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Date:   Thu, 8 Jul 2021 15:08:32 +0530
From:   Bharata B Rao <bharata@linux.ibm.com>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Guenter Roeck <linux@roeck-us.net>,
        Arnd Bergmann <arnd@arndb.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH] powerpc: preempt: Don't touch the idle task's
 preempt_count during hotplug
Message-ID: <YObHmEzAPZLx5O6B@in.ibm.com>
Reply-To: bharata@linux.ibm.com
References: <20210707183831.2106509-1-valentin.schneider@arm.com>
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On Wed, Jul 07, 2021 at 07:38:31PM +0100, Valentin Schneider wrote:
> Powerpc currently resets a CPU's idle task preempt_count to 0 before said
> task starts executing the secondary startup routine (and becomes an idle
> task proper).
> 
> This conflicts with commit
> 
>   f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> 
> which initializes all of the idle tasks' preempt_count to PREEMPT_DISABLED
> during smp_init(). Note that this was superfluous before said commit, as
> back then the hotplug machinery would invoke init_idle() via
> idle_thread_get(), which would have already reset the CPU's idle task's
> preempt_count to PREEMPT_ENABLED.
> 
> Get rid of this preempt_count write.
> 
> Cc: Guenter Roeck <linux@roeck-us.net>
> Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> Reported-by: Bharata B Rao <bharata@linux.ibm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  arch/powerpc/platforms/cell/smp.c    | 3 ---
>  arch/powerpc/platforms/pseries/smp.c | 5 +----
>  2 files changed, 1 insertion(+), 7 deletions(-)

The messages like "BUG: scheduling while atomic: swapper/1/0/0x00000000"
for each secondary CPU are no longer seen after this patch on powerpc.

Tested-by: Bharata B Rao <bharata@linux.ibm.com>