Re: _lwp_create change

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Wed, Apr 19, 2017 at 07:52:25PM -0400, Christos Zoulas wrote:
> This issue came up recently with the go language signal tests failing.
> The problem is that NetBSD copies the l_sigstk on lwp_creation and if
> threads happen to get signals at the same time, then they share the
> signal stack and they die (if sigaltstack is set). Go attempts to work
> around this by blocking the signals (setting the signal mask on lwp
> creation, and then calling sigaltstack() itself):

As discussed previously, I think the patch makes the behavior worse and
the Go implementation is kind of stupid as well. To correcly ensure that
signals always use the signal stack:

(1) Allocate a new signal stack.
(2) Mask all signals.
(3) Create the new thread.
(4) Switch the *old* stack to the new signal stack.
(5) Unmask signals again.

Variant:
(1) Allocate a new signal stack.
(2) Mask all signals.
(3) Switch the *old* stack to the new signal stack, remember the old
    setting.
(4) Create the new thread.
(5) Switch the *old* stack back to the original signal stack.
(6) Unmask signals again.

This is significantly more flexible than the proposed hack and requires
no code changes in the kernel. It does assume that no synchronous
signals happen between step (2) and (5) [or (2) and (6) respectively],
but if you want to segfault anyway, it likely doesn't really matter.
In either case, the new thread starts with the correct signal stack
without any race condition.

Joerg