Commit 6181db53 authored by Austin Clements's avatar Austin Clements

runtime: improve heapBitsSetType documentation

Currently the heapBitsSetType documentation says that there are no
races on the heap bitmap, but that isn't exactly true. There are no
*write-write* races, but there are read-write races. Expand the
documentation to explain this and why it's okay.

Change-Id: Ibd92b69bcd6524a40a9dd4ec82422b50831071ed
Reviewed-on: https://go-review.googlesource.com/23092Reviewed-by: default avatarRick Hudson <rlh@golang.org>
parent d8b08c3a
...@@ -847,10 +847,20 @@ func (s *mspan) countFree() int { ...@@ -847,10 +847,20 @@ func (s *mspan) countFree() int {
// malloc does not call heapBitsSetType when there are no pointers, // malloc does not call heapBitsSetType when there are no pointers,
// because all free objects are marked as noscan during // because all free objects are marked as noscan during
// heapBitsSweepSpan. // heapBitsSweepSpan.
//
// There can only be one allocation from a given span active at a time, // There can only be one allocation from a given span active at a time,
// so this code is not racing with other instances of itself, and // and the bitmap for a span always falls on byte boundaries,
// the bitmap for a span always falls on byte boundaries. // so there are no write-write races for access to the heap bitmap.
// Hence, it can access the bitmap with racing. // Hence, heapBitsSetType can access the bitmap without atomics.
//
// There can be read-write races between heapBitsSetType and things
// that read the heap bitmap like scanobject. However, since
// heapBitsSetType is only used for objects that have not yet been
// made reachable, readers will ignore bits being modified by this
// function. This does mean this function cannot transiently modify
// bits that belong to neighboring objects. Also, on weakly-ordered
// machines, callers must execute a store/store (publication) barrier
// between calling this function and making the object reachable.
// //
// TODO: This still has atomic accesses left over from when it could // TODO: This still has atomic accesses left over from when it could
// race with GC accessing mark bits in the bitmap. Remove these. // race with GC accessing mark bits in the bitmap. Remove these.
......
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