From: Steven Rostedt <rostedt@goodmis.org>
To: Florent Revest <revest@chromium.org>
Cc: linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
bpf@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org,
mhiramat@kernel.org, mark.rutland@arm.com, ast@kernel.org,
daniel@iogearbox.net, andrii@kernel.org, kpsingh@kernel.org,
jolsa@kernel.org, xukuohai@huaweicloud.com, lihuafei1@huawei.com
Subject: Re: [PATCH v2 04/10] ftrace: Store direct called addresses in their ops
Date: Thu, 16 Mar 2023 11:45:44 -0400 [thread overview]
Message-ID: <20230316114544.5db09039@gandalf.local.home> (raw)
In-Reply-To: <CABRcYmLLbRGZXWwEpyLW1YFz87tTPA8pCL7oLd4K6Hp9Etr5LA@mail.gmail.com>
On Thu, 16 Mar 2023 16:40:48 +0100
Florent Revest <revest@chromium.org> wrote:
> > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > > entry->direct = addr;
> > > }
> > > }
> > > + WRITE_ONCE(ops->direct_call, addr);
> >
> > I'm curious about the use of WRITE_ONCE(). It should not go outside the
> > mutex barrier.
>
> This WRITE_ONCE was originally suggested by Mark here:
> https://lore.kernel.org/all/Y9vW99htjOphDXqY@FVFF77S0Q05N.cambridge.arm.com/#t
>
> My understanding is that it's not so much about avoiding re-ordering
> but rather about avoiding store tearing since a ftrace_caller
> trampoline could concurrently read ops->direct_call. Does that make
> sense ?
Yes, but a comment needs to be added:
/* Prevent store tearing on some archs */
WRITE_ONCE(ops->direct_call, addr);
Or something to that affect. Otherwise I can see it confusing others in the
future. And probably me too, as I'll forget why it was a WRITE_ONCE() by
next month. ;-)
-- Steve
next prev parent reply other threads:[~2023-03-16 15:47 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-07 18:21 [PATCH v2 00/10] Add ftrace direct call for arm64 Florent Revest
2023-02-07 18:21 ` [PATCH v2 01/10] ftrace: Replace uses of _ftrace_direct APIs with _ftrace_direct_multi Florent Revest
2023-03-15 23:33 ` Steven Rostedt
2023-03-16 15:40 ` Florent Revest
2023-02-07 18:21 ` [PATCH v2 02/10] ftrace: Remove the legacy _ftrace_direct API Florent Revest
2023-02-07 18:21 ` [PATCH v2 03/10] ftrace: Rename _ftrace_direct_multi APIs to _ftrace_direct APIs Florent Revest
2023-02-07 18:21 ` [PATCH v2 04/10] ftrace: Store direct called addresses in their ops Florent Revest
2023-03-15 23:43 ` Steven Rostedt
2023-03-16 15:40 ` Florent Revest
2023-03-16 15:45 ` Steven Rostedt [this message]
2023-03-16 16:15 ` Florent Revest
2023-02-07 18:21 ` [PATCH v2 05/10] ftrace: Make DIRECT_CALLS work WITH_ARGS and !WITH_REGS Florent Revest
2023-02-07 18:21 ` [PATCH v2 06/10] ftrace: selftest: remove broken trace_direct_tramp Florent Revest
2023-03-15 23:51 ` Steven Rostedt
2023-03-16 15:41 ` Florent Revest
2023-02-07 18:21 ` [PATCH v2 07/10] arm64: ftrace: Add direct call support Florent Revest
2023-02-07 18:21 ` [PATCH v2 08/10] arm64: ftrace: Simplify get_ftrace_plt Florent Revest
2023-02-07 18:21 ` [PATCH v2 09/10] arm64: ftrace: Add direct call trampoline samples support Florent Revest
2023-02-07 18:21 ` [PATCH v2 10/10] selftests/bpf: Update the tests deny list on aarch64 Florent Revest
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