netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Message-ID: <20190124180109.GA27771@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Jan 2019 19:01:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     davem@...emloft.net, daniel@...earbox.net,
        jakub.kicinski@...ronome.com, netdev@...r.kernel.org,
        kernel-team@...com, mingo@...hat.com, will.deacon@....com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>, jannh@...gle.com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock


Thanks for having kernel/locking people on Cc...

On Wed, Jan 23, 2019 at 08:13:55PM -0800, Alexei Starovoitov wrote:

> Implementation details:
> - on !SMP bpf_spin_lock() becomes nop

Because no BPF program is preemptible? I don't see any assertions or
even a comment that says this code is non-preemptible.

AFAICT some of the BPF_RUN_PROG things are under rcu_read_lock() only,
which is not sufficient.

> - on architectures that don't support queued_spin_lock trivial lock is used.
>   Note that arch_spin_lock cannot be used, since not all archs agree that
>   zero == unlocked and sizeof(arch_spinlock_t) != sizeof(__u32).

I really don't much like direct usage of qspinlock; esp. not as a
surprise.

Why does it matter if 0 means unlocked; that's what
__ARCH_SPIN_LOCK_UNLOCKED is for.

I get the sizeof(__u32) thing, but why not key off of that?

> Next steps:
> - allow bpf_spin_lock in other map types (like cgroup local storage)
> - introduce BPF_F_LOCK flag for bpf_map_update() syscall and helper
>   to request kernel to grab bpf_spin_lock before rewriting the value.
>   That will serialize access to map elements.

So clearly this map stuff is shared between bpf proglets, otherwise
there would not be a need for locking. But what happens if one is from
task context and another from IRQ context?

I don't see a local_irq_save()/restore() anywhere. What avoids the
trivial lock inversion?


> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index a74972b07e74..2e98e4caf5aa 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(&qlock->val, 0);

And this is broken... That should've been atomic_set_release() at the
very least.

And this would again be the moment where I go pester you about the BPF
memory model :-)

> +#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,
> +};