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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-trace-kernel@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	David Vernet <void@manifault.com>,
	dthaler@microsoft.com, brauner@kernel.org, hch@infradead.org
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached
Date: Wed, 17 May 2023 13:08:59 -0700	[thread overview]
Message-ID: <CAHk-=wicngggxVpbnrYHjRTwGE0WYscPRM+L2HO2BF8ia1EXgQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wi4w9bPKFFGwLULjJf9hnkL941+c4HbeEVKNzqH04wqDA@mail.gmail.com>

On Wed, May 17, 2023 at 12:36 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> .. this is the patch that I think should go on top of it to fix the
> misleading "safe" and the incorrect RCU walk.
>
> NOTE! This adds that
>
>         lockdep_assert_held(&event_mutex);
>
> to user_event_enabler_update() too.

One more note: I think it would be really good to use different names
for the "links".

We have "mm->link", that is the list of mm's on the 'user_event_mms'
list, protected by the 'user_event_mms_lock' and RCU-walked.

And then we have 'enabler->link', which is the list of enables on the
'user_mm->enablers' list, protected by event_mutex, and _also_
occasionally RCU-walked.

And then we have 'validator->link', which isn't RCU-walked, and seems
to also be protected by the event_mutex (?).

This is all very confusing when looking at it as an outsider.
Particularly when you sometimes just see

        list_del_rcu(&mm->link);

and you have to figure "which 'link' are we talking about again?".

Also, I have to say, I found "mm->next" *really* confusing at first.

It turns out that "mm->next" is this list that is dynamically built up
by user_event_mm_get_all(), and is only a one-shot list that is valid
only as long as 'event_mutex' is held. But the only lock the code
*talks* about is the RCU lock, which does *not* protect that list, and
only exists as a way to walk that user_event_mms list without taking
any locks.

So user_event_enabler_update() actually relies on that 'event_mutex'
lock in another way than the obvious one that is about the
mm->enablers list that it *also* walks.

Again, that all seems to work, but it's *really* confusing how
"mm->next" always exists as a field, but is only usable and valid
while you hold that event_mutex and have called
user_event_mm_get_all() to gather the list.

I think both user_event_enabler_update() and user_event_mm_get_all()
should have a mention of how they require event_mutex and how that
->next list works.

Anyway, I still *think* the two patches I sent out are right, but I'm
just mentioning this confusion I had to deal with when trying to
decode what the rules were. Maybe all the above is obvious to
everybody else, but it took me a while to decipher (and maybe I
misread something).

             Linus

  parent reply	other threads:[~2023-05-17 20:09 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-08 16:37 [PATCH] tracing/user_events: Run BPF program if attached Beau Belgrave
2023-05-09 15:24 ` Alexei Starovoitov
2023-05-09 17:01   ` Steven Rostedt
2023-05-09 20:30     ` Steven Rostedt
2023-05-09 20:42       ` Steven Rostedt
2023-05-15 16:57       ` Alexei Starovoitov
2023-05-15 18:33         ` Steven Rostedt
2023-05-15 19:35           ` Beau Belgrave
2023-05-15 21:38             ` Steven Rostedt
2023-05-15 19:24         ` Beau Belgrave
2023-05-15 21:57           ` Steven Rostedt
2023-05-17  0:36           ` Alexei Starovoitov
2023-05-17  0:56             ` Linus Torvalds
2023-05-17  1:46               ` Linus Torvalds
2023-05-17  2:29                 ` Steven Rostedt
2023-05-17  3:03                   ` Linus Torvalds
2023-05-17 17:22                     ` Beau Belgrave
2023-05-17 18:15                       ` Linus Torvalds
2023-05-17 19:07                         ` Beau Belgrave
2023-05-17 19:26                           ` Linus Torvalds
2023-05-17 19:36                             ` Beau Belgrave
2023-05-17 19:36                             ` Linus Torvalds
2023-05-17 19:37                               ` Linus Torvalds
2023-05-17 23:00                                 ` Beau Belgrave
2023-05-17 23:14                                   ` Linus Torvalds
2023-05-17 23:25                                     ` Steven Rostedt
2023-05-18  0:14                                       ` Beau Belgrave
2023-05-18  0:23                                         ` Linus Torvalds
2023-05-17 20:08                               ` Linus Torvalds [this message]
2023-05-17  1:26             ` Steven Rostedt
2023-05-17 16:50               ` Beau Belgrave
2023-05-18  0:10                 ` Alexei Starovoitov
2023-05-18  0:19                   ` Beau Belgrave
2023-05-18  0:56                     ` Alexei Starovoitov
2023-05-18  1:18                       ` Beau Belgrave
2023-05-18  2:08                         ` Steven Rostedt
2023-05-18  3:14                           ` Alexei Starovoitov
2023-05-18 13:36                             ` Steven Rostedt
2023-05-18 17:28                               ` Beau Belgrave
2023-06-01  9:46                   ` Christian Brauner
2023-06-01 15:24                     ` Beau Belgrave
2023-06-01 15:57                       ` Christian Brauner
2023-06-01 16:29                         ` Beau Belgrave
2023-06-06 13:37                           ` Masami Hiramatsu
2023-06-06 17:05                             ` Beau Belgrave
2023-06-07 14:07                               ` Masami Hiramatsu
2023-06-07 19:26                                 ` Beau Belgrave
2023-06-08  0:25                                   ` Masami Hiramatsu
2023-05-17 17:51             ` Beau Belgrave
2023-06-06 13:57             ` Masami Hiramatsu
2023-06-06 16:57               ` Andrii Nakryiko
2023-06-06 20:57                 ` Beau Belgrave

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