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