linux-kernel - Re: [PATCH bpf-next v1 14/22] rqspinlock: Add macros for rqspinlock usage

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Message-ID: <62c08854-04cb-4e45-a9e1-e6200cb787fd@redhat.com>
Date: Wed, 8 Jan 2025 11:55:59 -0500
From: Waiman Long <llong@...hat.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Waiman Long <llong@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...nel.org>,
 Eduard Zingerman <eddyz87@...il.com>, "Paul E. McKenney"
 <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>,
 Barret Rhoden <brho@...gle.com>, Josh Don <joshdon@...gle.com>,
 Dohyun Kim <dohyunkim@...gle.com>, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v1 14/22] rqspinlock: Add macros for rqspinlock
 usage

On 1/7/25 8:59 AM, Kumar Kartikeya Dwivedi wrote:
> 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.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>   include/asm-generic/rqspinlock.h | 58 ++++++++++++++++++++++++++++++++
>   1 file changed, 58 insertions(+)
>
> diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h
> index dc436ab01471..53be8426373c 100644
> --- a/include/asm-generic/rqspinlock.h
> +++ b/include/asm-generic/rqspinlock.h
> @@ -12,8 +12,10 @@
>   #include <linux/types.h>
>   #include <vdso/time64.h>
>   #include <linux/percpu.h>
> +#include <asm/qspinlock.h>
>   
>   struct qspinlock;
> +typedef struct qspinlock rqspinlock_t;
>   
>   extern int resilient_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val, u64 timeout);
>   
> @@ -82,4 +84,60 @@ static __always_inline void release_held_lock_entry(void)
>   	this_cpu_dec(rqspinlock_held_locks.cnt);
>   }
>   
> +/**
> + * res_spin_lock - acquire a queued spinlock
> + * @lock: Pointer to queued spinlock structure
> + */
> +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, RES_DEF_TIMEOUT);
> +}
> +
> +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);
> +	/*
> +	 * Release barrier, ensuring ordering. See release_held_lock_entry.
> +	 */
> +unlock:
> +	queued_spin_unlock(lock);
> +	this_cpu_dec(rqspinlock_held_locks.cnt);
> +}
> +
> +#define raw_res_spin_lock_init(lock) ({ *(lock) = (struct qspinlock)__ARCH_SPIN_LOCK_UNLOCKED; })
> +
> +#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 */

Lockdep calls aren't included in the helper functions. That means all 
the *res_spin_lock*() calls will be outside the purview of lockdep. That 
also means a multi-CPU circular locking dependency involving a mixture 
of qspinlocks and rqspinlocks may not be detectable.

Cheers,
Longman