Re: Thread safety of coroutine-sigaltstack

[Paolo Bonzini <pbonzini@redhat.com>]

On 20/01/21 17:26, Max Reitz wrote:
> Now, this all seems obvious to me, but I’m wondering...  If 
> coroutine-sigaltstack really couldn’t create coroutines concurrently,
>  why wouldn’t we have noticed before?  I mean, this new backup case
> is kind of a stress test, yes, but surely we would have seen the
> problem already, right?  That’s why I’m not sure whether my analysis
> is correct.

Probably the BQL was hiding the issue?

> Anyway, I’ve attached a patch that wraps the whole
> SIGUSR2 handling section in a mutex, and that makes 256 pass reliably
> with Vladimir’s async backup series.  Besides being unsure whether
> the problem is really in coroutine-sigaltstack, I also don’t know
> whether getting out the big guns and wrapping everything in the mutex
> is the best solution.  So, it’s an RFC, I guess.

I think that's fine.  The coroutine pool will hide scalability issues.

Paolo