…a g"

Updates #18700 (backport)

Currently there are no diagnostics for mark root check during marking.
Fix this by printing out the same diagnostics we print during mark
termination.

Also, drop the allglock before throwing. Holding that across a throw
causes a self-deadlock with tracebackothers.

For #16083.

Change-Id: Ib605f3ae0c17e70704b31d8378274cfaa2307dc2
Reviewed-on: https://go-review.googlesource.com/35677
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

…k roots

Fixes #18700 (backport)

Currently we check that all roots are marked as soon as gcMarkDone
decides to transition from mark 1 to mark 2. However, issue #16083
indicates that there may be a race where we try to complete mark 1
while a worker is still scanning a stack, causing the root mark check
to fail.

We don't yet understand this race, but as a simple mitigation, move
the root check to after gcMarkDone performs a ragged barrier, which
will force any remaining workers to finish their current job.

Updates #16083. This may "fix" it, but it would be better to
understand and fix the underlying race.

Change-Id: I1af9ce67bd87ade7bc2a067295d79c28cd11abd2
Reviewed-on: https://go-review.googlesource.com/35678
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>