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| 2 | |
| 3 | * Resilient Queued Spin Lock defines
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| 4 | *
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| 5 | * (C) Copyright 2024-2025 Meta Platforms, Inc. and affiliates.
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| 6 | *
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| 7 | * Authors: Kumar Kartikeya Dwivedi <[email protected]>
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| 8 |
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| 9 | #ifndef __LINUX_RQSPINLOCK_H
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| 10 | #define __LINUX_RQSPINLOCK_H
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| 11 |
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| 12 | #include "../locking/qspinlock.h"
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| 13 |
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| 14 | |
| 15 | * try_cmpxchg_tail - Return result of cmpxchg of tail word with a new value
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| 16 | * @lock: Pointer to queued spinlock structure
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| 17 | * @tail: The tail to compare against
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| 18 | * @new_tail: The new queue tail code word
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| 19 | * Return: Bool to indicate whether the cmpxchg operation succeeded
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| 20 | *
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| 21 | * This is used by the head of the wait queue to clean up the queue.
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| 22 | * Provides relaxed ordering, since observers only rely on initialized
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| 23 | * state of the node which was made visible through the xchg_tail operation,
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| 24 | * i.e. through the smp_wmb preceding xchg_tail.
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| 25 | *
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| 26 | * We avoid using 16-bit cmpxchg, which is not available on all architectures.
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| 27 |
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| 28 | static __always_inline bool try_cmpxchg_tail(struct qspinlock *lock, u32 tail, u32 new_tail)
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| 29 | {
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| 30 | u32 old, new;
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| 31 |
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| 32 | old = atomic_read(&lock->val);
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| 33 | do {
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| 34 | |
| 35 | * Is the tail part we compare to already stale? Fail.
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| 36 |
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| 37 | if ((old & _Q_TAIL_MASK) != tail)
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| 38 | return false;
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| 39 | |
| 40 | * Encode latest locked/pending state for new tail.
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| 41 |
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| 42 | new = (old & _Q_LOCKED_PENDING_MASK) | new_tail;
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| 43 | } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
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| 44 |
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| 45 | return true;
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| 46 | }
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| 47 |
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| 48 | #endif
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