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(¶virt_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 (__ticket_t)(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 */ 224