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Date:   Wed, 3 Jun 2020 15:36:47 +0530
From:   Ritesh Harjani <>
To:     Hillf Danton <>
Cc:     syzbot <>,,,,,,,
Subject: Re: linux-next test error: BUG: using smp_processor_id() in
 preemptible [ADDR] code: syz-fuzzer/6792

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:
>> kernel config:
>> dashboard link:
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by:
>> 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?


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