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Normally, the caller to runtime.entersyscall() must not return before calling runtime.exitsyscall(), lest g->syscallsp become a dangling pointer. runtime.cgocallbackg() violates this constraint. To work around this, save g->syscallsp and g->syscallpc around cgo->Go callbacks, then restore them after calling runtime.entersyscall(), which restores the syscall stack frame pointer saved by cgocall. This allows the GC to correctly trace a goroutine that is currently returning from a Go->cgo->Go chain. This also adds a check to proc.c that panics if g->syscallsp is clearly invalid. It is not 100% foolproof, as it will not catch a case where the stack was popped then pushed back beyond g->syscallsp, but it does catch the present cgo issue and makes existing tests fail without the bugfix. Fixes #7978. LGTM=dvyukov, rsc R=golang-codereviews, dvyukov, minux, bradfitz, iant, gobot, rsc CC=golang-codereviews, rsc https://golang.org/cl/131910043 |
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