Commit 101acd2e authored by Kumar Kartikeya Dwivedi's avatar Kumar Kartikeya Dwivedi Committed by Alexei Starovoitov
Browse files

rqspinlock: Add macros for rqspinlock usage 



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: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-17-memxor@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent ecbd8047

include/asm-generic/rqspinlock.h

+87 −0
Original line number Diff line number Diff line
@@ -153,4 +153,91 @@ static __always_inline void release_held_lock_entry(void) 
	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 */