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

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Date:   Wed,  7 Jul 2021 19:38:31 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        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: [PATCH] powerpc: preempt: Don't touch the idle task's preempt_count during hotplug

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(-)

diff --git a/arch/powerpc/platforms/cell/smp.c b/arch/powerpc/platforms/cell/smp.c
index c855a0aeb49c..d7ab868aab54 100644
--- a/arch/powerpc/platforms/cell/smp.c
+++ b/arch/powerpc/platforms/cell/smp.c
@@ -78,9 +78,6 @@ static inline int smp_startup_cpu(unsigned int lcpu)
 
 	pcpu = get_hard_smp_processor_id(lcpu);
 
-	/* Fixup atomic count: it exited inside IRQ handler. */
-	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
-
 	/*
 	 * If the RTAS start-cpu token does not exist then presume the
 	 * cpu is already spinning.
diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index 096629f54576..7ebf3382816a 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -105,10 +105,7 @@ static inline int smp_startup_cpu(unsigned int lcpu)
 		return 1;
 	}
 
-	/* Fixup atomic count: it exited inside IRQ handler. */
-	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
-
-	/* 
+	/*
 	 * If the RTAS start-cpu token does not exist then presume the
 	 * cpu is already spinning.
 	 */
-- 
2.25.1