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Message-ID: <F792CF86EFE20D4AB8064279AFBA51C613E691B8@HKNPRD3002MB017.064d.mgd.msft.net>
Date: Mon, 1 Dec 2014 09:47:14 +0000
From: Dexuan Cui <decui@...rosoft.com>
To: Jason Wang <jasowang@...hat.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>,
KY Srinivasan <kys@...rosoft.com>,
"vkuznets@...hat.com" <vkuznets@...hat.com>,
Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
failure
> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Monday, December 1, 2014 16:23 PM
> To: Dexuan Cui
> Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org; driverdev-
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com; KY
> Srinivasan; vkuznets@...hat.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@...rosoft.com> wrote:
> >> -----Original Message-----
> >> From: Jason Wang [mailto:jasowang@...hat.com]
> >> Sent: Friday, November 28, 2014 18:13 PM
> >> To: Dexuan Cui
> >> Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> >> driverdev-
> >> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com; KY
> >> Srinivasan; vkuznets@...hat.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@...rosoft.com>
> >> wrote:
> >> >> -----Original Message-----
> >> >> From: Jason Wang [mailto:jasowang@...hat.com]
> >> >> Sent: Friday, November 28, 2014 14:47 PM
> >> >> To: Dexuan Cui
> >> >> Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> >> >> driverdev-
> >> >> devel@...uxdriverproject.org; olaf@...fle.de;
> >> apw@...onical.com; KY
> >> >> Srinivasan; vkuznets@...hat.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@...rosoft.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@...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))
> >> >> > + ;
> >> >>
> >> >> 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?
Thanks,
-- Dexuan
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