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

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Message-ID: <CA+55aFwCODeiXUPDR7-Y-=2xE2abmVuCnmVV=ezFqhO+JkaW=A@mail.gmail.com>
Date:	Thu, 14 May 2015 10:49:06 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, qemu-devel@...gnu.org,
	KVM list <kvm@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	zhang.zhanghailiang@...wei.com,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

On Thu, May 14, 2015 at 10:31 AM, Andrea Arcangeli <aarcange@...hat.com> 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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