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

From: Peter Zijlstra <[email-masked]>
Date: Wed, 30 Jan 2019 22:05:29 +0100

On Sun, Jan 27, 2019 at 06:50:02PM -0800, Alexei Starovoitov wrote:
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index a74972b07e74..e1d6aefbab50 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -221,6 +221,63 @@ const struct bpf_func_proto bpf_get_current_comm_proto = {
>  	.arg2_type	= ARG_CONST_SIZE,
>  };
>  
> +#ifndef CONFIG_QUEUED_SPINLOCKS
> +struct dumb_spin_lock {
> +	atomic_t val;
> +};
> +#endif
> +
> +notrace BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
> +{
> +#if defined(CONFIG_SMP)
> +#ifdef CONFIG_QUEUED_SPINLOCKS
> +	struct qspinlock *qlock = (void *)lock;
> +
> +	BUILD_BUG_ON(sizeof(*qlock) != sizeof(*lock));
> +	queued_spin_lock(qlock);
> +#else
> +	struct dumb_spin_lock *qlock = (void *)lock;
> +
> +	BUILD_BUG_ON(sizeof(*qlock) != sizeof(*lock));
> +	do {
> +		while (atomic_read(&qlock->val) != 0)
> +			cpu_relax();
> +	} while (atomic_cmpxchg(&qlock->val, 0, 1) != 0);
> +#endif
> +#endif
> +	return 0;
> +}
> +
> +const struct bpf_func_proto bpf_spin_lock_proto = {
> +	.func		= bpf_spin_lock,
> +	.gpl_only	= false,
> +	.ret_type	= RET_VOID,
> +	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
> +};
> +
> +notrace BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
> +{
> +#if defined(CONFIG_SMP)
> +#ifdef CONFIG_QUEUED_SPINLOCKS
> +	struct qspinlock *qlock = (void *)lock;
> +
> +	queued_spin_unlock(qlock);
> +#else
> +	struct dumb_spin_lock *qlock = (void *)lock;
> +
> +	atomic_set_release(&qlock->val, 0);
> +#endif
> +#endif
> +	return 0;
> +}
> +
> +const struct bpf_func_proto bpf_spin_unlock_proto = {
> +	.func		= bpf_spin_unlock,
> +	.gpl_only	= false,
> +	.ret_type	= RET_VOID,
> +	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
> +};
> +
>  #ifdef CONFIG_CGROUPS
>  BPF_CALL_0(bpf_get_current_cgroup_id)
>  {

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);
+}
+
+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);
+	} while (atomic_xchg(l, 1));
+}
+
+static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
+{
+	atomic_t *l = (void *)lock;
+	atomic_set_release(l, 0);
+}
+
+#endif
+
+notrace BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
+{
+	__bpf_spin_lock(lock);
+	return 0;
+}
+
+const struct bpf_func_proto bpf_spin_lock_proto = {
+	.func		= bpf_spin_lock,
+	.gpl_only	= false,
+	.ret_type	= RET_VOID,
+	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
+};
+
+notrace BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
+{
+	__bpf_spin_unlock(lock);
+	return 0;
+}
+
+const struct bpf_func_proto bpf_spin_unlock_proto = {
+	.func		= bpf_spin_unlock,
+	.gpl_only	= false,
+	.ret_type	= RET_VOID,
+	.arg1_type	= ARG_PTR_TO_SPIN_LOCK,
+};
+
 #ifdef CONFIG_CGROUPS
 BPF_CALL_0(bpf_get_current_cgroup_id)
 {