MIPS: ASID conflict after CPU hotplug

I am running SMP Linux 2.6.37-rc1 on BMIPS5000 (single core dual thread)
and observe some abnormalities when doing system suspend/resume which I
narrowed down to cpu hotplugging. The suspend brings the second thread
processor down and then restarts it, after which I see memory corruption
in userspace. I started digging and found out that problem occurs because
while doing execve() the child process is getting the same ASID as the
parent, which obviously corrupts parent's address space.

Further digging showed that activate_mm() calls get_new_mmu_context() to
get a new ASID, but at this time ASID field in entryHi is 1, and
asid_cache(cpu) is 0x100 (it was just reset to ASID_FIRST_VERSION when
the secondary TP was booting).

So, get_new_mmu_context() increments the asid_cache(cpu) value to
0x101, and thus puts 0x01 into entryHi. The result - ASID field does
not get changed as it was supposed to.

My solution is very simple - do not reset asid_cache(cpu) on TP warm
restart.

Patchwork: https://patchwork.linux-mips.org/patch/1797/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -1596,7 +1596,8 @@ void __cpuinit per_cpu_trap_init(void)
 	}
 #endif /* CONFIG_MIPS_MT_SMTC */
 
-	cpu_data[cpu].asid_cache = ASID_FIRST_VERSION;
+	if (!cpu_data[cpu].asid_cache)
+		cpu_data[cpu].asid_cache = ASID_FIRST_VERSION;
 
 	atomic_inc(&init_mm.mm_count);
 	current->active_mm = &init_mm;