From: Linus Torvalds <torvalds@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH] ring-buffer: Remove 32bit timestamp logic
Date: Thu, 14 Dec 2023 12:50:29 -0800 [thread overview]
Message-ID: <CAHk-=wieVSfyjTpe8L5kmwC4mk9dRge9dvyJiMZEkyz4-tOvow@mail.gmail.com> (raw)
In-Reply-To: <20231214153636.655e18ce@gandalf.local.home>
On Thu, 14 Dec 2023 at 12:35, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Thu, 14 Dec 2023 11:44:55 -0800
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Thu, 14 Dec 2023 at 08:55, Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > And yes, this does get called in NMI context.
> >
> > Not on an i486-class machine they won't. You don't have a local apic
> > on those, and they won't have any NMI sources under our control (ie
> > NMI does exist, but we're talking purely legacy NMI for "motherboard
> > problems" like RAM parity errors etc)
>
> Ah, so we should not worry about being in NMI context without a 64bit cmpxchg?
.. on x86.
Elsewhere, who knows?
It is *probably* true in most situations. '32-bit' => 'legacy' =>
'less likely to have fancy profiling / irq setups'.
But I really don't know.
> > So no. You need to forget about the whole "do a 64-bit cmpxchg on
> > 32-bit architectures" as being some kind of solution in the short
> > term.
>
> But do all archs have an implementation of cmpxchg64, even if it requires
> disabling interrupts? If not, then I definitely cannot remove this code.
We have a generic header file, so anybody who uses that would get the
fallback version, ie
arch_cmpxchg64 -> generic_cmpxchg64_local -> __generic_cmpxchg64_local
which does that irq disabling thing.
But no, not everybody is guaranteed to use that fallback. From a quick
look, ARC, hexagon and CSky don't do this, for example.
And then I got bored and stopped looking.
My guess is that *most* 32-bit architectures do not have a 64-bit
cmpxchg - not even the irq-safe one.
For the UP case you can do your own, of course.
Linus
next prev parent reply other threads:[~2023-12-14 20:50 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-14 2:11 [PATCH] ring-buffer: Remove 32bit timestamp logic Steven Rostedt
2023-12-14 2:46 ` Steven Rostedt
2023-12-14 6:53 ` Linus Torvalds
2023-12-14 16:56 ` Steven Rostedt
2023-12-14 19:44 ` Linus Torvalds
2023-12-14 20:36 ` Steven Rostedt
2023-12-14 20:50 ` Linus Torvalds [this message]
2023-12-14 21:01 ` Steven Rostedt
2023-12-17 17:40 ` David Laight
2023-12-14 2:49 ` Steven Rostedt
2023-12-14 3:05 ` Mathieu Desnoyers
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