linux-kernel - Re: [PATCH] scsi: storvsc: Prevent running tasklet for long

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Date:   Wed, 6 Jul 2022 02:53:58 -0700
From:   Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To:     Praveen Kumar <kumarpraveen@...ux.microsoft.com>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, linux-hyperv@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        ssengar@...rosoft.com, mikelley@...rosoft.com
Subject: Re: [PATCH] scsi: storvsc: Prevent running tasklet for long

On Wed, Jul 06, 2022 at 02:44:42PM +0530, Praveen Kumar wrote:
> On 05-07-2022 21:02, Saurabh Sengar wrote:
> > There can be scenarios where packets in ring buffer are continuously
> > getting queued from upper layer and dequeued from storvsc interrupt
> > handler, such scenarios can hold the foreach_vmbus_pkt loop (which is
> > executing as a tasklet) for a long duration. Theoretically its possible
> > that this loop executes forever. Add a condition to limit execution of
> > this tasklet for finite amount of time to avoid such hazardous scenarios.
> > 
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> > ---
> >  drivers/scsi/storvsc_drv.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> > index fe000da..0c428cb 100644
> > --- a/drivers/scsi/storvsc_drv.c
> > +++ b/drivers/scsi/storvsc_drv.c
> > @@ -60,6 +60,9 @@
> >  #define VMSTOR_PROTO_VERSION_WIN8_1	VMSTOR_PROTO_VERSION(6, 0)
> >  #define VMSTOR_PROTO_VERSION_WIN10	VMSTOR_PROTO_VERSION(6, 2)
> >  
> > +/* channel callback timeout in ms */
> > +#define CALLBACK_TIMEOUT		5
> 
> If I may, it would be good if we have the CALLBACK_TIMEOUT configurable based upon user's requirement with default value to '5'.
> I assume, this value '5' fits best to the use-case which we are trying to resolve here. Thanks.

Agree, how about adding a sysfs entry for this parameter

> 
> > +
> >  /*  Packet structure describing virtual storage requests. */
> >  enum vstor_packet_operation {
> >  	VSTOR_OPERATION_COMPLETE_IO		= 1,
> > @@ -1204,6 +1207,7 @@ static void storvsc_on_channel_callback(void *context)
> >  	struct hv_device *device;
> >  	struct storvsc_device *stor_device;
> >  	struct Scsi_Host *shost;
> > +	unsigned long expire = jiffies + msecs_to_jiffies(CALLBACK_TIMEOUT);
> >  
> >  	if (channel->primary_channel != NULL)
> >  		device = channel->primary_channel->device_obj;
> > @@ -1224,6 +1228,9 @@ static void storvsc_on_channel_callback(void *context)
> >  		u32 minlen = rqst_id ? sizeof(struct vstor_packet) :
> >  			sizeof(enum vstor_packet_operation);
> >  
> > +		if (time_after(jiffies, expire))
> > +			break;
> > +
> >  		if (pktlen < minlen) {
> >  			dev_err(&device->device,
> >  				"Invalid pkt: id=%llu, len=%u, minlen=%u\n",
> 
> Regards,
> 
> ~Praveen.