runtime/pprof: prevent a deadlock that SIGPROF might create on mips{,le}

64bit atomics on mips/mipsle are implemented using spinlocks. If SIGPROF
is received while the program is in the critical section, it will try to
write the sample using the same spinlock, creating a deadloop.
Prevent it by creating a counter of SIGPROFs during atomic64 and
postpone writing the sample(s) until called from elsewhere, with
pc set to _LostSIGPROFDuringAtomic64.

Added a test case, per Cherry's suggestion. Works around #20146.

Change-Id: Icff504180bae4ee83d78b19c0d9d6a80097087f9
Reviewed-on: https://go-review.googlesource.com/42652
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/runtime/cpuprof.go b/src/runtime/cpuprof.go
index c761e440b6..fb841a9f3d 100644
--- a/src/runtime/cpuprof.go
+++ b/src/runtime/cpuprof.go
@@ -163,6 +163,15 @@ func (p *cpuProfile) addExtra() {
 	}
 }
 
+func (p *cpuProfile) addLostAtomic64(count uint64) {
+	hdr := [1]uint64{count}
+	lostStk := [2]uintptr{
+		funcPC(_LostSIGPROFDuringAtomic64) + sys.PCQuantum,
+		funcPC(_System) + sys.PCQuantum,
+	}
+	cpuprof.log.write(nil, 0, hdr[:], lostStk[:])
+}
+
 // CPUProfile panics.
 // It formerly provided raw access to chunks of
 // a pprof-format profile generated by the runtime.