Put the reader into the wait queue
  42          */
  43         arch_spin_lock(&lock->wait_lock);
  44         atomic_add(_QR_BIAS, &lock->cnts);
  45
  46         /*
  47          * The ACQUIRE semantics of the following spinning code ensure
  48          * that accesses can't leak upwards out of our subsequent critical
  49          * section in the case that the lock is currently held for write.
  50          */
  51         atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  52
  53         /*
  54          * Signal the next one in queue to become queue head
  55          */
  56         arch_spin_unlock(&lock->wait_lock);
  57
  58         trace_contention_end(lock, 0);
  59 }
  60 EXPORT_SYMBOL(queued_read_lock_slowpath);
  61
  62 /**
  63  * queued_write_lock_slowpath - acquire write lock of a queued rwlock
  64  * @lock : Pointer to queued rwlock structure
  65  */
  66 void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
  67 {
  68         int cnts;
  69
  70         trace_contention_begin(lock, LCB_F_SPIN | LCB_F_WRITE);
  71
  72         /* Put the writer into the wait queue */
  73         arch_spin_lock(&lock->wait_lock);
  74
  75         /* Try to acquire the lock directly if no reader is present */
  76         if (!(cnts = atomic_read(&lock->cnts)) &&
  77             atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
  78                 goto unlock;
  79
  80         /* Set the waiting flag to notify readers that a writer is pending */
  81         atomic_or(_QW_WAITING, &lock->cnts);
  82
  83         /* When no more readers or writers, set the locked flag */
  84         do {
  85                 cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
  86         } while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
  87 unlock:
  88         arch_spin_unlock(&lock->wait_lock);
  89
  90         trace_contention_end(lock, 0);
  91 }
  92 EXPORT_SYMBOL(queued_write_lock_slowpath);