Ravi Bangoria <ravi.bango...@linux.ibm.com> writes: > Powerpc hw triggers watchpoint before executing the instruction. > To make trigger-after-execute behavior, kernel emulates the > instruction. If the instruction is 'load something into non- > volatile register', exception handler should restore emulated > register state while returning back, otherwise there will be > register state corruption. Ex, Adding a watchpoint on a list > can corrput the list: > > # cat /proc/kallsyms | grep kthread_create_list > c00000000121c8b8 d kthread_create_list > > Add watchpoint on kthread_create_list->next: > > # perf record -e mem:0xc00000000121c8c0 > > Run some workload such that new kthread gets invoked. Ex, I > just logged out from console: > > list_add corruption. next->prev should be prev (c000000001214e00), \ > but was c00000000121c8b8. (next=c00000000121c8b8). > WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0 > CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69 > ... > NIP __list_add_valid+0xb4/0xc0 > LR __list_add_valid+0xb0/0xc0 > Call Trace: > __list_add_valid+0xb0/0xc0 (unreliable) > __kthread_create_on_node+0xe0/0x260 > kthread_create_on_node+0x34/0x50 > create_worker+0xe8/0x260 > worker_thread+0x444/0x560 > kthread+0x160/0x1a0 > ret_from_kernel_thread+0x5c/0x70
This all depends on what code the compiler generates for the list access. Can you include a disassembly of the relevant code in your kernel so we have an example of the bad case. > diff --git a/arch/powerpc/kernel/exceptions-64s.S > b/arch/powerpc/kernel/exceptions-64s.S > index 9481a11..96de0d1 100644 > --- a/arch/powerpc/kernel/exceptions-64s.S > +++ b/arch/powerpc/kernel/exceptions-64s.S > @@ -1753,7 +1753,7 @@ handle_dabr_fault: > ld r5,_DSISR(r1) > addi r3,r1,STACK_FRAME_OVERHEAD > bl do_break > -12: b ret_from_except_lite > +12: b ret_from_except This probably warrants a comment explaining why we can't use the (badly named) "lite" version. cheers