3.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jose Abreu <Jose.Abreu@xxxxxxxxxxxx> commit 1ee55a8f7f6b7ca4c0c59e0b4b4e3584a085c2d3 upstream. I recently came upon a scenario where I would get a double fault machine check exception tiriggered by a kernel module. However the ensuing crash stacktrace (ksym lookup) was not working correctly. Turns out that machine check auto-disables MMU while modules are allocated in kernel vaddr spapce. This patch re-enables the MMU before start printing the stacktrace making stacktracing of modules work upon a fatal exception. Signed-off-by: Jose Abreu <joabreu@xxxxxxxxxxxx> Reviewed-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx> [vgupta: moved code into low level handler to avoid in 2 places] Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/arc/kernel/entry.S | 6 ++++++ arch/arc/mm/tlb.c | 3 --- 2 files changed, 6 insertions(+), 3 deletions(-) --- a/arch/arc/kernel/entry.S +++ b/arch/arc/kernel/entry.S @@ -315,6 +315,12 @@ ENTRY(EV_MachineCheck) lr r0, [efa] mov r1, sp + ; hardware auto-disables MMU, re-enable it to allow kernel vaddr + ; access for say stack unwinding of modules for crash dumps + lr r3, [ARC_REG_PID] + or r3, r3, MMU_ENABLE + sr r3, [ARC_REG_PID] + lsr r3, r2, 8 bmsk r3, r3, 7 brne r3, ECR_C_MCHK_DUP_TLB, 1f --- a/arch/arc/mm/tlb.c +++ b/arch/arc/mm/tlb.c @@ -689,9 +689,6 @@ void do_tlb_overlap_fault(unsigned long local_irq_save(flags); - /* re-enable the MMU */ - write_aux_reg(ARC_REG_PID, MMU_ENABLE | read_aux_reg(ARC_REG_PID)); - /* loop thru all sets of TLB */ for (set = 0; set < mmu->sets; set++) {