runtime: don't use FP when calling nextSample in the Plan 9 sighandler
In the Go signal handler on Plan 9, when a signal with
the _SigThrow flag is received, we call startpanic before
printing the stack trace.
The startpanic function calls systemstack which calls
startpanic_m. In the startpanic_m function, we call
allocmcache to allocate _g_.m.mcache. The problem is
that allocmcache calls nextSample, which does a floating
point operation to return a sampling point for heap profiling.
However, Plan 9 doesn't support floating point in the
signal handler.
This change adds a new function nextSampleNoFP, only
called when in the Plan 9 signal handler, which is
similar to nextSample, but avoids floating point.
Change-Id: Iaa30437aa0f7c8c84d40afbab7567ad3bd5ea2de
Reviewed-on: https://go-review.googlesource.com/16307
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index b86d41f..23c15da 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -826,6 +826,13 @@
// distributed random number and applying the cumulative distribution
// function for an exponential.
func nextSample() int32 {
+ if GOOS == "plan9" {
+ // Plan 9 doesn't support floating point in note handler.
+ if g := getg(); g == g.m.gsignal {
+ return nextSampleNoFP()
+ }
+ }
+
period := MemProfileRate
// make nextSample not overflow. Maximum possible step is
@@ -855,6 +862,20 @@
return int32(qlog*(minusLog2*float64(period))) + 1
}
+// nextSampleNoFP is similar to nextSample, but uses older,
+// simpler code to avoid floating point.
+func nextSampleNoFP() int32 {
+ // Set first allocation sample size.
+ rate := MemProfileRate
+ if rate > 0x3fffffff { // make 2*rate not overflow
+ rate = 0x3fffffff
+ }
+ if rate != 0 {
+ return int32(int(fastrand1()) % (2 * rate))
+ }
+ return 0
+}
+
type persistentAlloc struct {
base unsafe.Pointer
off uintptr
