From: Steven Rostedt <rostedt@goodmis.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Peter Zijlstra <peterz@infradead.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Ingo Molnar <mingo@kernel.org>, Robert Richter <rric@kernel.org>,
Gabriel Krisman Bertazi <krisman@collabora.com>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
netdev <netdev@vger.kernel.org>,
bpf@vger.kernel.org
Subject: Re: [PATCH] tracepoint: Do not warn on EEXIST or ENOENT
Date: Sat, 26 Jun 2021 22:52:33 -0400 [thread overview]
Message-ID: <20210626225233.2baae8be@rorschach.local.home> (raw)
In-Reply-To: <fc5d0f90-502d-b217-0ad6-0d17cae12ff7@i-love.sakura.ne.jp>
On Sun, 27 Jun 2021 10:10:24 +0900
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:
> On 2021/06/27 3:22, Steven Rostedt wrote:
> >> If BPF is expected to register the same tracepoint with the same
> >> callback and data more than once, then let's add a call to do that
> >> without warning. Like I said, other callers expect the call to succeed
> >> unless it's out of memory, which tends to cause other problems.
> >
> > If BPF is OK with registering the same probe more than once if user
> > space expects it, we can add this patch, which allows the caller (in
> > this case BPF) to not warn if the probe being registered is already
> > registered, and keeps the idea that a probe registered twice is a bug
> > for all other use cases.
>
> I think BPF will not register the same tracepoint with the same callback and
> data more than once, for bpf(BPF_RAW_TRACEPOINT_OPEN) cleans the request up
> by calling bpf_link_cleanup() and returns -EEXIST. But I think BPF relies on
> tracepoint_add_func() returning -EEXIST without crashing the kernel.
Which is the only user that does so, and what this patch addresses.
> > That's because (before BPF) there's no place in the kernel that tries
> > to register the same tracepoint multiple times, and was considered a
> > bug if it happened, because there's no ref counters to deal with adding
> > them multiple times.
>
> I see. But does that make sense? Since func_add() can fail with -ENOMEM,
> all places (even before BPF) needs to be prepared for failures.
Yes. -ENOMEM means that there's no resources to create a tracepoint.
But if the tracepoint already exsits, that means the accounting for
what tracepoints are running has been corrupted.
>
> >
> > If the tracepoint is already registered (with the given function and
> > data), then something likely went wrong.
>
> That can be prepared on the caller side of tracepoint_add_func() rather than
> tracepoint_add_func() side.
Not sure what you mean by that.
>
> >
> >> (3) And tracepoint_add_func() is triggerable via request from userspace.
> >
> > Only via BPF correct?
> >
> > I'm not sure how it works, but can't BPF catch that it is registering
> > the same tracepoint again?
>
> There is no chance to check whether some tracepoint is already registered, for
> tracepoints_mutex is the only lock which gives us a chance to check whether
> some tracepoint is already registered.
>
> Should bpf() syscall hold a global lock (like tracepoints_mutex) which will serialize
> the entire code in order to check whether some tracepoint is already registered?
> That might severely damage concurrency.
I think that the patch I posted handles what you want. For BPF it
returns without warning, but for all other cases, it warns. Does it fix
your issue?
-- Steve
prev parent reply other threads:[~2021-06-27 2:52 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-26 13:58 [PATCH] tracepoint: Do not warn on EEXIST or ENOENT Tetsuo Handa
2021-06-26 14:18 ` Steven Rostedt
2021-06-26 15:13 ` Tetsuo Handa
2021-06-26 15:17 ` Tetsuo Handa
2021-06-26 15:41 ` Steven Rostedt
2021-06-26 18:22 ` Steven Rostedt
2021-06-26 18:42 ` Mathieu Desnoyers
2021-06-26 23:35 ` Steven Rostedt
2021-06-27 1:10 ` Tetsuo Handa
2021-06-27 2:52 ` Steven Rostedt [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210626225233.2baae8be@rorschach.local.home \
--to=rostedt@goodmis.org \
--cc=andrii@kernel.org \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=gustavoars@kernel.org \
--cc=krisman@collabora.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mingo@kernel.org \
--cc=netdev@vger.kernel.org \
--cc=penguin-kernel@i-love.sakura.ne.jp \
--cc=peterz@infradead.org \
--cc=rric@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).