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

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Message-Id: <1417078861.18179.2@smtp.corp.redhat.com>
Date:	Thu, 27 Nov 2014 09:09:01 +0008
From:	Jason Wang <jasowang@...hat.com>
To:	Dexuan Cui <decui@...rosoft.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 v2] hv: hv_fcopy: drop the obsolete message on transfer
 failure



On Thu, Nov 27, 2014 at 4:50 PM, Dexuan Cui <decui@...rosoft.com> wrote:
>>  -----Original Message-----
>>  From: Jason Wang [mailto:jasowang@...hat.com]
>>  Sent: Thursday, November 27, 2014 15:15 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 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@...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"
>>  >
>>  >  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.ܨ}Ơz&j:+v.zZ++zfh~iz.w?&)ߢ.f^jǫym@Aa.0h.i

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