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Date: Wed, 7 Feb 2024 09:58:04 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
	Daniel Bristot de Oliveira <bristot@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Leonardo Bras <leobras@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch 05/12] timekeeping_notify: use stop_machine_fail when
 appropriate
Message-ID: <ZcN+XM4223HS42LM@tpad>
References: <20240206184911.248214633@redhat.com>
 <20240206185709.955049547@redhat.com>
 <87h6ikmqqo.ffs@tglx>
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In-Reply-To: <87h6ikmqqo.ffs@tglx>

On Wed, Feb 07, 2024 at 12:57:19PM +0100, Thomas Gleixner wrote:
> On Tue, Feb 06 2024 at 15:49, Marcelo Tosatti wrote:
> > Change timekeeping_notify to use stop_machine_fail when appropriate,
> > which will fail in case the target CPU is tagged as block interference
> > CPU.
> 
> You completely fail to explain 'appropriate'. There is zero reason for
> this churn, really.

The churn is so that we can return an error to
current_clocksource_store (sysfs handler for writes to
/sys/devices/system/clocksource/clocksource0/current_clocksource).

Yeah, should probably separate patches that prepare code for 
returning errors from the actual change between smp_call_function_single
and smp_call_function_single_fail (started that for the
rdmsr_on_cpu/wrmsr_on_cpu helpers).

OK, so i am assuming you are good with the general idea of the patch
(and have no better one) which is:

idx = block_interf_srcu_read_lock();
ret = smp_call_function_single_fail(cpu, remote_fn, ...);  (or stop_machine_fail)
block_interf_srcu_read_unlock(idx);