On Tue, Oct 18, 2016 at 2:53 PM, Ian Lance Taylor <i...@golang.org> wrote:
> On Tue, Oct 18, 2016 at 2:45 PM, Joshua Liebow-Feeser <he...@joshlf.com> > wrote: > > > > On Tue, Oct 18, 2016 at 2:40 PM, Joshua Liebow-Feeser <he...@joshlf.com> > > wrote: > >> > >> > >> > >> On Tue, Oct 18, 2016 at 2:16 PM, Ian Lance Taylor <i...@golang.org> > wrote: > >>> > >>> On Tue, Oct 18, 2016 at 12:30 PM, Joshua Liebow-Feeser < > he...@joshlf.com> > >>> wrote: > >>> > > >>> > I'm playing around with implementing a wait-free channel in the > runtime > >>> > package, and as part of this, it'd be really nice to have double-word > >>> > compare-and-swap (CAS). Barring that, however, for my purposes, it > >>> > would > >>> > actually be fine to have a one-word value that encodes both a pointer > >>> > and > >>> > some extra information using bit packing. The problem, though, is > that > >>> > if I > >>> > store this value as, for example, a uintptr, the GC may not realize > >>> > that > >>> > it's a pointer. So my question is: are there any bits in a pointer > >>> > which, > >>> > when modified, won't mess with the GC? Note that since this is > >>> > implemented > >>> > in the runtime, I'm totally OK with relying on behavior specific to > the > >>> > current GC implementation. > >>> > >>> See runtime/lfstack*.go. > >> > >> Awesome, thanks! > > > > > > Actually, quick follow-up. I noticed that the lfstack implementation > > side-steps the GC issue by just not keeping pointers. That might work > for me > > if I just store runtime.g pointers, but that raises another question: can > > the GC ever free g's, or are they just explicitly freed when a goroutine > > quits? That is, is it safe for me to store a pointer/counter hybrid like > in > > lfstack - where that pointer is a *g - and assume that the GC won't > collect > > the g from out from under me? > > For the specific case of a g, this is safe at the moment. The current > Go runtime caches all g's and never releases them. See gfget and > gfput in runtime/proc.go. > OK great. And they won't ever be moved? (Come to think of it, is pointer rewriting only ever a thing on the stack?) > > Ian > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
