linux-kernel - Re: [PATCH 2/4] swait: add the missing killable swaits

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Date:   Thu, 29 Jun 2017 11:59:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>, mfuzzey@...keon.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Daniel Wagner <wagi@...om.org>,
        David Woodhouse <dwmw2@...radead.org>,
        jewalt@...innovations.com, rafal@...ecki.pl,
        Arend Van Spriel <arend.vanspriel@...adcom.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Li, Yi" <yi1.li@...ux.intel.com>, atull@...nel.org,
        Moritz Fischer <moritz.fischer@...us.com>,
        Petr Mladek <pmladek@...e.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        "Coelho, Luciano" <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        David Howells <dhowells@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Alan Cox <alan@...ux.intel.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "stable # 4 . 6" <stable@...r.kernel.org>
Subject: Re: [PATCH 2/4] swait: add the missing killable swaits

On Thu, Jun 29, 2017 at 11:33 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
> On Thu, 29 Jun 2017, Linus Torvalds wrote:
>
>> I actually think swait is pure garbage. Most users only wake up one
>> process anyway, and using swait for that is stupid. If you only wake
>> up one, you might as well just have a single process pointer, not a
>> wait list at all, and then use "wake_up_process()".
>
> But you still need the notion of a queue, even if you wake one task
> at a time... I'm probably missing your point here.

The *reason* they wake up only one seems to be that there really is
just one. It's some per-cpu idle thread for kvm, and for RCU it's the
RCU workqueue thread.

So the queue literally looks suspiciously pointless.

But I might be wrong, and there can actually be multiple entries. If
there are, I don't see why the wake-up-one semantics the code uses
would be valid, though.

                   Linus