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

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Date:   Wed, 4 Jan 2017 22:03:28 +0000
From:   Long Li <longli@...rosoft.com>
To:     Greg KH <greg@...ah.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: Greg KH [mailto:greg@...ah.com]
> Sent: Wednesday, January 4, 2017 12:51 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
> 
> 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.
> 
> Please wrap your changelog at the proper column.

Will do in V2.
> 
> And what happens when the hypercall does not succeed?  How is the kernel
> going to recover from that?

Sorry I should have used better wording in the patch. It should be "Retry infinitely on transient failures for hypercall". The host guarantees that it will return something other than transient failures in a reasonable small time frame. I will fix the comment in V2.

> 
> >
> > 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;
> > -	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.
> >  			 */
> > -			ret = -EAGAIN;
> > -			break;
> 
> Document you are falling through please, otherwise someone will "fix"
> this later.
Will add comment in V2.

> 
> >  		case HV_STATUS_INSUFFICIENT_MEMORY:
> >  		case HV_STATUS_INSUFFICIENT_BUFFERS:
> > -			ret = -ENOMEM;
> > +			/*
> > +			 * Temporary failure out of resources
> > +			 */
> >  			break;
> >  		case HV_STATUS_SUCCESS:
> >  			return ret;
> > @@ -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);
> 
> If it is impossible, why do you have this line at all?

I will remove this line. There is no way for the code to get here.

> 
> What is this trying to solve?  Do you need to increase the time spent waiting?
> We all know things break, please allow the kernel to stay alive if at all
> possible.

The purpose is to wait until the host returns a non-transient status code for a hypercall. However, we don't know how many transient failures we are getting before the host returns a final status code. So use the infinite loop to wait until the host returns the final status code.

Thanks for reviewing. I will send V2 to address the comment.

Long

> 
> thanks,
> 
> greg k-h