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Austin Clements authored
Memory for stacks is manually managed by the runtime and, currently (with one exception) we free stack spans immediately when the last stack on a span is freed. However, the garbage collector assumes that spans can never transition from non-free to free during scan or mark. This disagreement makes it possible for the garbage collector to mark uninitialized objects and is blocking us from re-enabling the bad pointer test in the garbage collector (issue #9880). For example, the following sequence will result in marking an uninitialized object: 1. scanobject loads a pointer slot out of the object it's scanning. This happens to be one of the special pointers from the heap into a stack. Call the pointer p and suppose it points into X's stack. 2. X, running on another thread, grows its stack and frees its old stack. 3. The old stack happens to be large or was the last stack in its span, so X frees this span, setting it to state _MSpanFree. 4. The span gets reused as a heap span. 5. scanobject calls heapBitsForObject, which loads the span containing p, which is now in state _MSpanInUse, but doesn't necessarily have an object at p. The not-object at p gets marked, and at this point all sorts of things can go wrong. We already have a partial solution to this. When shrinking a stack, we put the old stack on a queue to be freed at the end of garbage collection. This was done to address exactly this problem, but wasn't a complete solution. This commit generalizes this solution to both shrinking and growing stacks. For stacks that fit in the stack pool, we simply don't free the span, even if its reference count reaches zero. It's fine to reuse the span for other stacks, and this enables that. At the end of GC, we sweep for cached stack spans with a zero reference count and free them. For larger stacks, we simply queue the stack span to be freed at the end of GC. Ideally, we would reuse these large stack spans the way we can small stack spans, but that's a more invasive change that will have to wait until after the freeze. Fixes #11267. Change-Id: Ib7f2c5da4845cc0268e8dc098b08465116972a71 Reviewed-on: https://go-review.googlesource.com/11502Reviewed-by:
Russ Cox <rsc@golang.org>
d57056ba