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From:   "Travis, Mike" <mike.travis@hpe.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bp@alien8.de>,
        "Dave Hansen" <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Wahl, Steve" <steve.wahl@hpe.com>,
        "x86@kernel.org" <x86@kernel.org>
CC:     "Sivanich, Dimitri" <dimitri.sivanich@hpe.com>,
        Andy Shevchenko <andy@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Anderson, Russ" <russ.anderson@hpe.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "platform-driver-x86@vger.kernel.org" 
        <platform-driver-x86@vger.kernel.org>,
        "Travis, Mike" <mike.travis@hpe.com>
Subject: Re: [PATCH] x86/platform/uv: Dont use smp_processor_id while
 preemptable
Thread-Topic: [PATCH] x86/platform/uv: Dont use smp_processor_id while
 preemptable
Thread-Index: AQHYbImZaP+RHM7abU6A9/aPZMC2cq0xUfKAgAB0aZo=
Date:   Thu, 26 May 2022 22:53:38 +0000
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References: <20220520203755.266337-1-mike.travis@hpe.com>
 <902b4745-4ca0-9b29-6563-ed49f924a5e1@intel.com>
In-Reply-To: <902b4745-4ca0-9b29-6563-ed49f924a5e1@intel.com>
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Dave Hansen <dave.hansen@intel.com> wrote:=0A=
  On 5/20/22 13:37, Mike Travis wrote:=0A=
  > To avoid a "BUG: using smp_processor_id() in preemptible" debug=0A=
  > warning message, disable preemption around use of the processor id.=0A=
=0A=
>I'm sure this gets rid of the warning.  But, could you please take a=0A=
>quick look at the callers and ensure that they can handle if this read=0A=
>comes from another CPU?=0A=
>=0A=
>In other words, what would actually go wrong if uv_read_rtc() got=0A=
>preempted in this region?  What would this actually fix?=0A=
=0A=
I talked with the author of this driver and among the preemption scenarios =
it always returns the same system time.  And due to the round robin aspect =
of the request scheduling, getting an earlier time when a second CPU reads =
it's time is not possible.  So getting rid of the debug warning message see=
ms to be the only benefit from this patch.=0A=