From: Dexuan Cui <decui@microsoft.com>
To: Jason Wang <jasowang@redhat.com>
CC: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "driverdev-devel@linuxdriverproject.org" 
	<driverdev-devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        KY Srinivasan <kys@microsoft.com>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        Haiyang Zhang <haiyangz@microsoft.com>
Subject: RE: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer
 failure
Thread-Topic: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer
 failure
Thread-Index: AQHQCiCqFBUCGSyvReGOHhAi+beE2px0MH3A
Date: Thu, 27 Nov 2014 11:44:17 +0000
Message-ID: <F792CF86EFE20D4AB8064279AFBA51C613E646F7@HKNPRD3002MB017.064d.mgd.msft.net>
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> -----Original Message-----
> From: Jason Wang [mailto:jasowang@redhat.com]
> Sent: Thursday, November 27, 2014 17:01 PM
> To: Dexuan Cui
> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org; driverdev-
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com; KY
> Srinivasan; vkuznets@redhat.com; Haiyang Zhang
> Subject: RE: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> On Thu, Nov 27, 2014 at 4:50 PM, Dexuan Cui <decui@microsoft.com> wrote:
> >>  -----Original Message-----
> >>  From: Jason Wang [mailto:jasowang@redhat.com]
> >>  Sent: Thursday, November 27, 2014 15:15 PM
> >>  To: Dexuan Cui
> >>  Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> >> driverdev-
> >>  devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com; KY
> >>  Srinivasan; vkuznets@redhat.com; Haiyang Zhang
> >>  Subject: Re: [PATCH v2] hv: hv_fcopy: drop the obsolete message on
> >> transfer
> >>  failure
> >>  ----- Original Message -----
> >>  > In the case the user-space daemon crashes, hangs or is killed, we
> >>  > need to down the semaphore, otherwise, after the daemon starts
> >> next
> >>  > time, the obsolete data in fcopy_transaction.message or
> >>  > fcopy_transaction.fcopy_msg will be used immediately.
> >>  >
> >>  > Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> >>  > Cc: K. Y. Srinivasan <kys@microsoft.com>
> >>  > Signed-off-by: Dexuan Cui <decui@microsoft.com>
> >>  > ---
> >>  >
> >>  > v2: I removed the "FCP" prefix as Greg asked.
> >>  >
> >>  >     I also updated the output message a little:
> >>  >     "FCP: failed to acquire the semaphore" -->
> >>  >     "can not acquire the semaphore: it is benign"
> >>  >
> >>  >  drivers/hv/hv_fcopy.c | 9 +++++++++
> >>  >  1 file changed, 9 insertions(+)
> >>  >
> >>  > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
> >>  > index 23b2ce2..c518ad9 100644
> >>  > --- a/drivers/hv/hv_fcopy.c
> >>  > +++ b/drivers/hv/hv_fcopy.c
> >>  > @@ -86,6 +86,15 @@ static void fcopy_work_func(struct work_struct
> >>  *dummy)
> >>  >  	 * process the pending transaction.
> >>  >  	 */
> >>  >  	fcopy_respond_to_host(HV_E_FAIL);
> >>  > +
> >>  > +	/* In the case the user-space daemon crashes, hangs or is
> >> killed, we
> >>  > +	 * need to down the semaphore, otherwise, after the daemon
> >> starts
> >>  next
> >>  > +	 * time, the obsolete data in fcopy_transaction.message or
> >>  > +	 * fcopy_transaction.fcopy_msg will be used immediately.
> >>  > +	 */
> >>
> >>  Looks still racy, what happens if the daemon start before
> >> down_trylock()
> >>  but after fcopy_respont_to_host() here?
> > Jason,
> > Thanks for pointing this out!
> > IMO we can resolve this by adding down_trylock() in fcopy_release().
> > What's your opinion?
> 
> 
> Looks better and need to cancel the timeout also here?
OK, let me post a v3.

> >
> >
> >>
> >>  > +	if (down_trylock(&fcopy_transaction.read_sema))
> >>  > +		pr_debug("can not acquire the semaphore: it is benign\n");
> >>
> >>  typo
> >>  > +
> >>  >  }
> > Sorry -- what typo do you mean?
> 
> s/benign/begin/ ?
I meant the issue(can't get the semaphore) is benign.

I think we can just remove the message, as KY suggested.
Instead, I'll add a comment for it.

Thanks,
-- Dexuan

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