commit 2d8c1995b9c4123ab7ac35a1ba1497b290755d4d
author Ian Lance Taylor <iant@golang.org> Fri Nov 15 10:05:13 2019 -0800
committer Ian Lance Taylor <iant@golang.org> Tue Nov 19 02:41:53 2019 +0000
tree adc9d10519ff2283666ab56f65a5656e548e7ef5
parent 3efa09f3862b68f0dfa5012455f475836e040e91

runtime: release timersLock while running timer

Dan Scales pointed out a theoretical deadlock in the runtime.

The timer code runs timer functions while holding the timers lock for a P.
The scavenger queues up a timer function that calls wakeScavenger,
which acquires the scavenger lock.

The scavengeSleep function acquires the scavenger lock,
then calls resetTimer which can call addInitializedTimer
which acquires the timers lock for the current P.

So there is a potential deadlock, in that the scavenger lock and
the timers lock for some P may both be acquired in different order.
It's not clear to me whether this deadlock can ever actually occur.

Issue 35532 describes another possible deadlock.

The pollSetDeadline function acquires pd.lock for some poll descriptor,
and in some cases calls resettimer which can in some cases acquire
the timers lock for the current P.

The timer code runs timer functions while holding the timers lock for a P.
The timer function for poll descriptors winds up in netpolldeadlineimpl
which acquires pd.lock.

So again there is a potential deadlock, in that the pd lock for some
poll descriptor and the timers lock for some P may both be acquired in
different order. I think this can happen if we change the deadline
for a network connection exactly as the former deadline expires.

Looking at the code, I don't see any reason why we have to hold
the timers lock while running a timer function.
This CL implements that change.

Updates #6239
Updates #27707
Fixes #35532

Change-Id: I17792f5a0120e01ea07cf1b2de8434d5c10704dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/207348
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>