On Wed, Mar 18, 2015 at 10:45:55PM -0400, Waiman Long wrote: > On 03/16/2015 09:16 AM, Peter Zijlstra wrote: > I do have some concern about this call site patching mechanism as the > modification is not atomic. The spin_unlock() calls are in many places in > the kernel. There is a possibility that a thread is calling a certain > spin_unlock call site while it is being patched by another one with the > alternative() function call. > > So far, I don't see any problem with bare metal where paravirt_patch_insns() > is used to patch it to the move instruction. However, in a virtual guest > enivornment where paravirt_patch_call() was used, there were situations > where the system panic because of page fault on some invalid memory in the > kthread. If you look at the paravirt_patch_call(), you will see: > > : > b->opcode = 0xe8; /* call */ > b->delta = delta; > > If another CPU reads the instruction at the call site at the right moment, > it will get the modified call instruction, but not the new delta value. It > will then jump to a random location. I believe that was causing the system > panic that I saw. > > So I think it is kind of risky to use it here unless we can guarantee that > call site patching is atomic wrt other CPUs. Just look at where the patching is done: init/main.c:start_kernel() check_bugs() alternative_instructions() apply_paravirt() We're UP and not holding any locks, disable IRQs (see text_poke_early()) and have NMIs 'disabled'. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html