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Subject: Re: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash
 bucket.
To:     NeilBrown <neilb@suse.com>
Cc:     Thomas Graf <tgraf@suug.ch>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <155416000985.9540.14182958463813560577.stgit@noble.brown>
 <155416006521.9540.5662092375167065834.stgit@noble.brown>
 <20190410193418.GA32402@roeck-us.net>
 <87r2a9xt79.fsf@notabene.neil.brown.name>
 <87lg0hxe67.fsf@notabene.neil.brown.name>
 <87imvlxcxc.fsf@notabene.neil.brown.name>
From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <5aaaf7c9-0d1c-f058-f841-917fcb61d955@roeck-us.net>
Date:   Thu, 11 Apr 2019 05:44:46 -0700
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On 4/10/19 11:40 PM, NeilBrown wrote:
> On Thu, Apr 11 2019, NeilBrown wrote:
> 
>> On Thu, Apr 11 2019, NeilBrown wrote:
>>
>>> On Wed, Apr 10 2019, Guenter Roeck wrote:
>>>
>>>> Hi,
>>>>
>> .....
>>>>
>>>> This patch causes my qemu q800 boot test to crash reliably.
>>>>
>> ....
>>>> Code: 4a89 6604 4280 60ea 2c2b 000c 2748 000c <2869> 000c 082c 0003 0002 6728 4878 0014 7620 4873 3800 486e ffec 4eb9 002e 5b88
>>>
>>> Thanks for testing and for the report.
>> .....
>>>
>>> .... and after googling a bit I see that 68000 require 2-byte alignment,
>>> but not 4-byte.  Oh..
>>>
>>> That means there aren't two spare bits in an address, so I cannot use
>>> one for the NULLS and one for a lock bit.  Bother.
>>>
>>> I might be able to find a different way forward, but for now I think we
>>> need to drop this series.
>>
>> I have found a way forward that I like.  It only requires one bit per
>> address to be over-loaded.
>>
>> The following patch implements it and works for me.
>> Could you please confirm that it fixes your problem on m68k ??
> 
> Sorry, that was on the wrong base.
> 
> Please try this one, against current net-next.
> 

First of all, excellent analysis!

With this patch applied:

Build reference: next-20190410-1-ge294005789ed

Building mcf5208evb:m5208:m5208evb_defconfig:initrd ... running .... passed
Building q800:m68040:mac_defconfig:initrd ... running .... passed
Building q800:m68040:mac_defconfig:rootfs ... running .... passed


I also reconfirmed the crash with next-20190410.

With that, feel free to add:

Tested-by: Guenter Roeck <linux@roeck-us.net>

Thanks,
Guenter