• Keith Randall's avatar
    reflect: ensure correct scanning of return values · ef503739
    Keith Randall authored
    During a call to a reflect-generated function or method (via
    makeFuncStub or methodValueCall), when should we scan the return
    values?
    
    When we're starting a reflect call, the space on the stack for the
    return values is not initialized yet, as it contains whatever junk was
    on the stack of the caller at the time. The return space must not be
    scanned during a GC.
    
    When we're finishing a reflect call, the return values are
    initialized, and must be scanned during a GC to make sure that any
    pointers in the return values are found and their referents retained.
    
    When the GC stack walk comes across a reflect call in progress on the
    stack, it needs to know whether to scan the results or not. It doesn't
    know the progress of the reflect call, so it can't decide by
    itself. The reflect package needs to tell it.
    
    This CL adds another slot in the frame of makeFuncStub and
    methodValueCall so we can put a boolean in there which tells the
    runtime whether to scan the results or not.
    
    This CL also adds the args length to reflectMethodValue so the
    runtime can restrict its scanning to only the args section (not the
    results) if the reflect package says the results aren't ready yet.
    
    Do a delicate dance in the reflect package to set the "results are
    valid" bit. We need to make sure we set the bit only after we've
    copied the results back to the stack. But we must set the bit before
    we drop reflect's copy of the results. Otherwise, we might have a
    state where (temporarily) no one has a live copy of the results.
    That's the state we were observing in issue #27695 before this CL.
    
    The bitmap used by the runtime currently contains only the args.
    (Actually, it contains all the bits, but the size is set so we use
    only the args portion.) This is safe for early in a reflect call, but
    unsafe late in a reflect call. The test issue27695.go demonstrates
    this unsafety. We change the bitmap to always include both args
    and results, and decide at runtime which portion to use.
    
    issue27695.go only has a test for method calls. Function calls were ok
    because there wasn't a safepoint between when reflect dropped its copy
    of the return values and when the caller is resumed. This may change
    when we introduce safepoints everywhere.
    
    This truncate-to-only-the-args was part of CL 9888 (in 2015). That
    part of the CL fixed the problem demonstrated in issue27695b.go but
    introduced the problem demonstrated in issue27695.go.
    
    TODO, in another CL: simplify FuncLayout and its test. stack return
    value is now identical to frametype.ptrdata + frametype.gcdata.
    
    Fixes #27695
    
    Change-Id: I2d49b34e34a82c6328b34f02610587a291b25c5f
    Reviewed-on: https://go-review.googlesource.com/137440
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarAustin Clements <austin@google.com>
    ef503739
value.go 78.1 KB