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

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, November 20, 2014 6:41 PM
> To: KY Srinivasan; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; driverdev-devel@linuxdriverproject.org;
> olaf@aepfle.de; apw@canonical.com; jasowang@redhat.com
> Cc: Haiyang Zhang; Vitaly Kuznetsov
> Subject: RE: [PATCH] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> 
> > -----Original Message-----
> > From: KY Srinivasan
> > Sent: Friday, November 21, 2014 1:58 AM
> > To: Dexuan Cui; gregkh@linuxfoundation.org;
> > linux-kernel@vger.kernel.org; driverdev-devel@linuxdriverproject.org;
> > olaf@aepfle.de; apw@canonical.com; jasowang@redhat.com
> > Cc: Haiyang Zhang
> > Subject: RE: [PATCH] hv: hv_fcopy: drop the obsolete message on
> > transfer failure
> > > -----Original Message-----
> > > From: Dexuan Cui
> > > Sent: Wednesday, November 19, 2014 11:48 PM
> > > To: KY Srinivasan; gregkh@linuxfoundation.org; linux-
> > > kernel@vger.kernel.org; driverdev-devel@linuxdriverproject.org;
> > > olaf@aepfle.de; apw@canonical.com; jasowang@redhat.com
> > > Cc: Haiyang Zhang
> > > 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. :-)
> >
> > Agreed; you may want to get rid of the pr_debug() call though.
> >
> > Thanks,
> >
> > K. Y
> 
> The pr_debug() is added intentionally according to suggestion of Redhat's
> Vitaly Kuznetsov in the bugzilla:
> https://bugzilla.redhat.com/show_bug.cgi?id=1162100#c5
> 
> The function is declared with__must_check in include/linux/semaphore.h:
> extern int __must_check down_trylock(struct semaphore *sem);
> 
> Without checking the return value, we'll get these warning if the "Kernel
> hacking" options are enabled:
> 
> drivers/hv/hv_fcopy.c: In function 'fcopy_work_func':
> drivers/hv/hv_fcopy.c:95:2: warning: ignoring return value of 'down_trylock',
> declared with attribute warn_unused_result [-Wunused-result]
>   (void)down_trylock(&fcopy_transaction.read_sema);
>   ^
> 
> In practice, the message I add should be very rare since it's very unlikely to
> fail to get the semaphore in this timeout case -- and in case this happens, it's
> actually OK, because the driver has told the host user the PowerShell
> command should fail.
Clearly, we don't want to ignore the return value (to avoid the warning); but that does not mean
that we need to print a message that is of questionable value. That said, I am fine with the code that
you currently have as this message is going to be very rare.

Regards,

K. Y 
> 
> Thanks,
> -- Dexuan