[release-branch.go1.14] runtime: don't send preemption signal if there is a signal pending
If multiple threads call preemptone to preempt the same M, it may
send many signals to the same M such that it hardly make
progress, causing live-lock problem. Only send a signal if there
isn't already one pending.
Updates #37741.
Fixes #37833.
Change-Id: Id94adb0b95acbd18b23abe637a8dcd81ab41b452
Reviewed-on: https://go-review.googlesource.com/c/go/+/223737
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit
0c0e8f224d5724e317952f77d215a752a3a7b7d9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/223939
Reviewed-by: Austin Clements <austin@google.com>