From 4f188c2d1c88d68d3f74a82c148cde804573d691 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Wed, 22 Jul 2015 15:14:54 -0400 Subject: [PATCH] runtime: disallow GC assists in non-preemptible contexts Currently it's possible to perform GC work on a system stack or when locks are held if there's an allocation that triggers an assist. This is generally a bad idea because of the fragility of these contexts, and it's incompatible with two changes we're about to make: one is to yield after signaling mark completion (which we can't do from a non-preemptible context) and the other is to make assists block if there's no other way for them to pay off the assist debt. This commit simply skips the assist if it's called from a non-preemptible context. The allocation will still count toward the assist debt, so it will be paid off by a later assist. There should be little allocation from non-preemptible contexts, so this shouldn't harm the overall assist mechanism. Change-Id: I7bf0e6c73e659fe6b52f27437abf39d76b245c79 Reviewed-on: https://go-review.googlesource.com/12649 Reviewed-by: Russ Cox --- src/runtime/mgcmark.go | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go index 9212b2edc6e..62f08814390 100644 --- a/src/runtime/mgcmark.go +++ b/src/runtime/mgcmark.go @@ -160,6 +160,15 @@ func gcAssistAlloc(size uintptr, allowAssist bool) { return } + // Don't assist in non-preemptible contexts. These are + // generally fragile and won't allow the assist to block. + if getg() == gp.m.g0 { + return + } + if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" { + return + } + // Compute the amount of assist scan work we need to do. scanWork := int64(gcController.assistRatio*float64(gp.gcalloc)) - gp.gcscanwork // scanWork can be negative if the last assist scanned a large