linux-kernel - Re: [PATCH 4/6] x86-mce: Add spinlocks to prevent duplicated MCP and CMCI reports.

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Message-ID: <CA+8MBbKNQEk5UQnVf-G8zeMiYE2FpyztHE6_RoDs3RM_8mrVow@mail.gmail.com>
Date:	Thu, 10 Jul 2014 11:57:06 -0700
From:	Tony Luck <tony.luck@...il.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Havard Skinnemoen <hskinnemoen@...gle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ewout van Bekkum <ewout@...gle.com>
Subject: Re: [PATCH 4/6] x86-mce: Add spinlocks to prevent duplicated MCP and
 CMCI reports.

On Thu, Jul 10, 2014 at 11:44 AM, Borislav Petkov <bp@...en8.de> wrote:
>         if (atomic_dec_and_test(&mce_banks[i].poll))
>                 m.status = mce_rdmsrl(MSR_IA32_MCx_STATUS(i));
>
>         atomic_add_unless(&mce_banks[i].poll, 1, 1);
>
> so that you have only one CPU read the status register of mce_banks[i].
>
> For non-shared banks, this will always work because no other CPU will
> dec that variable anyway.

You don't know if the bank is shared or not.  If it is not shared, then the
above code might skip reading the bank because some other cpu is busy
reading that bank number - and seeing its own private bank.

-Tony
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