linux-kernel - RE: [RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU

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Message-ID: <MW2PR2101MB1052A2E44557B29C191F557DD7CA0@MW2PR2101MB1052.namprd21.prod.outlook.com>
Date:   Sun, 29 Mar 2020 03:43:26 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Andrea Parri <parri.andrea@...il.com>,
        vkuznets <vkuznets@...hat.com>
CC:     Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the
 offer&rescind works to a specific CPU

From: Andrea Parri <parri.andrea@...il.com> Sent: Saturday, March 28, 2020 10:09 AM
> 
> > In case we believe that OFFER -> RESCINF sequence is always ordered
> > by the host AND we don't care about other offers in the queue the
> > suggested locking is OK: we're guaranteed to process RESCIND after we
> > finished processing OFFER for the same channel. However, waiting for
> > 'offer_in_progress == 0' looks fishy so I'd suggest we at least add a
> > comment explaining that the wait is only needed to serialize us with
> > possible OFFER for the same channel - and nothing else. I'd personally
> > still slightly prefer the algorythm I suggested as it guarantees we take
> > channel_mutex with offer_in_progress == 0 -- even if there are no issues
> > we can think of today (not strongly though).
> 
> Does it?  offer_in_progress is incremented without channel_mutex...
> 
> IAC, I have no objections to apply the changes you suggested.  To avoid
> misunderstandings: vmbus_bus_suspend() presents a similar usage...  Are
> you suggesting that I apply similar changes there?
> 
> Alternatively:  FWIW, the comment in vmbus_onoffer_rescind() does refer
> to "The offer msg and the corresponding rescind msg...".  I am all ears
> if you have any concrete suggestions to improve these comments.
> 

Given that waiting for 'offer_in_progress == 0' is the current code, I think
there's an argument to made for not changing it if the change isn't strictly
necessary.  This patch set introduces enough change that *is* necessary. :-)

Michael