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Austin Clements authored
Currently shinkstack is only safe during STW because it adjusts channel-related stack pointers and moves send/receive stack slots without synchronizing with the channel code. Make it safe to use when the world isn't stopped by: 1) Locking all channels the G is blocked on while adjusting the sudogs and copying the area of the stack that may contain send/receive slots. 2) For any stack frames that may contain send/receive slot, using an atomic CAS to adjust pointers to prevent races between adjusting a pointer in a receive slot and a concurrent send writing to that receive slot. In principle, the synchronization could be finer-grained. For example, we considered synchronizing around the sudogs, which would allow channel operations involving other Gs to continue if the G being shrunk was far enough down the send/receive queue. However, using the channel lock means no additional locks are necessary in the channel code. Furthermore, the stack shrinking code holds the channel lock for a very short time (much less than the time required to shrink the stack). This does not yet make stack shrinking concurrent; it merely makes doing so safe. This has negligible effect on the go1 and garbage benchmarks. For #12967. Change-Id: Ia49df3a8a7be4b36e365aac4155a2416b94b988c Reviewed-on: https://go-review.googlesource.com/20042Reviewed-by:
Keith Randall <khr@golang.org> Run-TryBot: Austin Clements <austin@google.com>
276b1777