Line data    Source code

       1             : /* SPDX-License-Identifier: GPL-2.0-or-later */
       2             : /*
       3             :  * Queued spinlock
       4             :  *
       5             :  * A 'generic' spinlock implementation that is based on MCS locks. For an
       6             :  * architecture that's looking for a 'generic' spinlock, please first consider
       7             :  * ticket-lock.h and only come looking here when you've considered all the
       8             :  * constraints below and can show your hardware does actually perform better
       9             :  * with qspinlock.
      10             :  *
      11             :  * qspinlock relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no
      12             :  * weaker than RCtso if you're power), where regular code only expects atomic_t
      13             :  * to be RCpc.
      14             :  *
      15             :  * qspinlock relies on a far greater (compared to asm-generic/spinlock.h) set
      16             :  * of atomic operations to behave well together, please audit them carefully to
      17             :  * ensure they all have forward progress. Many atomic operations may default to
      18             :  * cmpxchg() loops which will not have good forward progress properties on
      19             :  * LL/SC architectures.
      20             :  *
      21             :  * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply)
      22             :  * do. Carefully read the patches that introduced
      23             :  * queued_fetch_set_pending_acquire().
      24             :  *
      25             :  * qspinlock also heavily relies on mixed size atomic operations, in specific
      26             :  * it requires architectures to have xchg16; something which many LL/SC
      27             :  * architectures need to implement as a 32bit and+or in order to satisfy the
      28             :  * forward progress guarantees mentioned above.
      29             :  *
      30             :  * Further reading on mixed size atomics that might be relevant:
      31             :  *
      32             :  *   http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf
      33             :  *
      34             :  * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
      35             :  * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
      36             :  *
      37             :  * Authors: Waiman Long <waiman.long@hpe.com>
      38             :  */
      39             : #ifndef __ASM_GENERIC_QSPINLOCK_H
      40             : #define __ASM_GENERIC_QSPINLOCK_H
      41             : 
      42             : #include <asm-generic/qspinlock_types.h>
      43             : #include <linux/atomic.h>
      44             : 
      45             : #ifndef queued_spin_is_locked
      46             : /**
      47             :  * queued_spin_is_locked - is the spinlock locked?
      48             :  * @lock: Pointer to queued spinlock structure
      49             :  * Return: 1 if it is locked, 0 otherwise
      50             :  */
      51             : static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
      52             : {
      53             :         /*
      54             :          * Any !0 state indicates it is locked, even if _Q_LOCKED_VAL
      55             :          * isn't immediately observable.
      56             :          */
      57   251984187 :         return atomic_read(&lock->val);
      58             : }
      59             : #endif
      60             : 
      61             : /**
      62             :  * queued_spin_value_unlocked - is the spinlock structure unlocked?
      63             :  * @lock: queued spinlock structure
      64             :  * Return: 1 if it is unlocked, 0 otherwise
      65             :  *
      66             :  * N.B. Whenever there are tasks waiting for the lock, it is considered
      67             :  *      locked wrt the lockref code to avoid lock stealing by the lockref
      68             :  *      code and change things underneath the lock. This also allows some
      69             :  *      optimizations to be applied without conflict with lockref.
      70             :  */
      71             : static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
      72             : {
      73             :         return !atomic_read(&lock.val);
      74             : }
      75             : 
      76             : /**
      77             :  * queued_spin_is_contended - check if the lock is contended
      78             :  * @lock : Pointer to queued spinlock structure
      79             :  * Return: 1 if lock contended, 0 otherwise
      80             :  */
      81             : static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
      82             : {
      83           0 :         return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
      84             : }
      85             : /**
      86             :  * queued_spin_trylock - try to acquire the queued spinlock
      87             :  * @lock : Pointer to queued spinlock structure
      88             :  * Return: 1 if lock acquired, 0 if failed
      89             :  */
      90             : static __always_inline int queued_spin_trylock(struct qspinlock *lock)
      91             : {
      92             :         int val = atomic_read(&lock->val);
      93             : 
      94             :         if (unlikely(val))
      95             :                 return 0;
      96             : 
      97             :         return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
      98             : }
      99             : 
     100             : extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
     101             : 
     102             : #ifndef queued_spin_lock
     103             : /**
     104             :  * queued_spin_lock - acquire a queued spinlock
     105             :  * @lock: Pointer to queued spinlock structure
     106             :  */
     107             : static __always_inline void queued_spin_lock(struct qspinlock *lock)
     108             : {
     109             :         int val = 0;
     110             : 
     111             :         if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
     112             :                 return;
     113             : 
     114             :         queued_spin_lock_slowpath(lock, val);
     115             : }
     116             : #endif
     117             : 
     118             : #ifndef queued_spin_unlock
     119             : /**
     120             :  * queued_spin_unlock - release a queued spinlock
     121             :  * @lock : Pointer to queued spinlock structure
     122             :  */
     123             : static __always_inline void queued_spin_unlock(struct qspinlock *lock)
     124             : {
     125             :         /*
     126             :          * unlock() needs release semantics:
     127             :          */
     128             :         smp_store_release(&lock->locked, 0);
     129             : }
     130             : #endif
     131             : 
     132             : #ifndef virt_spin_lock
     133             : static __always_inline bool virt_spin_lock(struct qspinlock *lock)
     134             : {
     135             :         return false;
     136             : }
     137             : #endif
     138             : 
     139             : /*
     140             :  * Remapping spinlock architecture specific functions to the corresponding
     141             :  * queued spinlock functions.
     142             :  */
     143             : #define arch_spin_is_locked(l)          queued_spin_is_locked(l)
     144             : #define arch_spin_is_contended(l)       queued_spin_is_contended(l)
     145             : #define arch_spin_value_unlocked(l)     queued_spin_value_unlocked(l)
     146             : #define arch_spin_lock(l)               queued_spin_lock(l)
     147             : #define arch_spin_trylock(l)            queued_spin_trylock(l)
     148             : #define arch_spin_unlock(l)             queued_spin_unlock(l)
     149             : 
     150             : #endif /* __ASM_GENERIC_QSPINLOCK_H */