linux-kernel - Re: [PATCH] powerpc: preempt: Don't touch the idle task's preempt

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Date:   Thu, 8 Jul 2021 15:08:32 +0530
From:   Bharata B Rao <bharata@...ux.ibm.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Guenter Roeck <linux@...ck-us.net>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] powerpc: preempt: Don't touch the idle task's
 preempt_count during hotplug

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@...ck-us.net>
> Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> Reported-by: Bharata B Rao <bharata@...ux.ibm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@....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@...ux.ibm.com>