• Austin Clements's avatar
    runtime: remove write barriers from newstack, gogo · 3beaf26e
    Austin Clements authored
    Currently, newstack and gogo have write barriers for maintaining the
    context register saved in g.sched.ctxt. This is troublesome, because
    newstack can be called from go:nowritebarrierrec places that can't
    allow write barriers. It happens to be benign because g.sched.ctxt
    will always be nil on entry to newstack *and* it so happens the
    incoming ctxt will also always be nil in these contexts (I
    think/hope), but this is playing with fire. It's also desirable to
    mark newstack go:nowritebarrierrec to prevent any other, non-benign
    write barriers from creeping in, but we can't do that right now
    because of this one write barrier.
    
    Fix all of this by observing that g.sched.ctxt is really just a saved
    live pointer register. Hence, we can shade it when we scan g's stack
    and otherwise move it back and forth between the actual context
    register and g.sched.ctxt without write barriers. This means we can
    save it in morestack along with all of the other g.sched, eliminate
    the save from newstack along with its troublesome write barrier, and
    eliminate the shenanigans in gogo to invoke the write barrier when
    restoring it.
    
    Once we've done all of this, we can mark newstack
    go:nowritebarrierrec.
    
    Fixes #22385.
    For #22460.
    
    Change-Id: I43c24958e3f6785b53c1350e1e83c2844e0d1522
    Reviewed-on: https://go-review.googlesource.com/72553
    Run-TryBot: Austin Clements <austin@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarRick Hudson <rlh@golang.org>
    Reviewed-by: 's avatarCherry Zhang <cherryyz@google.com>
    3beaf26e
stack.go 34.2 KB