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 v3] hv: hv_fcopy: drop the obsolete message on transfer
 failure
Thread-Topic: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
 failure
Thread-Index: AQHQDVAjTepp2fiJgUel+58IpcrHxpx6jwLQ
Date: Mon, 1 Dec 2014 11:00:51 +0000
Message-ID: <F792CF86EFE20D4AB8064279AFBA51C613E69318@HKNPRD3002MB017.064d.mgd.msft.net>
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> -----Original Message-----
> From: Jason Wang [mailto:jasowang@redhat.com]
> Sent: Monday, December 1, 2014 18:18 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 v3] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> 
> 
> 
> On Mon, Dec 1, 2014 at 5:47 PM, Dexuan Cui <decui@microsoft.com> wrote:
> >>  -----Original Message-----
> >>  From: Jason Wang [mailto:jasowang@redhat.com]
> >>  Sent: Monday, December 1, 2014 16:23 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 v3] hv: hv_fcopy: drop the obsolete message on
> >> transfer
> >>  failure
> >>  On Fri, Nov 28, 2014 at 7:54 PM, Dexuan Cui <decui@microsoft.com>
> >> wrote:
> >>  >>  -----Original Message-----
> >>  >>  From: Jason Wang [mailto:jasowang@redhat.com]
> >>  >>  Sent: Friday, November 28, 2014 18:13 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 v3] hv: hv_fcopy: drop the obsolete message
> >> on
> >>  >> transfer
> >>  >>  failure
> >>  >>  On Fri, Nov 28, 2014 at 4:36 PM, Dexuan Cui
> >> <decui@microsoft.com>
> >>  >> wrote:
> >>  >>  >>  -----Original Message-----
> >>  >>  >>  From: Jason Wang [mailto:jasowang@redhat.com]
> >>  >>  >>  Sent: Friday, November 28, 2014 14:47 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 v3] hv: hv_fcopy: drop the obsolete
> >> message
> >>  >> on
> >>  >>  >> transfer
> >>  >>  >>  failure
> >>  >>  >>  On Thu, Nov 27, 2014 at 9:09 PM, Dexuan Cui
> >>  >> <decui@microsoft.com>
> >>  >>  >> wrote:
> >>  >>  >>  > 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.
> >>  >>  >>  >
> >>  >>  >>  > Cc: Jason Wang <jasowang@redhat.com>
> >>  >>  >>  > Cc: 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"
> >>  >>  >>  >
> >>  >>  >>  > 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))
> >>  >>  >>  > +		;
> >>  >>  >>
> >>  >>  >>  Sorry, I'm not quite understand how if () ; can help here.
> >>  >>  >>
> >>  >>  >>  Btw, a question not relate to this patch.
> >>  >>  >>
> >>  >>  >>  What happens if a daemon is resume from SIGSTOP and expires
> >> the
> >>  >>  >> check
> >>  >>  >>  here?
> >>  >>  > Hi Jason,
> >>  >>  > My idea is: here we need down_trylock(), but in case we can't
> >> get
> >>  >> the
> >>  >>  > semaphore, it's OK anyway:
> >>  >>  >
> >>  >>  > Scenario 1):
> >>  >>  > 1.1: when the daemon is blocked on the pread(), the daemon
> >>  >> receives
> >>  >>  > SIGSTOP;
> >>  >>  > 1.2: the host user runs the PowerShell Copy-VMFile command;
> >>  >>  > 1.3.1: the driver reports the failure to the host user in 5s
> >> and
> >>  >>  > 1.3.2: the driver down()-es the semaphore;
> >>  >>  > 1.4: the daemon receives SIGCONT and it will be still blocked
> >> on
> >>  >> the
> >>  >>  > pread().
> >>  >>  > Without the down_trylock(), in 1.4, the daemon can receive an
> >>  >>  > obsolete message.
> >>  >>  > NOTE: in this scenario, the daemon is not killed.
> >>  >>  >
> >>  >>  > Scenario 2):
> >>  >>  > In senario 1), if the daemon receives SIGCONT between 1.3.1
> >> and
> >>  >> 1.3.2
> >>  >>  > and
> >>  >>  > do down() in fcopy_read(), it will receive the message but:
> >> the
> >>  >>  > driver has
> >>  >>  > reported the failure to the host user and the driver's 1.3.2
> >> can't
> >>  >>  > get the
> >>  >>  > semaphore -- IMO this is acceptably OK, though in the VM, an
> >>  >>  > incomplete
> >>  >>  > file will be left there.
> >>  >>  > BTW, I think in the daemon's hv_start_fcopy() we should add a
> >>  >>  > close(target_fd) before open()-ing a new one.
> >>  >>
> >>  >>  Right, but how about the case when resuming from SIGSTOP but no
> >>  >> timeout?
> >>  > Sorry, I don't understand this:
> >>  > if no timeout, fcopy_read() will get the semaphore and
> >> fcopy_write()
> >>  > will try to cancel fcopy_work.
> >>
> >>  Yes.
> >>  >
> >>  >
> >>  >>  Looks like in this case userspace() may wait in
> >> down_interruptible()
> >>  >>  until timeout. We probably need something like this:
> >>  >>
> >>  >>          if (down_interruptible(&fcopy_transaction.read_sema)) {
> >>  >>                  up(&fcopy_transaction.read_sema);
> >>  >>                  return -EINTR;
> >>  >>          }
> >>  > until "timeout"?
> >>  > if the daemon can't get the semaphore, it can only be wake by a
> >>  > signal(the
> >>  > daemon doesn't install handler, so by default most signals will
> >> kill
> >>  > the daemon).
> >>  > In case a signal waking up the daemon doesn't kill the daemon, why
> >>  > should
> >>  > we do up()?
> >>
> >>  True, no need since we do down_trylock() in release().
> >>
> >>  Btw, there's no EINTR handling in handling pread() return value,
> >>  may add such one which should be useful for something like
> >> debugging.
> >>
> >>  >
> >>  >
> >>  >>
> >>  >>  This should synchronize with the timeout work for sure.
> >>  >>  But how about only schedule it after this?
> >>  >>  It does not may sense to start the timer during interrupt
> >>  >>  since the file may not even opened and it may take time
> >>  >>  to handle signals?
> >>  >>
> >>  >>  >
> >>  >>  >>  >
> >>  >>  >>  > +
> >>  >>  >>  >  }
> >>  >>  >>  >
> >>  >>  >>  >  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))
> >>  >>  >>  > +			;
> >>  >>  >>
> >>  >>  >>  And this.
> >>  >>  >
> >>  >>  > Scenario 3):
> >>  >>  > When the daemon exits(e.g., SIGKILL received), if there is a
> >>  >>  > fcopy_work
> >>  >>  > pending (scheduled but not start to run yet), we should
> >> cancel the
> >>  >>  > work (as you suggested) and down() the semaphore, otherwise,
> >> the
> >>  >>  > obsolete message will be received by the next instance of the
> >>  >> daemon.
> >>  >>
> >>  >>  Yes
> >>  >>  >
> >>  >>  >
> >>  >>  > Scenario 4):  in the driver's hv_fcopy_onchannelcallback():
> >>  >>  >         schedule_delayed_work(&fcopy_work, 5*HZ);
> >>  >>  >         ----> if fcopy_release() is running on another vcpu,
> >> just
> >>  >>  > before the next line?
> >>  >>  >         fcopy_send_data();
> >>  >>  >
> >>  >>  > In this case, fcopy_release() can cancel fcopy_work, but
> >>  >>  > can't get the semaphore since it hasn't been up()-ed.
> >>  >>  > Hmm, in this case,   fcopy_send_data() will do up()  later,
> >> and
> >>  >> we'll
> >>  >>  > buffer an obsolete message in the driver, and the message
> >> will be
> >>  >>  > fetched by the next instance of the daemon...
> >>  >>  >
> >>  >>  > Looks we need a spinlock here?
> >>  >>
> >>  >>  Unless fcopy_release() can wait for all data for current
> >> transation
> >>  >>  to be received. Spinlock won't help.
> >>  >>
> >>  >>  But an idea is let the daemon the handle such cases. E.g make
> >> sure
> >>  >> the
> >>  >>  processing begins with START_COPY and end with
> >>  COMPLETE/CANCEL_COPY.
> >>  >>  Drop all requests that does not start with START_COPY.
> >>  >>
> >>  >>  Thought?
> >>  > Good idea.
> >>  > I also think we should reinforce the concept of state machine in
> >> the
> >>  > daemon code.
> >>
> >>  Yes, it needs.
> > I agree.
> > Obviously we can do something to make the daemon/driver work better
> > in the corner cases.
> >
> >>  >
> >>  > The daemon/driver communication has so many corner cases...
> >>
> >>  Looks so, let's first address the issue mentioned in this patch.
> > OK.
> >
> >>  I don't have any more comments other than changing
> >>
> >>   if(down_trylock(&fcopy_transaction.read_sema))
> >>       ;
> >>
> >>  to
> >>
> >>  down_trylock(&fcopy_transaction.read_sema);
> > Hi Jason,
> > This is to address Vitaly's comment in the bugzilla:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1162100#c5
> >
> > down_trylock(&fcopy_transaction.read_sema) will
> >
> > "
> > produces the following compile warning:
> > 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);
> > "
> >
> > Actually I personally don't care about the warning, because we only
> > see it when we run some kind of code checker program.  :-)
> >
> > I can change my v3  to the "normal" style you prefer, if
> > there is no strong objection from Vitaly?
> 
> Ah, I see the point. Then no objection for this patch.
> 
> Since Vitaly said he does not has objection.
> 
> Acked-by: Jason Wang <jasowang@redhat.com>

I realized non-trivial effort might be needed to better handle corner cases.
This is only the first step.

E.g., I found a new corner case:
after we unload the hv_utils/fcopy driver, the fcopy daemon can
exit and fcopy_release() will be invoked -- at this time fcopy_release() has
become obsolete and invalid, and as a result, a kernel panic can happen:  I
did see the panic once.

-- Dexuan
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