[PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Dexuan Cui]

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.

Reviewed-by: 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"

 drivers/hv/hv_fcopy.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
index 23b2ce2..c518ad9 100644
--- a/drivers/hv/hv_fcopy.c
+++ b/drivers/hv/hv_fcopy.c
@@ -86,6 +86,15 @@ 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.
+	 */
+	if (down_trylock(&fcopy_transaction.read_sema))
+		pr_debug("can not acquire the semaphore: it is benign\n");
+
 }
 
 static int fcopy_handle_handshake(u32 version)
-- 
1.9.1

Re: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Jason Wang]

----- Original Message -----
> 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.
> 
> Reviewed-by: 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"
> 
>  drivers/hv/hv_fcopy.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
> index 23b2ce2..c518ad9 100644
> --- a/drivers/hv/hv_fcopy.c
> +++ b/drivers/hv/hv_fcopy.c
> @@ -86,6 +86,15 @@ 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.
> +	 */

Looks still racy, what happens if the daemon start before down_trylock()
but after fcopy_respont_to_host() here?

> +	if (down_trylock(&fcopy_transaction.read_sema))
> +		pr_debug("can not acquire the semaphore: it is benign\n");

typo
> +
>  }
>  
>  static int fcopy_handle_handshake(u32 version)
> --
> 1.9.1
> 
> --
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RE: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Dexuan Cui]

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> -----Original Message-----
> From: Jason Wang [mailto:jasowang@redhat.com]
> Sent: Thursday, November 27, 2014 15:15 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 v2] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> ----- Original Message -----
> > 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.
> >
> > Reviewed-by: 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"
> >
> >  drivers/hv/hv_fcopy.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
> > index 23b2ce2..c518ad9 100644
> > --- a/drivers/hv/hv_fcopy.c
> > +++ b/drivers/hv/hv_fcopy.c
> > @@ -86,6 +86,15 @@ 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.
> > +	 */
> 
> Looks still racy, what happens if the daemon start before down_trylock()
> but after fcopy_respont_to_host() here?
Jason,
Thanks for pointing this out!
IMO we can resolve this by adding down_trylock() in fcopy_release().
What's your opinion?

> 
> > +	if (down_trylock(&fcopy_transaction.read_sema))
> > +		pr_debug("can not acquire the semaphore: it is benign\n");
> 
> typo
> > +
> >  }
Sorry -- what typo do you mean?

Thanks,
-- Dexuan
ÿôèº{.nÇ+‰·Ÿ®‰­†+%ŠËÿ±éݶ\x17¥Šwÿº{.nÇ+‰·¥Š{±þG«éÿŠ{ayº\x1dʇڙë,j\a­¢f£¢·hšïêÿ‘êçz_è®\x03(­éšŽŠÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?™¨è­Ú&£ø§~á¶iO•æ¬z·švØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?–I¥

RE: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Jason Wang]

On Thu, Nov 27, 2014 at 4:50 PM, Dexuan Cui <decui@microsoft.com> wrote:
>>  -----Original Message-----
>>  From: Jason Wang [mailto:jasowang@redhat.com]
>>  Sent: Thursday, November 27, 2014 15:15 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 v2] hv: hv_fcopy: drop the obsolete message on 
>> transfer
>>  failure
>>  ----- Original Message -----
>>  > 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.
>>  >
>>  > Reviewed-by: 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"
>>  >
>>  >  drivers/hv/hv_fcopy.c | 9 +++++++++
>>  >  1 file changed, 9 insertions(+)
>>  >
>>  > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
>>  > index 23b2ce2..c518ad9 100644
>>  > --- a/drivers/hv/hv_fcopy.c
>>  > +++ b/drivers/hv/hv_fcopy.c
>>  > @@ -86,6 +86,15 @@ 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.
>>  > +	 */
>>  
>>  Looks still racy, what happens if the daemon start before 
>> down_trylock()
>>  but after fcopy_respont_to_host() here?
> Jason,
> Thanks for pointing this out!
> IMO we can resolve this by adding down_trylock() in fcopy_release().
> What's your opinion?


Looks better and need to cancel the timeout also here?
> 
> 
>>  
>>  > +	if (down_trylock(&fcopy_transaction.read_sema))
>>  > +		pr_debug("can not acquire the semaphore: it is benign\n");
>>  
>>  typo
>>  > +
>>  >  }
> Sorry -- what typo do you mean?

s/benign/begin/ ?
> 
> Thanks,
> -- Dexuan
> �NrybXǧv^)޺{.n+{zX\x17ܨ}Ơz&j:+v\azZ++zfh~iz\x1ew?&)ߢ^[f^jǫym@Aa\x7f\f0h\x0fi\x7f

RE: [PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Dexuan Cui]

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> -----Original Message-----
> From: Jason Wang [mailto:jasowang@redhat.com]
> Sent: Thursday, November 27, 2014 17:01 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 v2] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> On Thu, Nov 27, 2014 at 4:50 PM, Dexuan Cui <decui@microsoft.com> wrote:
> >>  -----Original Message-----
> >>  From: Jason Wang [mailto:jasowang@redhat.com]
> >>  Sent: Thursday, November 27, 2014 15:15 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 v2] hv: hv_fcopy: drop the obsolete message on
> >> transfer
> >>  failure
> >>  ----- Original Message -----
> >>  > 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.
> >>  >
> >>  > Reviewed-by: 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"
> >>  >
> >>  >  drivers/hv/hv_fcopy.c | 9 +++++++++
> >>  >  1 file changed, 9 insertions(+)
> >>  >
> >>  > diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
> >>  > index 23b2ce2..c518ad9 100644
> >>  > --- a/drivers/hv/hv_fcopy.c
> >>  > +++ b/drivers/hv/hv_fcopy.c
> >>  > @@ -86,6 +86,15 @@ 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.
> >>  > +	 */
> >>
> >>  Looks still racy, what happens if the daemon start before
> >> down_trylock()
> >>  but after fcopy_respont_to_host() here?
> > Jason,
> > Thanks for pointing this out!
> > IMO we can resolve this by adding down_trylock() in fcopy_release().
> > What's your opinion?
> 
> 
> Looks better and need to cancel the timeout also here?
OK, let me post a v3.

> >
> >
> >>
> >>  > +	if (down_trylock(&fcopy_transaction.read_sema))
> >>  > +		pr_debug("can not acquire the semaphore: it is benign\n");
> >>
> >>  typo
> >>  > +
> >>  >  }
> > Sorry -- what typo do you mean?
> 
> s/benign/begin/ ?
I meant the issue(can't get the semaphore) is benign.

I think we can just remove the message, as KY suggested.
Instead, I'll add a comment for it.

Thanks,
-- Dexuan

ÿôèº{.nÇ+‰·Ÿ®‰­†+%ŠËÿ±éݶ\x17¥Šwÿº{.nÇ+‰·¥Š{±þG«éÿŠ{ayº\x1dʇڙë,j\a­¢f£¢·hšïêÿ‘êçz_è®\x03(­éšŽŠÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?™¨è­Ú&£ø§~á¶iO•æ¬z·švØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?–I¥