…visible to checkdead
If no M is available, startm first grabs an idle P, then drops
sched.lock and calls newm to start a new M to run than P.
Unfortunately, that leaves a window in which a G (e.g., returning from a
syscall) may find no idle P, add to the global runq, and then in stopm
discover that there are no running M's, a condition that should be
impossible with runnable G's.
To avoid this condition, we pre-allocate the new M ID in startm before
dropping sched.lock. This ensures that checkdead will see the M as
running, and since that new M must eventually run the scheduler, it will
handle any pending work as necessary.
Outside of startm, most other calls to newm/allocm don't have a P at
all. The only exception is startTheWorldWithSema, which always has an M
if there is 1 P (i.e., the currently running M), and if there is >1 P
the findrunnable spinning dance ensures the problem never occurs.
This has been tested with strategically placed sleeps in the runtime to
help induce the correct race ordering, but the timing on this is too
narrow for a test that can be checked in.
Run-TryBot: Michael Pratt <email@example.com>
TryBot-Result: Gobot Gobot <firstname.lastname@example.org>
Reviewed-by: Austin Clements <email@example.com>
(cherry picked from commit 85afa2e)