MIPS r4k cache operations with SMP enabled

From: Chris Packham
Date: Mon May 27 2019 - 22:56:22 EST


I'm trying to port a fairly old Broadcom integrated chip (BCM6818) to
the latest Linux kernel using the mips/bmips support.

The chip has a BMIPS4355 core. This has two "thread processors" (cpu
cores) with separate I-caches but a shared D-cache.

I've got things booting but I encounter the following BUG()

BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
caller is blast_dcache16+0x24/0x154
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.1.0-at1 #5
Stack : 00000036 8008d0d0 806a0000 807c0000 80754e10 0000000b 80754684
80900000 8f828424 807986e7 8071348c 00000000 10008f00 8f831c30
00000000 00000000 80920000 00000056 00002335 00000000 807a0000
6d6d3a20 00000000 00000056 73776170 00000000 ffffffff 10008f01
80790000 00002cc2 ffffffff 80900000 00000010 8f83198c 00000000
Call Trace:
[<8001c208>] show_stack+0x30/0x100
[<8063282c>] dump_stack+0x9c/0xd0
[<802f1cec>] debug_smp_processor_id+0xfc/0x110
[<8002e274>] blast_dcache16+0x24/0x154
[<80122978>] map_vm_area+0x58/0x70
[<80123888>] __vmalloc_node_range+0x1fc/0x2b4
[<80123b54>] vmalloc+0x44/0x50
[<807d15d0>] jffs2_zlib_init+0x24/0x94
[<807d1354>] jffs2_compressors_init+0x10/0x30
[<807d151c>] init_jffs2_fs+0x68/0xf8
[<8001016c>] do_one_initcall+0x7c/0x1f0
[<807bee30>] kernel_init_freeable+0x17c/0x258
[<80650d1c>] kernel_init+0x10/0xf8
[<80015e6c>] ret_from_kernel_thread+0x14/0x1c

In blast_dcache16 current_cpu_data is used which invokes
smp_processor_id() triggering the BUG(). I can fix this by sprinkling
preempt_disable/preempt_enable through arch/mips/mm/c-r4k.c but that
seems kind of wrong. Does anyone have any suggestion as to the right way
to avoid this BUG()?