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| 1 | +/* SPDX-License-Identifier: GPL-2.0 */ |
| 2 | + |
| 3 | +/* |
| 4 | + * 'Generic' ticket-lock implementation. |
| 5 | + * |
| 6 | + * It relies on atomic_fetch_add() having well defined forward progress |
| 7 | + * guarantees under contention. If your architecture cannot provide this, stick |
| 8 | + * to a test-and-set lock. |
| 9 | + * |
| 10 | + * It also relies on atomic_fetch_add() being safe vs smp_store_release() on a |
| 11 | + * sub-word of the value. This is generally true for anything LL/SC although |
| 12 | + * you'd be hard pressed to find anything useful in architecture specifications |
| 13 | + * about this. If your architecture cannot do this you might be better off with |
| 14 | + * a test-and-set. |
| 15 | + * |
| 16 | + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence |
| 17 | + * uses atomic_fetch_add() which is RCsc to create an RCsc hot path, along with |
| 18 | + * a full fence after the spin to upgrade the otherwise-RCpc |
| 19 | + * atomic_cond_read_acquire(). |
| 20 | + * |
| 21 | + * The implementation uses smp_cond_load_acquire() to spin, so if the |
| 22 | + * architecture has WFE like instructions to sleep instead of poll for word |
| 23 | + * modifications be sure to implement that (see ARM64 for example). |
| 24 | + * |
| 25 | + */ |
| 26 | + |
| 27 | +#ifndef __ASM_GENERIC_TICKET_SPINLOCK_H |
| 28 | +#define __ASM_GENERIC_TICKET_SPINLOCK_H |
| 29 | + |
| 30 | +#include <linux/atomic.h> |
| 31 | +#include <asm-generic/spinlock_types.h> |
| 32 | + |
| 33 | +static __always_inline void ticket_spin_lock(arch_spinlock_t *lock) |
| 34 | +{ |
| 35 | + u32 val = atomic_fetch_add(1<<16, &lock->val); |
| 36 | + u16 ticket = val >> 16; |
| 37 | + |
| 38 | + if (ticket == (u16)val) |
| 39 | + return; |
| 40 | + |
| 41 | + /* |
| 42 | + * atomic_cond_read_acquire() is RCpc, but rather than defining a |
| 43 | + * custom cond_read_rcsc() here we just emit a full fence. We only |
| 44 | + * need the prior reads before subsequent writes ordering from |
| 45 | + * smb_mb(), but as atomic_cond_read_acquire() just emits reads and we |
| 46 | + * have no outstanding writes due to the atomic_fetch_add() the extra |
| 47 | + * orderings are free. |
| 48 | + */ |
| 49 | + atomic_cond_read_acquire(&lock->val, ticket == (u16)VAL); |
| 50 | + smp_mb(); |
| 51 | +} |
| 52 | + |
| 53 | +static __always_inline bool ticket_spin_trylock(arch_spinlock_t *lock) |
| 54 | +{ |
| 55 | + u32 old = atomic_read(&lock->val); |
| 56 | + |
| 57 | + if ((old >> 16) != (old & 0xffff)) |
| 58 | + return false; |
| 59 | + |
| 60 | + return atomic_try_cmpxchg(&lock->val, &old, old + (1<<16)); /* SC, for RCsc */ |
| 61 | +} |
| 62 | + |
| 63 | +static __always_inline void ticket_spin_unlock(arch_spinlock_t *lock) |
| 64 | +{ |
| 65 | + u16 *ptr = (u16 *)lock + IS_ENABLED(CONFIG_CPU_BIG_ENDIAN); |
| 66 | + u32 val = atomic_read(&lock->val); |
| 67 | + |
| 68 | + smp_store_release(ptr, (u16)val + 1); |
| 69 | +} |
| 70 | + |
| 71 | +static __always_inline int ticket_spin_value_unlocked(arch_spinlock_t lock) |
| 72 | +{ |
| 73 | + u32 val = lock.val.counter; |
| 74 | + |
| 75 | + return ((val >> 16) == (val & 0xffff)); |
| 76 | +} |
| 77 | + |
| 78 | +static __always_inline int ticket_spin_is_locked(arch_spinlock_t *lock) |
| 79 | +{ |
| 80 | + arch_spinlock_t val = READ_ONCE(*lock); |
| 81 | + |
| 82 | + return !ticket_spin_value_unlocked(val); |
| 83 | +} |
| 84 | + |
| 85 | +static __always_inline int ticket_spin_is_contended(arch_spinlock_t *lock) |
| 86 | +{ |
| 87 | + u32 val = atomic_read(&lock->val); |
| 88 | + |
| 89 | + return (s16)((val >> 16) - (val & 0xffff)) > 1; |
| 90 | +} |
| 91 | + |
| 92 | +/* |
| 93 | + * Remapping spinlock architecture specific functions to the corresponding |
| 94 | + * ticket spinlock functions. |
| 95 | + */ |
| 96 | +#define arch_spin_is_locked(l) ticket_spin_is_locked(l) |
| 97 | +#define arch_spin_is_contended(l) ticket_spin_is_contended(l) |
| 98 | +#define arch_spin_value_unlocked(l) ticket_spin_value_unlocked(l) |
| 99 | +#define arch_spin_lock(l) ticket_spin_lock(l) |
| 100 | +#define arch_spin_trylock(l) ticket_spin_trylock(l) |
| 101 | +#define arch_spin_unlock(l) ticket_spin_unlock(l) |
| 102 | + |
| 103 | +#endif /* __ASM_GENERIC_TICKET_SPINLOCK_H */ |
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