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

From: KY Srinivasan
Date: Mon Dec 01 2014 - 10:55:06 EST



> -----Original Message-----
> From: Dexuan Cui
> Sent: Monday, December 1, 2014 3:01 AM
> To: Jason Wang
> Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; driverdev-
> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; KY
> Srinivasan; vkuznets@xxxxxxxxxx; Haiyang Zhang
> Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
> failure
>
> > -----Original Message-----
> > From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> > Sent: Monday, December 1, 2014 18:18 PM
> > To: Dexuan Cui
> > Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > driverdev- devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> > apw@xxxxxxxxxxxxx; KY Srinivasan; vkuznets@xxxxxxxxxx; 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@xxxxxxxxxxxxx>
> wrote:
> > >> -----Original Message-----
> > >> From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> > >> Sent: Monday, December 1, 2014 16:23 PM
> > >> To: Dexuan Cui
> > >> Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > >> driverdev-
> > >> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx;
> > >> KY Srinivasan; vkuznets@xxxxxxxxxx; 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@xxxxxxxxxxxxx>
> > >> wrote:
> > >> >> -----Original Message-----
> > >> >> From: Jason Wang [mailto:jasowang@xxxxxxxxxx] >> Sent:
> > >> Friday, November 28, 2014 18:13 PM >> To: Dexuan Cui >> Cc:
> > >> gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; >>
> > >> driverdev- >> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> > >> apw@xxxxxxxxxxxxx; KY >> Srinivasan; vkuznets@xxxxxxxxxx; 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@xxxxxxxxxxxxx> >> wrote:
> > >> >> >> -----Original Message----- >> >> From: Jason Wang
> > >> [mailto:jasowang@xxxxxxxxxx] >> >> Sent: Friday, November 28,
> > >> 2014 14:47 PM >> >> To: Dexuan Cui >> >> Cc:
> > >> gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; >> >>
> > >> driverdev- >> >> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> > >> >> apw@xxxxxxxxxxxxx; KY >> >> Srinivasan; vkuznets@xxxxxxxxxx;
> > >> 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@xxxxxxxxxxxxx> >> >> 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@xxxxxxxxxx> >> >> > Cc:
> > >> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> >> >> > Cc: K. Y.
> > >> Srinivasan <kys@xxxxxxxxxxxxx> >> >> > Signed-off-by: Dexuan Cui
> > >> <decui@xxxxxxxxxxxxx> >> >> > --- >> >> > >> >> > 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 âfcopy_work_funcâ:
> > > drivers/hv/hv_fcopy.c:95:2: warning: ignoring return value of
> > > âdown_trylockâ, 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@xxxxxxxxxx>
>
> 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.

You should not be able to unload the driver when there is still an open reference from the
Daemon.

K. Y
>
> -- Dexuan

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