Re: linux-next test error: BUG: using smp_processor_id() in preemptible [ADDR] code: syz-fuzzer/6792

[Ritesh Harjani]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > 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