On 1/11/19 11:57 AM, Josh Poimboeuf wrote: > On Fri, Jan 11, 2019 at 05:46:36PM +0100, Alexandre Chartre wrote: >> >> >> On 01/11/2019 04:28 PM, Josh Poimboeuf wrote: >>> On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote: >>>> To avoid any issue with live patching the call instruction, what about >>>> toggling between two call instructions: one would be the currently active >>>> call, while the other would currently be inactive but to be used after a >>>> change is made. You can safely patch the inactive call and then toggle >>>> the call flow (using a jump label) between the active and inactive calls. >>>> >>>> So instead of having a single call instruction: >>>> >>>> call function >>>> >>>> You would have: >>>> >>>> STATIC_JUMP_IF_TRUE label, key >>>> call function1 >>>> jmp done >>>> label: >>>> call function2 >>>> done: >>>> >>>> If the key is set so that function1 is currently called then you can >>>> safely update the call instruction for function2. Once this is done, >>>> just flip the key to make the function2 call active. On a next update, >>>> you would, of course, have to switch and update the call for function1. >>> >>> What about the following race? >>> >>> CPU1 CPU2 >>> static key is false, doesn't jump >>> task gets preempted before calling function1 >>> change static key to true >>> start patching "call function1" >>> task resumes, sees inconsistent call instruction >>> >> >> If the function1 call is active then it won't be changed, you will change >> function2. However, I presume you can still have a race but if the function >> is changed twice before calling function1: >> >> CPU1 CPU2 >> static key is false, doesn't jump >> task gets preempted before calling function1 >> -- first function change -- >> patch "call function2" >> change static key to true >> -- second function change -- >> start patching "call >> function1" >> task resumes, sees inconsistent call instruction >> >> So right, that's a problem. > > Right, that's what I meant to say :-) >
could you use something like synchronize_rcu_tasks() between successive updates to guarantee nobody's stuck in the middle of the call instruction update? Yes its really slow but the update path is slow anyways. Thanks, -Jason
