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Currently, markroot delays scanning mark worker stacks until mark termination by putting the mark worker G directly on the rescan list when it encounters one during the mark phase. Without this, since mark workers are non-preemptible, two mark workers that attempt to scan each other's stacks can deadlock. However, this is annoyingly asymmetric and causes some real problems. First, markroot does not own the G at that point, so it's not technically safe to add it to the rescan list. I haven't been able to find a specific problem this could cause, but I suspect it's the root cause of issue #17099. Second, this will interfere with the hybrid barrier, since there is no stack rescanning during mark termination with the hybrid barrier. This commit switches to a different approach. We move the mark worker's call to gcDrain to the system stack and set the mark worker's status to _Gwaiting for the duration of the drain to indicate that it's preemptible. This lets another mark worker scan its G stack while the drain is running on the system stack. We don't return to the G stack until we can switch back to _Grunning, which ensures we don't race with a stack scan. This lets us eliminate the special case for mark worker stack scans and scan them just like any other goroutine. The only subtlety to this approach is that we have to disable stack shrinking for mark workers; they could be referring to captured variables from the G stack, so it's not safe to move their stacks. Updates #17099 and #17503. Change-Id: Ia5213949ec470af63e24dfce01df357c12adbbea Reviewed-on: https://go-review.googlesource.com/31820 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rick Hudson <rlh@golang.org> |
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