Merge: Do not decrement idle task preempt count in CPU offline

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/4343

Description: Do not decrement idle task preempt count in CPU offline

JIRA: https://issues.redhat.com/browse/RHEL-38427

CVE: CVE-2021-47454

Build Info: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=61490750

Tested: Verified Brew build test kernel RPMs

Signed-off-by: Mamatha Inamdar <minamdar@redhat.com>

commit 787252a
Author: Nathan Lynch <nathanl@linux.ibm.com>
Date:   Fri Oct 15 12:39:02 2021 -0500

    powerpc/smp: do not decrement idle task preempt count in CPU offline

    With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we
    get:

    BUG: scheduling while atomic: swapper/1/0/0x00000000
    no locks held by swapper/1/0.
    CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
    Call Trace:
     dump_stack_lvl+0xac/0x108
     __schedule_bug+0xac/0xe0
     __schedule+0xcf8/0x10d0
     schedule_idle+0x3c/0x70
     do_idle+0x2d8/0x4a0
     cpu_startup_entry+0x38/0x40
     start_secondary+0x2ec/0x3a0
     start_secondary_prolog+0x10/0x14

    This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
    preempt count, for reasons explained in commit a7c2bb8 ("powerpc:
    Re-enable preemption before cpu_die()"), specifically "start_secondary()
    expects a preempt_count() of 0."

    However, since commit 2c669ef ("powerpc/preempt: Don't touch the idle
    task's preempt_count during hotplug") and commit f1a0a37 ("sched/core:
    Initialize the idle task with preemption disabled"), that justification no
    longer holds.

    The idle task isn't supposed to re-enable preemption, so remove the
    vestigial preempt_enable() from the CPU offline path.

    Tested with pseries and powernv in qemu, and pseries on PowerVM.

    Fixes: 2c669ef ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug")
    Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
    Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
    Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20211015173902.2278118-1-nathanl@linux.ibm.com

Signed-off-by: Mamatha Inamdar <minamdar@redhat.com>

Approved-by: Steve Best <sbest@redhat.com>
Approved-by: Tony Camuso <tcamuso@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Scott Weaver <scweaver@redhat.com>

Author: swvrrh

arch/powerpc/kernel/smp.c

@@ -1779,8 +1779,6 @@ void __cpu_die(unsigned int cpu)
 
 void arch_cpu_idle_dead(void)
 {
-	sched_preempt_enable_no_resched();
-
 	/*
 	 * Disable on the down path. This will be re-enabled by
 	 * start_secondary() via start_secondary_resume() below