8cb350d69a
CL 310850 dropped work re-checks on non-spinning Ms to fix #43997. This introduced a new race condition: a non-spinning M may drop its P and then park at the same time a spinning M attempts to wake a P to handle some new work. The spinning M fails to find an idle P (because the non-spinning M hasn't quite made its P idle yet), and does nothing assuming that the system is fully loaded. This results in loss of work conservation. In the worst case we could have a complete deadlock if injectglist fails to wake anything just as all Ps are going idle. sched.needspinning adds new synchronization to cover this case. If work submission fails to find a P, it sets needspinning to indicate that a spinning M is required. When non-spinning Ms prepare to drop their P, they check needspinning and abort going idle to become a spinning M instead. This addresses the race without extra spurious wakeups. In the normal (non-racing case), an M will become spinning via the normal path and clear the flag. injectglist must change in addition to wakep because it is a similar form of work submission, notably used following netpoll at a point when we might not have a P that would guarantee the work runs. Fixes #45867 Change-Id: Ieb623a6d4162fb8c2be7b4ff8acdebcc3a0d69a8 Reviewed-on: https://go-review.googlesource.com/c/go/+/389014 TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Run-TryBot: Michael Pratt <mpratt@google.com> Auto-Submit: Michael Pratt <mpratt@google.com> |
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