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rqspinlock: Add macros for rqspinlock usage

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Introduce helper macros that wrap around the rqspinlock slow path and
provide an interface analogous to the raw_spin_lock API. Note that
in case of error conditions, preemption and IRQ disabling is
automatically unrolled before returning the error back to the caller.

Ensure that in absence of CONFIG_QUEUED_SPINLOCKS support, we fallback
to the test-and-set implementation.

Add some comments describing the subtle memory ordering logic during
unlock, and why it's safe.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-17-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Kumar Kartikeya Dwivedi
2025-03-15 21:05:32 -07:00
committed by Alexei Starovoitov
parent ecbd804752
commit 101acd2e78
1 changed files with 87 additions and 0 deletions
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87
include/asm-generic/rqspinlock.h
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@@ -153,4 +153,91 @@ dec:
this_cpu_dec(rqspinlock_held_locks.cnt);
}
#ifdef CONFIG_QUEUED_SPINLOCKS
/**
* res_spin_lock - acquire a queued spinlock
* @lock: Pointer to queued spinlock structure
*
* Return:
* * 0 - Lock was acquired successfully.
* * -EDEADLK - Lock acquisition failed because of AA/ABBA deadlock.
* * -ETIMEDOUT - Lock acquisition failed because of timeout.
*/
static __always_inline int res_spin_lock(rqspinlock_t *lock)
{
int val = 0;
if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL))) {
grab_held_lock_entry(lock);
return 0;
}
return resilient_queued_spin_lock_slowpath(lock, val);
}
#else
#define res_spin_lock(lock) resilient_tas_spin_lock(lock)
#endif /* CONFIG_QUEUED_SPINLOCKS */
static __always_inline void res_spin_unlock(rqspinlock_t *lock)
{
struct rqspinlock_held *rqh = this_cpu_ptr(&rqspinlock_held_locks);
if (unlikely(rqh->cnt > RES_NR_HELD))
goto unlock;
WRITE_ONCE(rqh->locks[rqh->cnt - 1], NULL);
unlock:
/*
* Release barrier, ensures correct ordering. See release_held_lock_entry
* for details. Perform release store instead of queued_spin_unlock,
* since we use this function for test-and-set fallback as well. When we
* have CONFIG_QUEUED_SPINLOCKS=n, we clear the full 4-byte lockword.
*
* Like release_held_lock_entry, we can do the release before the dec.
* We simply care about not seeing the 'lock' in our table from a remote
* CPU once the lock has been released, which doesn't rely on the dec.
*
* Unlike smp_wmb(), release is not a two way fence, hence it is
* possible for a inc to move up and reorder with our clearing of the
* entry. This isn't a problem however, as for a misdiagnosis of ABBA,
* the remote CPU needs to hold this lock, which won't be released until
* the store below is done, which would ensure the entry is overwritten
* to NULL, etc.
*/
smp_store_release(&lock->locked, 0);
this_cpu_dec(rqspinlock_held_locks.cnt);
}
#ifdef CONFIG_QUEUED_SPINLOCKS
#define raw_res_spin_lock_init(lock) ({ *(lock) = (rqspinlock_t)__ARCH_SPIN_LOCK_UNLOCKED; })
#else
#define raw_res_spin_lock_init(lock) ({ *(lock) = (rqspinlock_t){0}; })
#endif
#define raw_res_spin_lock(lock) \
({ \
int __ret; \
preempt_disable(); \
__ret = res_spin_lock(lock); \
if (__ret) \
preempt_enable(); \
__ret; \
})
#define raw_res_spin_unlock(lock) ({ res_spin_unlock(lock); preempt_enable(); })
#define raw_res_spin_lock_irqsave(lock, flags) \
({ \
int __ret; \
local_irq_save(flags); \
__ret = raw_res_spin_lock(lock); \
if (__ret) \
local_irq_restore(flags); \
__ret; \
})
#define raw_res_spin_unlock_irqrestore(lock, flags) ({ raw_res_spin_unlock(lock); local_irq_restore(flags); })
#endif /* __ASM_GENERIC_RQSPINLOCK_H */