> -----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 ‘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@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. -- Dexuan _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
