linux-kernel - Re: [PATCH] MIPS: Fix using smp_processor_id() in preemptible in show

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Date:   Thu, 25 Nov 2021 19:32:39 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Xuefeng Li <lixuefeng@...ngson.cn>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Fix using smp_processor_id() in preemptible in
 show_cpuinfo()

On 11/25/2021 05:34 PM, Thomas Bogendoerfer wrote:
> On Mon, Nov 22, 2021 at 06:17:37PM +0800, Tiezhu Yang wrote:
>> There exists the following issue under DEBUG_PREEMPT:
>>
>>  BUG: using smp_processor_id() in preemptible [00000000] code: systemd/1
>>  caller is show_cpuinfo+0x460/0xea0
>>  ...
>>  Call Trace:
>>  [<ffffffff8020f0dc>] show_stack+0x94/0x128
>>  [<ffffffff80e6cab4>] dump_stack_lvl+0x94/0xd8
>>  [<ffffffff80e74c5c>] check_preemption_disabled+0x104/0x110
>>  [<ffffffff802209c8>] show_cpuinfo+0x460/0xea0
>>  [<ffffffff80539d54>] seq_read_iter+0xfc/0x4f8
>>  [<ffffffff804fcc10>] new_sync_read+0x110/0x1b8
>>  [<ffffffff804ff57c>] vfs_read+0x1b4/0x1d0
>>  [<ffffffff804ffb18>] ksys_read+0xd0/0x110
>>  [<ffffffff8021c090>] syscall_common+0x34/0x58
>>
>> We can see the following call trace:
>>  show_cpuinfo()
>>    cpu_has_fpu
>>      current_cpu_data
>>        smp_processor_id()
>>
>>  $ addr2line -f -e vmlinux 0xffffffff802209c8
>>  show_cpuinfo
>>  arch/mips/kernel/proc.c:188
>>
>>  $ head -188 arch/mips/kernel/proc.c | tail -1
>> 	 if (cpu_has_fpu)
>>
>>  arch/mips/include/asm/cpu-features.h
>>  #  define cpu_has_fpu		(current_cpu_data.options & MIPS_CPU_FPU)
>>
>>  arch/mips/include/asm/cpu-info.h
>>  #define current_cpu_data cpu_data[smp_processor_id()]
>>
>> Based on the above analysis, fix the issue by disabling preemption
>> around cpu_has_fpu in show_cpuinfo().
>>
>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>
> missing
>
> Fixes: 626bfa037299 ("MIPS: kernel: proc: add CPU option reporting")

Thank you, will add in v2.

>
>> ---
>>  arch/mips/kernel/proc.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/mips/kernel/proc.c b/arch/mips/kernel/proc.c
>> index 376a6e2..c6c2661 100644
>> --- a/arch/mips/kernel/proc.c
>> +++ b/arch/mips/kernel/proc.c
>> @@ -185,8 +185,10 @@ static int show_cpuinfo(struct seq_file *m, void *v)
>>  		seq_puts(m, " tx39_cache");
>>  	if (cpu_has_octeon_cache)
>>  		seq_puts(m, " octeon_cache");
>> +	preempt_disable();
>>  	if (cpu_has_fpu)
>>  		seq_puts(m, " fpu");
>> +	preempt_enable();
>
> what about using raw_cpu_has_fpu() instead ?

OK, no problem, using raw_cpu_has_fpu can also fix the issue and looks 
better, I will send v2 soon.

Thanks,
Tiezhu

>
> Thomas.
>