Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/split_lock: Make life miserable for split lockers | Date | Tue, 08 Mar 2022 15:59:39 +0100 |
| |
Tony,
On Mon, Mar 07 2022 at 16:37, Tony Luck wrote: > On Mon, Mar 07, 2022 at 11:30:35PM +0100, Thomas Gleixner wrote: >> On Wed, Feb 16 2022 at 17:27, Tony Luck wrote: >> > Questions for this RFC: >> > >> > 1) Does this need to be a new option? Maybe just update the >> > existing "warn" mode to add this level of extra pain. >> >> That's fine. Warn is the default today, right? > > Yes. Warn is the current default. > Does "That's fine" mean ok to change exiting warn code to add > this level of pain? Or OK to add a new option?
Add pain to the existing warn code.
>> The question is whether this is something to worry about. If so, then we >> need to go back to the drawing board. > > I don't think it is worth worrying about. The case you describe is > a process that is about to be preempted when the #AC trap happens. > In that case this CPU (in fact both HT threads on this core) get > two jiffies of free split locks. Cases from here: > > 1) The original process gets to run on either of these threads > before the timeout. They get to execute their split lock and carry > on running. > > 2) The process is scheduled on a different core during the two jiffie > window. They take an #AC trap and block on the semaphore until the > original core releases. Then they get their chance to run on this new > core. > > 3) The original process doesn't get rescheduled for two jiffies, then > runs somewhere. The original core has released the sempahore and re-enabled > split lock checking. So the process takes #AC, gets the semaphore, kernel > disables split lock checking ... and we try again. > > Now it is possible that the process may repeatedly be preempted in between > getting the semaphore and actually getting all the way to user space > to split a lock ... but can only happen if there are multiple processes > splitting locks. The goal of this patch is to be mean to all of them. If > we happen to be extra mean to some of them, well so be it.
Fair enough.
I still do not like the inconsistent state between the TIF flag and the SLD MSR.
Thanks,
tglx
| |