On 17/06/16 19:33, Alex Bennée wrote: > Last time I went through the MTTCG code the access to the > break/watchpoint code was annotated with "RCU?". The code currently > gets away with avoiding locks for the gdbstub as the guest execution > state is usually halted. However when used for modelling architectural > debug registers there is no such protection.
I'm not so sure if there's any architecture which permits changing breakpoins/watchpoints of one core from another. > The patch series changes things in stages. > > First we move the break/watchpoints into an array which is more > amenable to RCU control that the QLIST. We then control the life time > of references to break/watchpoint data by removing long held > references in the target code and getting information when needed from > the core. Then we stop dynamically allocation the watch/breakpoint > data and store it directly in the array which makes iteration across > the list a bit more cache friendly than referenced pointers. Finally > addition and removal of elements of the array is put under RCU > control. This ensures there is always a safe array of data to check > in the run-loop. I a little bit unsure if we really want to complicate things with RCU. Why don't we simply protect the lists with a mutex given that there's no contention expected? BTW, as it comes to debugging, I suppose we don't expect great performance anyway. Kind regards, Sergey
