Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

[Linus Torvalds]

On Thu, May 14, 2015 at 10:31 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> +static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
> +                                          struct userfaultfd_wake_range *range)
> +{
> +       if (waitqueue_active(&ctx->fault_wqh))
> +               __wake_userfault(ctx, range);
> +}

Pretty much every single time people use this "if
(waitqueue_active())" model, it tends to be a bug, because it means
that there is zero serialization with people who are just about to go
to sleep. It's fundamentally racy against all the "wait_event()" loops
that carefully do memory barriers between testing conditions and going
to sleep, because the memory barriers now don't exist on the waking
side.

So I'm making a new rule: if you use waitqueue_active(), I want an
explanation for why it's not racy with the waiter. A big comment about
the memory ordering, or about higher-level locks that are held by the
caller, or something.

                     Linus
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