[release-branch.go1.17] runtime: fix net poll races

The netpoll code was written long ago, when the
only multiprocessors that Go ran on were x86.
It assumed that an atomic store would trigger a
full memory barrier and then used that barrier
to order otherwise racy access to a handful of fields,
including pollDesc.closing.

On ARM64, this code has finally failed, because
the atomic store is on a value completely unrelated
to any of the racily-accessed fields, and the ARMv8
hardware, unlike x86, is clever enough not to do a
full memory barrier for a simple atomic store.
We are seeing a constant background rate of trybot
failures where the net/http tests deadlock - a netpollblock
has clearly happened after the pollDesc has begun to close.

The code that does the racy reads is netpollcheckerr,
which needs to be able to run without acquiring a lock.
This CL fixes the race, without introducing unnecessary
inefficiency or deadlock, by arranging for every updater
of the relevant fields to publish a summary as a single
atomic uint32, and then having netpollcheckerr use a
single atomic load to fetch the relevant bits and then
proceed as before.

For #45211
Fixes #50611

Change-Id: Ib6788c8da4d00b7bda84d55ca3fdffb5a64c1a0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/378234
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Trust: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 17b2fb1b656a275906b5071c562439d50a27f167)
Reviewed-on: https://go-review.googlesource.com/c/go/+/392714
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

1 files changed, 1 insertions, 4 deletions

diff --git a/src/runtime/netpoll_aix.go b/src/runtime/netpoll_aix.go
index 4590ed81a60..90950af4447 100644
--- a/src/runtime/netpoll_aix.go
+++ b/src/runtime/netpoll_aix.go
@@ -212,10 +212,7 @@ retry:
 			pfd.events &= ^_POLLOUT
 		}
 		if mode != 0 {
-			pds[i].everr = false
-			if pfd.revents == _POLLERR {
-				pds[i].everr = true
-			}
+			pds[i].setEventErr(pfd.revents == _POLLERR)
 			netpollready(&toRun, pds[i], mode)
 			n--
 		}