linux-kernel - RE: [PATCH] Retry infinitely for hypercall

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Date:   Wed, 4 Jan 2017 22:05:23 +0000
From:   Long Li <longli@...rosoft.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] Retry infinitely for hypercall



> -----Original Message-----
> From: Dan Carpenter [mailto:dan.carpenter@...cle.com]
> Sent: Wednesday, January 4, 2017 1:48 PM
> To: Long Li <longli@...rosoft.com>
> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; devel@...uxdriverproject.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH] Retry infinitely for hypercall
> 
> Fix the subsystem prefix in the subject.
> 
> On Wed, Jan 04, 2017 at 02:39:31PM -0800, Long Li wrote:
> > From: Long Li <longli@...rosoft.com>
> >
> > Hyper-v host guarantees that a hypercall will succeed. Retry infinitely to
> avoid returning transient failures to upper layer.
> >
> > Signed-off-by: Long Li <longli@...rosoft.com>
> > ---
> >  drivers/hv/connection.c | 17 ++++++++---------
> >  1 file changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c index
> > 6ce8b87..4bcb099 100644
> > --- a/drivers/hv/connection.c
> > +++ b/drivers/hv/connection.c
> > @@ -439,7 +439,6 @@ int vmbus_post_msg(void *buffer, size_t buflen)  {
> >  	union hv_connection_id conn_id;
> >  	int ret = 0;
> 
> Btw, when you disable GCC's uninitialized variable checking by storing bogus
> values in "ret", it's eventually going to bite you in the bum.
> Eventually you're going to get a bug that should have been detected through
> static analysis if only you hadn't disabled it.
> 
> > -	int retries = 0;
> >  	u32 usec = 1;
> >
> >  	conn_id.asu32 = 0;
> > @@ -447,10 +446,10 @@ int vmbus_post_msg(void *buffer, size_t buflen)
> >
> >  	/*
> >  	 * hv_post_message() can have transient failures because of
> > -	 * insufficient resources. Retry the operation a couple of
> > -	 * times before giving up.
> > +	 * insufficient resources. We retry infinitely on these failures
> > +	 * because host guarantees hypercall will eventually succeed.
> >  	 */
> > -	while (retries < 20) {
> > +	while (1) {
> >  		ret = hv_post_message(conn_id, 1, buffer, buflen);
> >
> >  		switch (ret) {
> > @@ -459,11 +458,11 @@ int vmbus_post_msg(void *buffer, size_t buflen)
> >  			 * We could get this if we send messages too
> >  			 * frequently.
> >  			 */
> 
> Move the comment above the code it's commenting about.
> 
> 		/*
> 		 * We could get INVALID_CONNECTION_ID if we flood the
> 		 * host with too many messages.
> 		 */
> 		case HV_STATUS_INVALID_CONNECTION_ID:
> 		case HV_STATUS_INSUFFICIENT_MEMORY:
> 		case HV_STATUS_INSUFFICIENT_BUFFERS:
> 			break;
> 
> 
> 
> > -			ret = -EAGAIN;
> > -			break;
> >  		case HV_STATUS_INSUFFICIENT_MEMORY:
> >  		case HV_STATUS_INSUFFICIENT_BUFFERS:
> > -			ret = -ENOMEM;
> > +			/*
> > +			 * Temporary failure out of resources
> > +			 */
> >  			break;
> >  		case HV_STATUS_SUCCESS:
> >  			return ret;
> 
> 			return 0;
> 
> Better to be more explicit.  When I looked at this I got briefly confused if this
> function was supposed to return HV_ statuses or standard kernel error
> codes.  It turns out that HV_STATUS_SUCCESS is zero the success returns
> map directly to linux kernel code for success but it's clearer to be explicit.
> 
> > @@ -472,12 +471,12 @@ int vmbus_post_msg(void *buffer, size_t buflen)
> >  			return -EINVAL;
> >  		}
> 
> > -		retries++;
> >  		udelay(usec);
> >  		if (usec < 2048)
> >  			usec *= 2;
> >  	}
> > -	return ret;
> > +	/* Impossible to get here */
> > +	BUG_ON(1);
> 
> Remove the comment and the BUG_ON().
> 
> regards,
> dan carpenter

Thanks, I will fix those in V2.

Long