[release-branch.go1.14] runtime: don't send preemption signal if there is a signal...

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>

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 99eb19eb0c02e270eba165905bd6169736dc3bbc..2a872bfba9258805ad6f6bf74619c08906eaf4d7 100644 (file)
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -540,6 +540,10 @@ type m struct {
        // requested, but fails. Accessed atomically.
        preemptGen uint32
 
+       // Whether this is a pending preemption signal on this M.
+       // Accessed atomically.
+       signalPending uint32
+
        dlogPerM
 
        mOS

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index d2e669380510cc3ed487f7378f5e1e5545dd4011..f18e6b5785d4e43f9a904e8d6a031e25670ffa70 100644 (file)
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -333,6 +333,7 @@ func doSigPreempt(gp *g, ctxt *sigctxt) {
 
        // Acknowledge the preemption.
        atomic.Xadd(&gp.m.preemptGen, 1)
+       atomic.Store(&gp.m.signalPending, 0)
 }
 
 const preemptMSupported = pushCallSupported
@@ -359,7 +360,14 @@ func preemptM(mp *m) {
                // required).
                return
        }
-       signalM(mp, sigPreempt)
+       if atomic.Cas(&mp.signalPending, 0, 1) {
+               // If multiple threads are preempting the same M, it may send many
+               // signals to the same M such that it hardly make progress, causing
+               // live-lock problem. Apparently this could happen on darwin. See
+               // issue #37741.
+               // Only send a signal if there isn't already one pending.
+               signalM(mp, sigPreempt)
+       }
 }
 
 // sigFetchG fetches the value of G safely when running in a signal handler.