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

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Message-ID: <F792CF86EFE20D4AB8064279AFBA51C613E577E6@HKNPRD3002MB017.064d.mgd.msft.net>
Date:	Thu, 20 Nov 2014 07:47:41 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	KY Srinivasan <kys@...rosoft.com>,
	"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>
CC:	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH] hv: hv_fcopy: drop the obsolete message on transfer
 failure

> -----Original Message-----
> From: KY Srinivasan
> Sent: Thursday, November 20, 2014 6:59 AM
> > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c index
> > 23b2ce2..177122a 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.
> > + */
> > +if (down_trylock(&fcopy_transaction.read_sema))
> > +pr_debug("FCP: failed to acquire the semaphore\n");
> > +
> >  }
> 
> When the daemon is killed, we currently reset the state in the release
> function. Why can't we cleanup the semaphore state (initialize) here as well.
> 
> K. Y

Hi KY,
1) The down_trylock() here is necessary: the daemon can fail to respond in 5
seconds due to many reasons, e.g., the VM's CPU and I/O are too busy. In this
case, the daemon may become running later(NOTE: in this example, the daemon
is not killed), but from the host user's point of view, the PowerShell
copy-vmfile command has failed, so here we have to 'down' the semaphore
anyway, otherwise, the daemon can get obsolete data.

2) If we add a line
sema_init(&fcopy_transaction.read_sema, 0);
in fcopy_release(), it seems OK at a glance, but we have to handle the race
condition: the above down_trylock() and the sema_init() can, in theory, run
simultaneously on different virtual CPUs.  It's tricky to address this.

3) So I think we can reuse the same semaphore without an actually unnecessary
re-initialization. :-)

Thanks,
-- Dexuan
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