linux-kernel - Re: [PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of find_get

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Date:   Thu, 21 Jul 2022 21:58:00 +0100
From:   Lee Jones <lee@...nel.org>
To:     Yonghong Song <yhs@...com>
Cc:     Jiri Olsa <olsajiri@...il.com>, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, bpf@...r.kernel.org
Subject: Re: [PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of
 find_get_pid()

On Thu, 21 Jul 2022, Yonghong Song wrote:

> 
> 
> On 7/21/22 5:14 AM, Jiri Olsa wrote:
> > On Thu, Jul 21, 2022 at 12:59:09PM +0100, Lee Jones wrote:
> > > On Thu, 21 Jul 2022, Jiri Olsa wrote:
> > > 
> > > > On Thu, Jul 21, 2022 at 12:14:30PM +0100, Lee Jones wrote:
> > > > > The documentation for find_pid() clearly states:
> > 
> > typo find_vpid
> > 
> > > > > 
> > > > >    "Must be called with the tasklist_lock or rcu_read_lock() held."
> > > > > 
> > > > > Presently we do neither.
> > 
> > just curious, did you see crash related to this or you just spot that
> > 
> > > > > 
> > > > > In an ideal world we would wrap the in-lined call to find_vpid() along
> > > > > with get_pid_task() in the suggested rcu_read_lock() and have done.
> > > > > However, looking at get_pid_task()'s internals, it already does that
> > > > > independently, so this would lead to deadlock.
> > > > 
> > > > hm, we can have nested rcu_read_lock calls, right?
> > > 
> > > I assumed not, but that might be an oversight on my part.
> 
> From kernel documentation, nested rcu_read_lock is allowed.
> https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html
> 
> RCU's grace-period guarantee allows updaters to wait for the completion of
> all pre-existing RCU read-side critical sections. An RCU read-side critical
> section begins with the marker rcu_read_lock() and ends with the marker
> rcu_read_unlock(). These markers may be nested, and RCU treats a nested set
> as one big RCU read-side critical section. Production-quality
> implementations of rcu_read_lock() and rcu_read_unlock() are extremely
> lightweight, and in fact have exactly zero overhead in Linux kernels built
> for production use with CONFIG_PREEMPT=n.
> 
> > > 
> > > Would that be your preference?
> > 
> > seems simpler than calling get/put for ppid
> 
> The current implementation seems okay since we can hide
> rcu_read_lock() inside find_get_pid(). We can also avoid
> nested rcu_read_lock(), which is although allowed but
> not pretty.

Right, this was my thinking.

Happy to go with whatever you guys decide though.

Make the call and I'll rework, or not.

-- 
Lee Jones [李琼斯]