RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer failure
From: Jason Wang
Date: Fri Nov 28 2014 - 05:13:24 EST
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?
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;
}
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?
Thanks
Please share your idea.
>
> + fcopy_respond_to_host(HV_E_FAIL);
> + }
> return 0;
> }
>
> --
> 1.9.1
>
-- Dexuan
ïïNrybXÇv^)Þ{.n+{zXÜ}Æz&j:+vzZ++zfh~iz w?&)ßf^jÇym@Aa 0hi
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