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.


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