linux-kernel - RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer failure

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Message-ID: <F792CF86EFE20D4AB8064279AFBA51C61EA7CFF9@SIXPRD3002MB028.064d.mgd.msft.net>
Date:	Wed, 14 Jan 2015 01:43:45 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
 failure

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
> Sent: Tuesday, January 13, 2015 21:52 PM
> To: Dexuan Cui; KY Srinivasan
> Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org; driverdev-
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> jasowang@...hat.com; Haiyang Zhang
> Subject: Re: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> 
> Dexuan Cui <decui@...rosoft.com> writes:
> 
> > 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.
> 
> It seems this patch got lost and I don't see it in recent 'resend'
> series. K.Y., Dexuan, can you please take a look?
Hi Vitaly, Jason,
The patch can't fix all the corner cases (it would need non-trivial efforts for that)
as we discussed, but I think it would be better for us to have it as it can indeed fix
an obvious issue and doesn't introduce new issues.

I think I can document the known discussed corner cases in the patch as TODOs
and resend the patch.

Please let me know if you have different opinions. :-) 

Thanks,
-- Dexuan

> 
> >
> > Cc: Jason Wang <jasowang@...hat.com>
> > Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> > Cc: K. Y. Srinivasan <kys@...rosoft.com>
> > Signed-off-by: Dexuan Cui <decui@...rosoft.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"
> >
> > v3: I added the code in fcopy_release() as Jason Wang suggested.
> >     I removed the pr_debug (it isn't so meaningful)and added a comment
> instead.
> >
> >  drivers/hv/hv_fcopy.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
> > index 23b2ce2..faa6ba6 100644
> > --- a/drivers/hv/hv_fcopy.c
> > +++ b/drivers/hv/hv_fcopy.c
> > @@ -86,6 +86,18 @@ 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.
> > +	 *
> > +	 * NOTE: fcopy_read() happens to get the semaphore (very rare)? We're
> > +	 * still OK, because we've reported the failure to the host.
> > +	 */
> > +	if (down_trylock(&fcopy_transaction.read_sema))
> > +		;
> > +
> >  }
> >
> >  static int fcopy_handle_handshake(u32 version)
> > @@ -351,6 +363,13 @@ static int fcopy_release(struct inode *inode, struct
> file *f)
> >  	 */
> >  	in_hand_shake = true;
> >  	opened = false;
> > +
> > +	if (cancel_delayed_work_sync(&fcopy_work)) {
> > +		/* We haven't up()-ed the semaphore(very rare)? */
> > +		if (down_trylock(&fcopy_transaction.read_sema))
> > +			;
> > +		fcopy_respond_to_host(HV_E_FAIL);
> > +	}
> >  	return 0;
> >  }
> 
> --
>   Vitaly
--
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