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Date:   Thu, 25 Nov 2021 10:34:20 +0100
From:   Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To:     Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Xuefeng Li <lixuefeng@loongson.cn>, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] MIPS: Fix using smp_processor_id() in preemptible in
 show_cpuinfo()
Message-ID: <20211125093420.GB6537@alpha.franken.de>
References: <1637576257-11590-1-git-send-email-yangtiezhu@loongson.cn>
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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@loongson.cn>

missing 

Fixes: 626bfa037299 ("MIPS: kernel: proc: add CPU option reporting")

> ---
>  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 ?

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
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