arch/x86/include/asm/spinlock.h

   1 #ifndef _ASM_X86_SPINLOCK_H
   2 #define _ASM_X86_SPINLOCK_H
   3
   4 #include <linux/jump_label.h>
   5 #include <linux/atomic.h>
   6 #include <asm/page.h>
   7 #include <asm/processor.h>
   8 #include <linux/compiler.h>
   9 #include <asm/paravirt.h>
  10 #include <asm/bitops.h>
  11
  12 /*
  13  * Your basic SMP spinlocks, allowing only a single CPU anywhere
  14  *
  15  * Simple spin lock operations.  There are two variants, one clears IRQ's
  16  * on the local processor, one does not.
  17  *
  18  * These are fair FIFO ticket locks, which support up to 2^16 CPUs.
  19  *
  20  * (the type definitions are in asm/spinlock_types.h)
  21  */
  22
  23 #ifdef CONFIG_X86_32
  24 # define LOCK_PTR_REG "a"
  25 #else
  26 # define LOCK_PTR_REG "D"
  27 #endif
  28
  29 #if defined(CONFIG_X86_32) && (defined(CONFIG_X86_PPRO_FENCE))
  30 /*
  31  * On PPro SMP, we use a locked operation to unlock
  32  * (PPro errata 66, 92)
  33  */
  34 # define UNLOCK_LOCK_PREFIX LOCK_PREFIX
  35 #else
  36 # define UNLOCK_LOCK_PREFIX
  37 #endif
  38
  39 /* How long a lock should spin before we consider blocking */
  40 #define SPIN_THRESHOLD  (1 << 15)
  41
  42 extern struct static_key paravirt_ticketlocks_enabled;
  43 static __always_inline bool static_key_false(struct static_key *key);
  44
  45 #ifdef CONFIG_PARAVIRT_SPINLOCKS
  46
  47 static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
  48 {
  49         set_bit(0, (volatile unsigned long *)&lock->tickets.head);
  50 }
  51
  52 #else  /* !CONFIG_PARAVIRT_SPINLOCKS */
  53 static __always_inline void __ticket_lock_spinning(arch_spinlock_t *lock,
  54                                                         __ticket_t ticket)
  55 {
  56 }
  57 static inline void __ticket_unlock_kick(arch_spinlock_t *lock,
  58                                                         __ticket_t ticket)
  59 {
  60 }
  61
  62 #endif /* CONFIG_PARAVIRT_SPINLOCKS */
  63 static inline int  __tickets_equal(__ticket_t one, __ticket_t two)
  64 {
  65         return !((one ^ two) & ~TICKET_SLOWPATH_FLAG);
  66 }
  67
  68 static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
  69                                                         __ticket_t head)
  70 {
  71         if (head & TICKET_SLOWPATH_FLAG) {
  72                 arch_spinlock_t old, new;
  73
  74                 old.tickets.head = head;
  75                 new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
  76                 old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
  77                 new.tickets.tail = old.tickets.tail;
  78
  79                 /* try to clear slowpath flag when there are no contenders */
  80                 cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
  81         }
  82 }
  83
  84 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
  85 {
  86         return __tickets_equal(lock.tickets.head, lock.tickets.tail);
  87 }
  88
  89 /*
  90  * Ticket locks are conceptually two parts, one indicating the current head of
  91  * the queue, and the other indicating the current tail. The lock is acquired
  92  * by atomically noting the tail and incrementing it by one (thus adding
  93  * ourself to the queue and noting our position), then waiting until the head
  94  * becomes equal to the the initial value of the tail.
  95  *
  96  * We use an xadd covering *both* parts of the lock, to increment the tail and
  97  * also load the position of the head, which takes care of memory ordering
  98  * issues and should be optimal for the uncontended case. Note the tail must be
  99  * in the high part, because a wide xadd increment of the low part would carry
 100  * up and contaminate the high part.
 101  */
 102 static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
 103 {
 104         register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
 105
 106         inc = xadd(&lock->tickets, inc);
 107         if (likely(inc.head == inc.tail))
 108                 goto out;
 109
 110         for (;;) {
 111                 unsigned count = SPIN_THRESHOLD;
 112
 113                 do {
 114                         inc.head = READ_ONCE(lock->tickets.head);
 115                         if (__tickets_equal(inc.head, inc.tail))
 116                                 goto clear_slowpath;
 117                         cpu_relax();
 118                 } while (--count);
 119                 __ticket_lock_spinning(lock, inc.tail);
 120         }
 121 clear_slowpath:
 122         __ticket_check_and_clear_slowpath(lock, inc.head);
 123 out:
 124         barrier();      /* make sure nothing creeps before the lock is taken */
 125 }
 126
 127 static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
 128 {
 129         arch_spinlock_t old, new;
 130
 131         old.tickets = READ_ONCE(lock->tickets);
 132         if (!__tickets_equal(old.tickets.head, old.tickets.tail))
 133                 return 0;
 134
 135         new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
 136         new.head_tail &= ~TICKET_SLOWPATH_FLAG;
 137
 138         /* cmpxchg is a full barrier, so nothing can move before it */
 139         return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
 140 }
 141
 142 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 143 {
 144         if (TICKET_SLOWPATH_FLAG &&
 145                 static_key_false(&paravirt_ticketlocks_enabled)) {
 146                 __ticket_t head;
 147
 148                 BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
 149
 150                 head = xadd(&lock->tickets.head, TICKET_LOCK_INC);
 151
 152                 if (unlikely(head & TICKET_SLOWPATH_FLAG)) {
 153                         head &= ~TICKET_SLOWPATH_FLAG;
 154                         __ticket_unlock_kick(lock, (head + TICKET_LOCK_INC));
 155                 }
 156         } else
 157                 __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
 158 }
 159
 160 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 161 {
 162         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 163
 164         return !__tickets_equal(tmp.tail, tmp.head);
 165 }
 166
 167 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 168 {
 169         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 170
 171         tmp.head &= ~TICKET_SLOWPATH_FLAG;
 172         return (tmp.tail - tmp.head) > TICKET_LOCK_INC;
 173 }
 174 #define arch_spin_is_contended  arch_spin_is_contended
 175
 176 static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
 177                                                   unsigned long flags)
 178 {
 179         arch_spin_lock(lock);
 180 }
 181
 182 static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
 183 {
 184         __ticket_t head = READ_ONCE(lock->tickets.head);
 185
 186         for (;;) {
 187                 struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 188                 /*
 189                  * We need to check "unlocked" in a loop, tmp.head == head
 190                  * can be false positive because of overflow.
 191                  */
 192                 if (__tickets_equal(tmp.head, tmp.tail) ||
 193                                 !__tickets_equal(tmp.head, head))
 194                         break;
 195
 196                 cpu_relax();
 197         }
 198 }
 199
 200 /*
 201  * Read-write spinlocks, allowing multiple readers
 202  * but only one writer.
 203  *
 204  * NOTE! it is quite common to have readers in interrupts
 205  * but no interrupt writers. For those circumstances we
 206  * can "mix" irq-safe locks - any writer needs to get a
 207  * irq-safe write-lock, but readers can get non-irqsafe
 208  * read-locks.
 209  *
 210  * On x86, we implement read-write locks using the generic qrwlock with
 211  * x86 specific optimization.
 212  */
 213
 214 #include <asm/qrwlock.h>
 215
 216 #define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
 217 #define arch_write_lock_flags(lock, flags) arch_write_lock(lock)
 218
 219 #define arch_spin_relax(lock)   cpu_relax()
 220 #define arch_read_relax(lock)   cpu_relax()
 221 #define arch_write_relax(lock)  cpu_relax()
 222
 223 #endif /* _ASM_X86_SPINLOCK_H */