Re: [PATCH 0/2] New fix for icache coherency race

[Paul Burton]

On Thu, Feb 26, 2015 at 02:16:01PM +0100, Lars Persson wrote:
> This patch set proposes an improved fix for the race condition that
> originally was fixed in commit 2a4a8b1e5d9d ("MIPS: Remove race window
> in page fault handling").
> 
> I have used the flush_icache_page API that is marked as deprecated in
> Documentation/cachetlb.txt. There are strong reasons to keep this API
> because it is not possible to implement an efficient and race-free
> lazy flushing using the other APIs.
> 
> You can refer to a discussion about the same issue in arch/arm where
> they chose to implement the solution in set_pte_at. In arch/mips we
> could not do this because we lack information about the executability
> of the mapping in set_pte_at() and thus we would have to flush all
> pages to be safe.
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2010-November/030915.html
> 
> Lars Persson (2):
>   Revert "MIPS: Remove race window in page fault handling"
>   MIPS: Fix race condition in lazy cache flushing.

FYI these 2 patches prevent a linux-next based kernel booting on the
Ingenic JZ4780-based CI20 board. I've not yet tried on the in-tree
JZ4740-based qi_lb60 to see whether it's also affected, nor have I yet
figured out what's going wrong. I'll hopefully dig into it tomorrow, but
just a heads up!

The boot failure (using an initramfs, so no DMA or much I/O at all):

    [    4.618013] Freeing unused kernel memory: 5160K (803d6000 - 808e0000)
    [    4.625211] CPU 0 Unable to handle kernel paging request at virtual address 00000000, epc == 80027924, ra == 8001db10
    [    4.635881] Oops[#1]:
    [    4.638149] CPU: 0 PID: 1 Comm: init Not tainted 4.0.0-rc7-next-20150408+ #334
    [    4.645354] task: 8dc39568 ti: 8dc3a000 task.ti: 8dc3a000
    [    4.650736] $ 0   : 00000000 00000001 00000001 00001000
    [    4.655965] $ 4   : 00000000 00000001 808e0000 8df0b610
    [    4.639347] $ 8   : 00000000 8d8f8160 00000000 00000000
    [    4.644575] $12   : 00000000 00000118 00000040 ffff0000
    [    4.649802] $16   : 81c649fc 77984000 00000004 8df013f8
    [    4.655030] $20   : 81c649fc 00000000 00000000 00040000
    [    4.638413] $24   : ff000000 80027920
    [    4.643642] $28   : 8dc3a000 8dc3bd38 ffffffbf 8001db10
    [    4.648871] Hi    : 3036f946
    [    4.651740] Lo    : eeea49fc
    [    4.654620] epc   : 80027924 r4k_blast_dcache_page_dc32+0x4/0x9c
    [    4.638763]     Not tainted
    [    4.641550] ra    : 8001db10 __update_cache+0xa4/0xd0
    [    4.646585] Status: 10000403 KERNEL EXL IE
    [    4.650767] Cause : 00800008
    [    4.653634] BadVA : 00000000
    [    4.656503] PrId  : 3ee1024f (Ingenic JZRISC)
    [    4.639002] Process init (pid: 1, threadinfo=8dc3a000, task=8dc39568, tls=00000000)
    [    4.646636] Stack : 3f916478 67caba37 58047621 77984000 58047621 77984000 8df0b610 800b777c
              00400000 00031ec0 00000001 00400000 8d8f8178 8ddae840 8de8fbdc 81c649fc
              8df013f8 8dc3be00 8de8fbe0 8008f7ac 8df09e38 8de9be40 fffffff8 8deeb000
              00000000 00000000 00000000 00000040 803b9924 8dcd9000 77984000 800e2200
              803ca320 800e1474 8df013f8 00000610 77984b50 00000000 8df00778 8df0b610
              ...
    [    4.638523] Call Trace:
    [    4.640962] [<80027924>] r4k_blast_dcache_page_dc32+0x4/0x9c
    [    4.646608] [<8001db10>] __update_cache+0xa4/0xd0
    [    4.651305] [<800b777c>] do_set_pte+0x14c/0x174
    [    4.655826] [<8008f7ac>] filemap_map_pages+0x2ac/0x384
    [    4.639107] [<800b7a54>] handle_mm_fault+0x2b0/0x1020
    [    4.644148] [<8001f460>] __do_page_fault+0x160/0x470
    [    4.649102] [<80013e24>] resume_userspace_check+0x0/0x10
    [    4.654395]
    [    4.655876]
    Code: 03e00008  00000000  24831000 <bc950000> bc950020  bc950040  bc950060  bc950080  bc9500a0
    [    4.643978] ---[ end trace 672ef517bf5944f0 ]---
    [    4.648581] Fatal exception: panic in 5 seconds

The next-20140408 based CI20 branch (currently including reverts of
these 2 patches) if anyone wants to reproduce:

    https://github.com/paulburton/linux/tree/wip-ci20-v4.1

Thanks,
    Paul

>  arch/mips/include/asm/cacheflush.h |   35 ++++++++++++++++++++---------------
>  arch/mips/include/asm/pgtable.h    |   10 ++++++----
>  arch/mips/mm/cache.c               |   27 ++++++++-------------------
>  3 files changed, 34 insertions(+), 38 deletions(-)
> 
> -- 
> 1.7.10.4
>

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