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Subject: Re: linux-next test error: BUG: using smp_processor_id() in
 preemptible [ADDR] code: syz-fuzzer/6792
To:     Hillf Danton <hdanton@sina.com>
Cc:     syzbot <syzbot+82f324bb69744c5f6969@syzkaller.appspotmail.com>,
        adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-next@vger.kernel.org,
        sfr@canb.auug.org.au, syzkaller-bugs@googlegroups.com,
        tytso@mit.edu
References: <20200602145256.9236-1-hdanton@sina.com>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Wed, 3 Jun 2020 15:36:47 +0530
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On 6/2/20 8:22 PM, Hillf Danton wrote:
> 
> Tue, 02 Jun 2020 04:20:16 -0700
>> syzbot found the following crash on:
>>
>> HEAD commit:    0e21d462 Add linux-next specific files for 20200602
>> git tree:       linux-next
>> console output: https://syzkaller.appspot.com/x/log.txt?x=127233ee100000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=ecc1aef35f550ee3
>> dashboard link: https://syzkaller.appspot.com/bug?extid=82f324bb69744c5f6969
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+82f324bb69744c5f6969@syzkaller.appspotmail.com
>>
>> BUG: using smp_processor_id() in preemptible [00000000] code: syz-fuzzer/6792
>> caller is ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
> 
> Fix 42f56b7a4a7d ("ext4: mballoc: introduce pcpu seqcnt for freeing PA
> to improve ENOSPC handling") by redefining discard_pa_seq to be a simple
> regular sequence counter to axe the need of percpu operation.

Why remove percpu seqcnt? IIUC, percpu are much better in case of a 
multi-threaded use case which could run and allocate blocks in parallel.
Whereas a updating a simple variable across different cpus may lead to 
cacheline bouncing problem.
Since in this case we can very well have a use case of multiple threads 
trying to allocate blocks at the same time, so why change this to a 
simple seqcnt from percpu seqcnt?

-ritesh