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: AdAKHzSfFBUCGSyvReGOHhAi+beE2g==
Date: Thu, 27 Nov 2014 08:50:35 +0000
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> -----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?

> 
> > +	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?

Thanks,
-- Dexuan
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