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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Beau Belgrave <beaub@linux.microsoft.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	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: Tue, 16 May 2023 17:56:02 -0700	[thread overview]
Message-ID: <CAHk-=whBKoovtifU2eCeyuBBee-QMcbxdXDLv0mu0k2DgxiaOw@mail.gmail.com> (raw)
In-Reply-To: <20230517003628.aqqlvmzffj7fzzoj@MacBook-Pro-8.local>

On Tue, May 16, 2023 at 5:36 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon, May 15, 2023 at 12:24:07PM -0700, Beau Belgrave wrote:
> > > >
> > > >   ret = pin_user_pages_remote(mm->mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT,
> > > >                               &page, NULL, NULL);
> > >
> > > ... which will call pin_user_pages_remote() in RCU CS.
> > > This looks buggy, since pin_user_pages_remote() may schedule.
> > >
> >
> > If it's possible to schedule, I can change this to cache the probe
> > callbacks under RCU then drop it. However, when would
> > pin_user_pages_remote() schedule with FOLL_NOFAULT?
>
> Are you saying that passing FOLL_NOFAULT makes it work in atomic context?

Absolutely not.

It may not fault missing pages in, but that does *not* make it atomic.

That code depends on all the usual MM locking, and it does not work at
all in the same way that "pagefault_disable()" does, for example. That
will fail on any fault and never take locks, and is designed to work
in atomic contexts. Very different.

So no, don't think you can call pin_user_pages_remote() or any other
GUP function from atomic context.

We do have "get_user_page[s]_fast_only()" and that is the only version
of GUP that is actually lock-free.

Also, just FYI, those special gup_user*fast_only()" functions simply
will not work on some architectures at all.

               Linus

  reply	other threads:[~2023-05-17  0:56 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 [this message]
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
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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