netdev - Re: [PATCH v2 bpf-next 3/4] bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW

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Message-ID: <20201116183749.6aaknb5ptvzlp7ss@kafai-mbp.dhcp.thefacebook.com>
Date:   Mon, 16 Nov 2020 10:37:49 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        <netdev@...r.kernel.org>, Song Liu <songliubraving@...com>
Subject: Re: [PATCH v2 bpf-next 3/4] bpf: Allow using bpf_sk_storage in
 FENTRY/FEXIT/RAW_TP

On Mon, Nov 16, 2020 at 10:00:04AM -0800, Jakub Kicinski wrote:
> On Mon, 16 Nov 2020 09:37:34 -0800 Martin KaFai Lau wrote:
> > On Sat, Nov 14, 2020 at 05:17:20PM -0800, Jakub Kicinski wrote:
> > > On Thu, 12 Nov 2020 13:13:13 -0800 Martin KaFai Lau wrote:  
> > > > This patch adds bpf_sk_storage_get_tracing_proto and
> > > > bpf_sk_storage_delete_tracing_proto.  They will check
> > > > in runtime that the helpers can only be called when serving
> > > > softirq or running in a task context.  That should enable
> > > > most common tracing use cases on sk.  
> > >   
> > > > +	if (!in_serving_softirq() && !in_task())  
> > > 
> > > This is a curious combination of checks. Would you mind indulging me
> > > with an explanation?  
> > The current lock usage in bpf_local_storage.c is only expected to
> > run in either of these contexts.
> 
> :)
> 
> Locks that can run in any context but preempt disabled or softirq
> disabled?
Not exactly. e.g. running from irq won't work.

> 
> Let me cut to the chase. Are you sure you didn't mean to check
> if (irq_count()) ?
so, no.

>From preempt.h:

/*
 * ...
 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
 * ...
 */
#define in_interrupt()          (irq_count())