Re: [PATCH v5 bpf-next 1/9] bpf: introduce bpf_spin_lock

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Jan 30, 2019 at 10:05:29PM +0100, Peter Zijlstra wrote:
> 
> Would something like the below work for you instead?
> 
> I find it easier to read, and the additional CONFIG symbol would give
> architectures (say ARM) an easy way to force the issue.
> 
> 
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -221,6 +221,72 @@ const struct bpf_func_proto bpf_get_curr
>  	.arg2_type	= ARG_CONST_SIZE,
>  };
>  
> +#if defined(CONFIG_QUEUED_SPINLOCKS) || defined(CONFIG_BPF_ARCH_SPINLOCK)
> +
> +static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
> +{
> +	arch_spinlock_t *l = (void *)lock;
> +	BUILD_BUG_ON(sizeof(*l) != sizeof(__u32));
> +	if (1) {
> +		union {
> +			__u32 val;
> +			arch_spinlock_t lock;
> +		} u = { .lock = __ARCH_SPIN_LOCK_UNLOCKED };
> +		compiletime_assert(u.val == 0, "__ARCH_SPIN_LOCK_UNLOCKED not 0");
> +	}
> +	arch_spin_lock(l);

And archs can select CONFIG_BPF_ARCH_SPINLOCK when they don't
use qspinlock and their arch_spinlock_t is compatible ?
Nice. I like the idea!
Probably needs a kconfig change somewhere too...
I'll play with it...

> +}
> +
> +static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
> +{
> +	arch_spinlock_t *l = (void *)lock;
> +	arch_spin_unlock(l);
> +}
> +
> +#else
> +
> +static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
> +{
> +	atomic_t *l = (void *)lock;
> +	do {
> +		atomic_cond_read_relaxed(l, !VAL);

wow. that's quite a macro magic.
Should it be
atomic_cond_read_relaxed(l, (!VAL));
like qspinlock.c does ?