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Linux/arch/x86/include/asm/spinlock.h

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  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 */
229 

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