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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Boqun Feng <boqun.feng@gmail.com>, Borislav Petkov <bp@alien8.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>, Dennis Zhou <dennis@kernel.org>,
	Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 0/3] Remove volatile from arch_raw_cpu_ptr() and revert the hacks.
Date: Thu, 24 Mar 2022 11:28:16 -0700	[thread overview]
Message-ID: <CAHk-=wgcM9vUjGTdoCbX4DuBM+53FFmHFqYZdF8mNzATwj8z0A@mail.gmail.com> (raw)
In-Reply-To: <20220324173927.2230447-1-bigeasy@linutronix.de>

On Thu, Mar 24, 2022 at 10:39 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> The following series does that. The assembly code looks okay. In a few
> simple test cases the this_cpu_ptr() usage is always created and is not
> moved passed preempt_enable() statement.

Hmm. I didn't think things through, and you're very correct in having
thought about preempt_enable/disable as needing barriers for that asm.

So the "volatile" does have that potential significance for that
arch_raw_cpu_ptr() in case it was what ordered it wrt preemption.

It turns out that it's all ok, because

 - even without the 'volatile', arch_raw_cpu_ptr() has an *input* that
is a memory location:

                     : "m" (this_cpu_off)

 - and prempt_{dis,en}able() fundamentally has a 'barrier()' in it

and as such it's not just random luck that the percpu thing is never
moved around preemption points. They are properly serialized even
without any 'volatile'.

But that "it's not just random luck" might be worth a comment.

So Ack on your series, but that additional comment might be worth it,
since I didn't even think of it until you mentioned it. Clearly much
too subtle.

Thanks,
              Linus

  parent reply	other threads:[~2022-03-24 18:28 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-21 11:11 [GIT PULL] locking changes for v5.18 Ingo Molnar
2022-03-22 22:05 ` Linus Torvalds
2022-03-22 22:19   ` Borislav Petkov
2022-03-22 22:58     ` Linus Torvalds
2022-03-23  7:11       ` Sebastian Andrzej Siewior
2022-03-23 11:09         ` [PATCH] locking/local_lock: Pretend to use the per-CPU variable if not needed Sebastian Andrzej Siewior
2022-03-23 17:17           ` Linus Torvalds
2022-03-24 17:39             ` [PATCH 0/3] Remove volatile from arch_raw_cpu_ptr() and revert the hacks Sebastian Andrzej Siewior
2022-03-24 17:39               ` [PATCH 1/3] x86/percpu: Remove volatile from arch_raw_cpu_ptr() Sebastian Andrzej Siewior
2022-03-24 17:39               ` [PATCH 2/3] Revert "locking/local_lock: Make the empty local_lock_*() function a macro." Sebastian Andrzej Siewior
2022-03-24 17:39               ` [PATCH 3/3] Revert "mm/page_alloc: mark pagesets as __maybe_unused" Sebastian Andrzej Siewior
2022-03-24 18:28               ` Linus Torvalds [this message]
2022-03-28 13:55                 ` [PATCH 0/3] Remove volatile from arch_raw_cpu_ptr() and revert the hacks Peter Zijlstra
2022-03-28 14:59                   ` Sebastian Andrzej Siewior
2022-03-28 14:58                 ` [PATCH v2 " Sebastian Andrzej Siewior
2022-03-28 14:58                   ` [PATCH v2 1/3] x86/percpu: Remove volatile from arch_raw_cpu_ptr() Sebastian Andrzej Siewior
2022-04-05  8:28                     ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-28 14:58                   ` [PATCH v2 2/3] Revert "locking/local_lock: Make the empty local_lock_*() function a macro." Sebastian Andrzej Siewior
2022-04-05  8:28                     ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-28 14:58                   ` [PATCH v2 3/3] Revert "mm/page_alloc: mark pagesets as __maybe_unused" Sebastian Andrzej Siewior
2022-04-05  8:28                     ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-23 11:21       ` [PATCH] x86/defconfig: Enable WERROR Borislav Petkov
2022-03-23 17:19         ` Linus Torvalds
2022-03-23 17:33           ` Borislav Petkov
2022-03-24  8:31           ` [PATCH] x86/config: Make the x86 defconfigs a bit more usable Ingo Molnar
2022-03-24  9:12             ` David Laight
2022-03-24 15:47             ` Nathan Chancellor
2022-03-25 11:52               ` Andy Shevchenko
2022-03-27 19:04                 ` Ingo Molnar
2022-03-27 19:03               ` Ingo Molnar
2022-03-28 15:41                 ` Nathan Chancellor
2022-09-02  8:50                   ` Ingo Molnar
2022-09-02  9:18                     ` Masahiro Yamada
2022-09-04  9:48                       ` Ingo Molnar
2022-09-05  2:16                         ` Masahiro Yamada
2022-09-05  9:54                           ` Ingo Molnar
     [not found]                             ` <CAK7LNAQyiNpbLuVjjQ8-GOQECtfQZqsNS8xH0E2ZkLAHYtXt7A@mail.gmail.com>
2022-09-10 17:28                               ` Linus Torvalds
2022-03-24  8:16         ` [tip: x86/urgent] x86/defconfig: Enable WERROR tip-bot2 for Borislav Petkov
2022-03-25 11:41       ` [GIT PULL] locking changes for v5.18 Andy Shevchenko
2022-03-25 12:23         ` Peter Zijlstra
2022-03-25 13:06           ` Andy Shevchenko
2022-03-25 17:29         ` Linus Torvalds
2022-03-25 17:53           ` Andy Shevchenko
2022-03-22 22:38   ` Peter Zijlstra
2022-03-22 23:26   ` Linus Torvalds
2022-03-24  8:40     ` Ingo Molnar
2022-03-24 10:19       ` Borislav Petkov
2022-03-24 23:19         ` Nick Desaulniers
2022-03-22 23:27 ` pr-tracker-bot

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