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

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