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_QUEUED_SPINLOCKS
  46 #include <asm/qspinlock.h>
  47 #else
  48
  49 #ifdef CONFIG_PARAVIRT_SPINLOCKS
  50
  51 static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
  52 {
  53         set_bit(0, (volatile unsigned long *)&lock->tickets.head);
  54 }
  55
  56 #else  /* !CONFIG_PARAVIRT_SPINLOCKS */
  57 static __always_inline void __ticket_lock_spinning(arch_spinlock_t *lock,
  58                                                         __ticket_t ticket)
  59 {
  60 }
  61 static inline void __ticket_unlock_kick(arch_spinlock_t *lock,
  62                                                         __ticket_t ticket)
  63 {
  64 }
  65
  66 #endif /* CONFIG_PARAVIRT_SPINLOCKS */
  67 static inline int  __tickets_equal(__ticket_t one, __ticket_t two)
  68 {
  69         return !((one ^ two) & ~TICKET_SLOWPATH_FLAG);
  70 }
  71
  72 static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
  73                                                         __ticket_t head)
  74 {
  75         if (head & TICKET_SLOWPATH_FLAG) {
  76                 arch_spinlock_t old, new;
  77
  78                 old.tickets.head = head;
  79                 new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
  80                 old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
  81                 new.tickets.tail = old.tickets.tail;
  82
  83                 /* try to clear slowpath flag when there are no contenders */
  84                 cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
  85         }
  86 }
  87
  88 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
  89 {
  90         return __tickets_equal(lock.tickets.head, lock.tickets.tail);
  91 }
  92
  93 /*
  94  * Ticket locks are conceptually two parts, one indicating the current head of
  95  * the queue, and the other indicating the current tail. The lock is acquired
  96  * by atomically noting the tail and incrementing it by one (thus adding
  97  * ourself to the queue and noting our position), then waiting until the head
  98  * becomes equal to the the initial value of the tail.
  99  *
 100  * We use an xadd covering *both* parts of the lock, to increment the tail and
 101  * also load the position of the head, which takes care of memory ordering
 102  * issues and should be optimal for the uncontended case. Note the tail must be
 103  * in the high part, because a wide xadd increment of the low part would carry
 104  * up and contaminate the high part.
 105  */
 106 static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
 107 {
 108         register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
 109
 110         inc = xadd(&lock->tickets, inc);
 111         if (likely(inc.head == inc.tail))
 112                 goto out;
 113
 114         for (;;) {
 115                 unsigned count = SPIN_THRESHOLD;
 116
 117                 do {
 118                         inc.head = READ_ONCE(lock->tickets.head);
 119                         if (__tickets_equal(inc.head, inc.tail))
 120                                 goto clear_slowpath;
 121                         cpu_relax();
 122                 } while (--count);
 123                 __ticket_lock_spinning(lock, inc.tail);
 124         }
 125 clear_slowpath:
 126         __ticket_check_and_clear_slowpath(lock, inc.head);
 127 out:
 128         barrier();      /* make sure nothing creeps before the lock is taken */
 129 }
 130
 131 static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
 132 {
 133         arch_spinlock_t old, new;
 134
 135         old.tickets = READ_ONCE(lock->tickets);
 136         if (!__tickets_equal(old.tickets.head, old.tickets.tail))
 137                 return 0;
 138
 139         new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
 140         new.head_tail &= ~TICKET_SLOWPATH_FLAG;
 141
 142         /* cmpxchg is a full barrier, so nothing can move before it */
 143         return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;
 144 }
 145
 146 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 147 {
 148         if (TICKET_SLOWPATH_FLAG &&
 149                 static_key_false(&paravirt_ticketlocks_enabled)) {
 150                 __ticket_t head;
 151
 152                 BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
 153
 154                 head = xadd(&lock->tickets.head, TICKET_LOCK_INC);
 155
 156                 if (unlikely(head & TICKET_SLOWPATH_FLAG)) {
 157                         head &= ~TICKET_SLOWPATH_FLAG;
 158                         __ticket_unlock_kick(lock, (head + TICKET_LOCK_INC));
 159                 }
 160         } else
 161                 __add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
 162 }
 163
 164 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 165 {
 166         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 167
 168         return !__tickets_equal(tmp.tail, tmp.head);
 169 }
 170
 171 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 172 {
 173         struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 174
 175         tmp.head &= ~TICKET_SLOWPATH_FLAG;
 176         return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
 177 }
 178 #define arch_spin_is_contended  arch_spin_is_contended
 179
 180 static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
 181                                                   unsigned long flags)
 182 {
 183         arch_spin_lock(lock);
 184 }
 185
 186 static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
 187 {
 188         __ticket_t head = READ_ONCE(lock->tickets.head);
 189
 190         for (;;) {
 191                 struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 192                 /*
 193                  * We need to check "unlocked" in a loop, tmp.head == head
 194                  * can be false positive because of overflow.
 195                  */
 196                 if (__tickets_equal(tmp.head, tmp.tail) ||
 197                                 !__tickets_equal(tmp.head, head))
 198                         break;
 199
 200                 cpu_relax();
 201         }
 202 }
 203 #endif /* CONFIG_QUEUED_SPINLOCKS */
 204
 205 /*
 206  * Read-write spinlocks, allowing multiple readers
 207  * but only one writer.
 208  *
 209  * NOTE! it is quite common to have readers in interrupts
 210  * but no interrupt writers. For those circumstances we
 211  * can "mix" irq-safe locks - any writer needs to get a
 212  * irq-safe write-lock, but readers can get non-irqsafe
 213  * read-locks.
 214  *
 215  * On x86, we implement read-write locks using the generic qrwlock with
 216  * x86 specific optimization.
 217  */
 218
 219 #include <asm/qrwlock.h>
 220
 221 #define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
 222 #define arch_write_lock_flags(lock, flags) arch_write_lock(lock)
 223
 224 #define arch_spin_relax(lock)   cpu_relax()
 225 #define arch_read_relax(lock)   cpu_relax()
 226 #define arch_write_relax(lock)  cpu_relax()
 227
 228 #endif /* _ASM_X86_SPINLOCK_H */