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 <asm/qrwlock.h>
  16
  17 /**
  18  * queued_read_lock_slowpath - acquire read lock of a queue rwlock
  19  * @lock: Pointer to queue rwlock structure
  20  */
  21 void 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         /*
  39          * Put the reader into the wait queue
  40          */
  41         arch_spin_lock(&lock->wait_lock);
  42         atomic_add(_QR_BIAS, &lock->cnts);
  43
  44         /*
  45          * The ACQUIRE semantics of the following spinning code ensure
  46          * that accesses can't leak upwards out of our subsequent critical
  47          * section in the case that the lock is currently held for write.
  48          */
  49         atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  50
  51         /*
  52          * Signal the next one in queue to become queue head
  53          */
  54         arch_spin_unlock(&lock->wait_lock);
  55 }
  56 EXPORT_SYMBOL(queued_read_lock_slowpath);
  57
  58 /**
  59  * queued_write_lock_slowpath - acquire write lock of a queue rwlock
  60  * @lock : Pointer to queue rwlock structure
  61  */
  62 void queued_write_lock_slowpath(struct qrwlock *lock)
  63 {
  64         /* Put the writer into the wait queue */
  65         arch_spin_lock(&lock->wait_lock);
  66
  67         /* Try to acquire the lock directly if no reader is present */
  68         if (!atomic_read(&lock->cnts) &&
  69             (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
  70                 goto unlock;
  71
  72         /* Set the waiting flag to notify readers that a writer is pending */
  73         atomic_add(_QW_WAITING, &lock->cnts);
  74
  75         /* When no more readers or writers, set the locked flag */
  76         do {
  77                 atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
  78         } while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
  79                                         _QW_LOCKED) != _QW_WAITING);
  80 unlock:
  81         arch_spin_unlock(&lock->wait_lock);
  82 }
  83 EXPORT_SYMBOL(queued_write_lock_slowpath);