From fae6eb5f2fc422447e3ca324c895503e9166d247 Mon Sep 17 00:00:00 2001 From: Russ Cox Date: Wed, 27 Mar 2024 10:08:25 -0400 Subject: runtime: fix timer race introduced in CL 573455 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit There is a short window when timers.adjust could miss a timer update. Close that window. Does not change benchmark. goos: linux goarch: amd64 pkg: time cpu: AMD Ryzen 9 7950X 16-Core Processor │ s7base.txt │ s7.txt │ │ sec/op │ sec/op vs base │ AdjustTimers10000-32 239.9µ ± 5% 237.6µ ± 8% ~ (p=0.631 n=10) AdjustTimers10000SingleThread-32 1.686m ± 8% 1.710m ± 5% ~ (p=0.481 n=10) AdjustTimers10000NoReset-32 194.1µ ± 1% 190.8µ ± 2% -1.69% (p=0.023 n=10) AdjustTimers10000NoSleep-32 226.2µ ± 3% 222.9µ ± 3% ~ (p=0.143 n=10) AdjustTimers10000NoResetNoSleep-32 182.9µ ± 1% 180.9µ ± 2% ~ (p=0.165 n=10) goos: darwin goarch: arm64 pkg: time cpu: Apple M3 Pro │ m3base.txt │ m3.txt │ │ sec/op │ sec/op vs base │ AdjustTimers10000-12 269.3µ ± 2% 267.0µ ± 2% ~ (p=0.529 n=10) AdjustTimers10000SingleThread-12 1.176m ± 1% 1.213m ± 1% +3.15% (p=0.000 n=10) AdjustTimers10000NoReset-12 262.6µ ± 2% 261.4µ ± 3% ~ (p=0.123 n=10) AdjustTimers10000NoSleep-12 247.8µ ± 1% 246.5µ ± 1% ~ (p=0.393 n=10) AdjustTimers10000NoResetNoSleep-12 231.0µ ± 1% 232.3µ ± 1% ~ (p=0.684 n=10) Change-Id: Ifdfcdd5a25046027912a8b306644bde7ec2d3214 Reviewed-on: https://go-review.googlesource.com/c/go/+/574741 Auto-Submit: Russ Cox LUCI-TryBot-Result: Go LUCI Reviewed-by: Ian Lance Taylor --- src/runtime/time.go | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/src/runtime/time.go b/src/runtime/time.go index b696c837ab..f9335c95f8 100644 --- a/src/runtime/time.go +++ b/src/runtime/time.go @@ -526,6 +526,9 @@ func (t *timer) modify(when, period int64, f func(arg any, seq uintptr, delay in // See comment in type timer above and in timers.adjust below. if when < t.whenHeap { wake = true + // Force timerModified bit out to t.astate before updating t.minWhenModified, + // to synchronize with t.ts.adjust. See comment in adjust. + t.astate.Store(t.state) t.ts.updateMinWhenModified(when) } } @@ -785,6 +788,15 @@ func (ts *timers) adjust(now int64, force bool) { // The wakeTime method implementation reads minWhenModified *before* minWhenHeap, // so that if the minWhenModified is observed to be 0, that means the minWhenHeap that // follows will include the information that was zeroed out of it. + // + // Originally Step 3 locked every timer, which made sure any timer update that was + // already in progress during Steps 1+2 completed and was observed by Step 3. + // All that locking was too expensive, so now we do an atomic load of t.astate to + // decide whether we need to do a full lock. To make sure that we still observe any + // timer update already in progress during Steps 1+2, t.modify sets timerModified + // in t.astate *before* calling t.updateMinWhenModified. That ensures that the + // overwrite in Step 2 cannot lose an update: if it does overwrite an update, Step 3 + // will see the timerModified and do a full lock. ts.minWhenHeap.Store(ts.wakeTime()) ts.minWhenModified.Store(0) -- cgit v1.2.3-54-g00ecf