diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go index 4fa14996c2e..c71f856f098 100644 --- a/src/runtime/malloc.go +++ b/src/runtime/malloc.go @@ -1016,6 +1016,14 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer { // Align tiny pointer for required (conservative) alignment. if size&7 == 0 { off = alignUp(off, 8) + } else if sys.PtrSize == 4 && size == 12 { + // Conservatively align 12-byte objects to 8 bytes on 32-bit + // systems so that objects whose first field is a 64-bit + // value is aligned to 8 bytes and does not cause a fault on + // atomic access. See issue 37262. + // TODO(mknyszek): Remove this workaround if/when issue 36606 + // is resolved. + off = alignUp(off, 8) } else if size&3 == 0 { off = alignUp(off, 4) } else if size&1 == 0 { diff --git a/src/runtime/malloc_test.go b/src/runtime/malloc_test.go index 5c97f548fda..4ba94d0494e 100644 --- a/src/runtime/malloc_test.go +++ b/src/runtime/malloc_test.go @@ -12,8 +12,10 @@ import ( "os" "os/exec" "reflect" + "runtime" . "runtime" "strings" + "sync/atomic" "testing" "time" "unsafe" @@ -168,6 +170,61 @@ func TestTinyAlloc(t *testing.T) { } } +var ( + tinyByteSink *byte + tinyUint32Sink *uint32 + tinyObj12Sink *obj12 +) + +type obj12 struct { + a uint64 + b uint32 +} + +func TestTinyAllocIssue37262(t *testing.T) { + // Try to cause an alignment access fault + // by atomically accessing the first 64-bit + // value of a tiny-allocated object. + // See issue 37262 for details. + + // GC twice, once to reach a stable heap state + // and again to make sure we finish the sweep phase. + runtime.GC() + runtime.GC() + + // Make 1-byte allocations until we get a fresh tiny slot. + aligned := false + for i := 0; i < 16; i++ { + tinyByteSink = new(byte) + if uintptr(unsafe.Pointer(tinyByteSink))&0xf == 0xf { + aligned = true + break + } + } + if !aligned { + t.Fatal("unable to get a fresh tiny slot") + } + + // Create a 4-byte object so that the current + // tiny slot is partially filled. + tinyUint32Sink = new(uint32) + + // Create a 12-byte object, which fits into the + // tiny slot. If it actually gets place there, + // then the field "a" will be improperly aligned + // for atomic access on 32-bit architectures. + // This won't be true if issue 36606 gets resolved. + tinyObj12Sink = new(obj12) + + // Try to atomically access "x.a". + atomic.StoreUint64(&tinyObj12Sink.a, 10) + + // Clear the sinks. + tinyByteSink = nil + tinyUint32Sink = nil + tinyObj12Sink = nil +} + func TestPageCacheLeak(t *testing.T) { defer GOMAXPROCS(GOMAXPROCS(1)) leaked := PageCachePagesLeaked()