In the Linux kernel, the following vulnerability has been resolved:
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: dumpstacklvl+0xac/0x108 _schedulebug+0xac/0xe0 _schedule+0xcf8/0x10d0 scheduleidle+0x3c/0x70 doidle+0x2d8/0x4a0 cpustartupentry+0x38/0x40 startsecondary+0x2ec/0x3a0 startsecondaryprolog+0x10/0x14
This is because powerpc's archcpuidledead() decrements the idle task's preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc: Re-enable preemption before cpudie()"), specifically "startsecondary() expects a preemptcount() of 0."
However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug") and commit f1a0a376ca0c ("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.