From 1a050f3cff3a93f634d2d929aba779585f39609c Mon Sep 17 00:00:00 2001 From: Simon Marlow Date: Tue, 16 Mar 2010 14:31:03 +0000 Subject: [PATCH] copy_tag_nolock(): fix write ordering and add a write_barrier() Fixes a rare crash in the parallel GC. If we copy a closure non-atomically during GC, as we do for all immutable values, then before writing the forwarding pointer we better make sure that the closure itself is visible to other threads that might follow the forwarding pointer. I imagine this doesn't happen very often, but I just found one case of it: in scavenge_stack, the RET_FUN case, after evacuating ret_fun->fun we then follow it and look up the info pointer. --- rts/sm/Evac.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/rts/sm/Evac.c b/rts/sm/Evac.c index 21017a6..d5c9b8a 100644 --- a/rts/sm/Evac.c +++ b/rts/sm/Evac.c @@ -140,14 +140,18 @@ copy_tag_nolock(StgClosure **p, const StgInfoTable *info, to = alloc_for_copy(size,gen); *p = TAG_CLOSURE(tag,(StgClosure*)to); - src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to); - + from = (StgPtr)src; to[0] = (W_)info; for (i = 1; i < size; i++) { // unroll for small i to[i] = from[i]; } + // if somebody else reads the forwarding pointer, we better make + // sure there's a closure at the end of it. + write_barrier(); + src->header.info = (const StgInfoTable *)MK_FORWARDING_PTR(to); + // if (to+size+2 < bd->start + BLOCK_SIZE_W) { // __builtin_prefetch(to + size + 2, 1); // } -- 1.7.10.4