|author||Austin Clements <firstname.lastname@example.org>||Thu Jul 16 16:27:09 2015 -0400|
|committer||Rob Pike <email@example.com>||Fri Jul 17 03:34:11 2015 +0000|
runtime: don't free large spans until heapBitsSweepSpan returns This fixes a race between 1) sweeping and freeing an unmarked large span and 2) reusing that span and allocating from it. This race arises because mSpan_Sweep returns spans for large objects to the heap *before* heapBitsSweepSpan clears the mark bit on the object in the span. Specifically, the following sequence of events can lead to an incorrectly zeroed bitmap byte, which causes the garbage collector to not trace any pointers in that object (the pointer bits for the first four words are cleared, and the scan bits are also cleared, so it looks like a no-scan object). 1) P0 calls mSpan_Sweep on a large span S0 with an unmarked object on it. 2) mSpan_Sweep calls heapBitsSweepSpan, which invokes the callback for the one (unmarked) object on the span. 3) The callback calls mHeap_Free, which makes span S0 available for allocation, but this is too early. 4) P1 grabs this S0 from the heap to use for allocation. 5) P1 allocates an object on this span and writes that object's type bits to the bitmap. 6) P0 returns from the callback to heapBitsSweepSpan. heapBitsSweepSpan clears the byte containing the mark, even though this span is now owned by P1 and this byte contains important bitmap information. This fixes this problem by simply delaying the mHeap_Free until after the heapBitsSweepSpan. I think the overall logic of mSpan_Sweep could be simplified now, but this seems like the minimal change. Fixes #11617. Change-Id: I6b1382c7e7cc35f81984467c0772fe9848b7522a Reviewed-on: https://go-review.googlesource.com/12320 Run-TryBot: Austin Clements <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com> Reviewed-by: Matthew Dempsky <firstname.lastname@example.org> Reviewed-by: Rob Pike <email@example.com>
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