[PATCH] MIPS: Fix using smp_processor_id() in preemptible in show_cpuinfo()

[Tiezhu Yang]

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@xxxxxxxxxxx>
---
 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();
 	if (cpu_has_32fpr)
 		seq_puts(m, " 32fpr");
 	if (cpu_has_cache_cdex_p)
-- 
2.1.0