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From: Borislav Petkov <bp@alien8.de>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Darren Hart <dvhart@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v5] x86/mce: Avoid infinite loop for copy from user recovery
Date: Thu, 28 Jan 2021 18:57:35 +0100	[thread overview]
Message-ID: <20210128175735.GB2120@zn.tnic> (raw)
In-Reply-To: <20210126223605.GA14355@agluck-desk2.amr.corp.intel.com>

On Tue, Jan 26, 2021 at 02:36:05PM -0800, Luck, Tony wrote:
> In some cases Linux might context switch to something else. Perhaps
> this task even gets picked up by another CPU to run the task work
> queued functions.  But I imagine that the context switch should act
> as a barrier ... shouldn't it?

I'm given to understand that the #MC from user is likely to schedule and
a context switch has a barrier character.

> After a few cycles of the test injection to user mode, I saw an
> overflow in the machine check bank. As if it hadn't been cleared
> from the previous iteration ...

This sounds weird. As if something else is happening which we haven't
thought of yet...

> When the tests were failing, code was on top of v5.11-rc3. Latest
> experiments moved to -rc5.  There's just a tracing fix from
> PeterZ between rc3 and rc5 to mce/core.c:
> 
> 737495361d44 ("x86/mce: Remove explicit/superfluous tracing")
> 
> which doesn't appear to be a candidate for the problems I saw.

Doesn't look like it.

> This is the bit that changed during my detour using atomic_t mce_count.
> I added the local variable to capture value from atomic_inc_return(), then
> used it later, instead of a bunch of atomic_read() calls.
> 
> I kept it this way because "if (count == 1)" is marginally easier to read
> than "if (current->mce_count++ == 0)"

Right.

So still no explanation why it would fail before. ;-\

Crazy idea: if you still can reproduce on -rc3, you could bisect: i.e.,
if you apply the patch on -rc3 and it explodes and if you apply the same
patch on -rc5 and it works, then that could be a start... Yeah, don't
have a better idea here. :-\

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

  reply	other threads:[~2021-01-28 17:59 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-08 22:22 [PATCH 0/2] Fix infinite machine check loop in futex_wait_setup() Tony Luck
2021-01-08 22:22 ` [PATCH 1/2] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-08 22:22 ` [PATCH 2/2] futex, x86/mce: Avoid double machine checks Tony Luck
2021-01-08 22:47   ` Peter Zijlstra
2021-01-08 23:08     ` Luck, Tony
2021-01-08 23:14       ` Peter Zijlstra
2021-01-08 23:20         ` Luck, Tony
2021-01-11 21:44 ` [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup() Tony Luck
2021-01-11 21:44   ` [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-11 22:11     ` Andy Lutomirski
2021-01-11 22:20       ` Luck, Tony
2021-01-12 17:00         ` Andy Lutomirski
2021-01-12 17:16           ` Luck, Tony
2021-01-12 17:21             ` Andy Lutomirski
2021-01-12 18:23               ` Luck, Tony
2021-01-12 18:57                 ` Andy Lutomirski
2021-01-12 20:52                   ` Luck, Tony
2021-01-12 22:04                     ` Andy Lutomirski
2021-01-13  1:50                       ` Luck, Tony
2021-01-13  4:15                         ` Andy Lutomirski
2021-01-13 10:00                           ` Borislav Petkov
2021-01-13 16:06                             ` Luck, Tony
2021-01-13 16:19                               ` Borislav Petkov
2021-01-13 16:32                                 ` Luck, Tony
2021-01-13 17:35                                   ` Borislav Petkov
2021-01-14 20:22     ` Borislav Petkov
2021-01-14 21:05       ` Luck, Tony
2021-01-11 21:44   ` [PATCH v2 2/3] x86/mce: Add new return value to get_user() for machine check Tony Luck
2021-01-11 21:44   ` [PATCH v2 3/3] futex, x86/mce: Avoid double machine checks Tony Luck
2021-01-14 17:22   ` [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup() Andy Lutomirski
2021-01-15  0:38   ` [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-15 15:27     ` Borislav Petkov
2021-01-15 19:34       ` Luck, Tony
2021-01-15 20:51         ` [PATCH v4] " Luck, Tony
2021-01-15 23:23           ` Luck, Tony
2021-01-19 10:56             ` Borislav Petkov
2021-01-19 23:57               ` Luck, Tony
2021-01-20 12:18                 ` Borislav Petkov
2021-01-20 17:17                   ` Luck, Tony
2021-01-21 21:09                   ` Luck, Tony
2021-01-25 22:55                     ` [PATCH v5] " Luck, Tony
2021-01-26 11:03                       ` Borislav Petkov
2021-01-26 22:36                         ` Luck, Tony
2021-01-28 17:57                           ` Borislav Petkov [this message]
2021-02-01 18:58                             ` Luck, Tony
2021-02-02 11:01                               ` Borislav Petkov
2021-02-02 16:04                                 ` Luck, Tony
2021-02-02 21:06                                   ` Borislav Petkov
2021-02-02 22:12                                     ` Luck, Tony
2021-01-18 15:39         ` [PATCH v3] " Borislav Petkov

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