> -----Original Message----- > From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx] > Sent: Tuesday, March 22, 2016 2:56 AM > To: KY Srinivasan <kys@xxxxxxxxxxxxx> > Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > jasowang@xxxxxxxxxx; stable@xxxxxxxxxxxxxxx > Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in > hv_need_to_signal_on_read() > > KY Srinivasan <kys@xxxxxxxxxxxxx> writes: > > >> -----Original Message----- > >> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx] > >> Sent: Monday, March 21, 2016 1:19 AM > >> To: KY Srinivasan <kys@xxxxxxxxxxxxx> > >> Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > >> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > >> jasowang@xxxxxxxxxx; stable@xxxxxxxxxxxxxxx > >> Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in > >> hv_need_to_signal_on_read() > >> > >> "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx> writes: > >> > >> > We need to issue a full memory barrier prior making a signalling > >> > decision. > >> > >> Any reason this should be mb()? This is pretty strong and will probably > >> lead to performace regression ... and, btw, we have another mb() in > >> hv_ringbuffer_read(). > >> > >> Could you please describe the scenarion you're trying to protect against > >> so we could search for a better solution? > > > > If the reading of the pend_sz (in the function > hv_need_to_signal_on_read) > > were to be reordered and read before we commit the new read index we > could > > have a problem. > > > If these are two reads we can add a lightweight barrier just preventing > compiler from reordering (e.g. smp_rmb()), right? > > > If the host were to set the pending_sz after we have sampled pending_sz > > and go to sleep before we commit the read index, we could miss sending > > the interrupt. > > so write and then we read and we need to prevent reordering... not sure > how to get rid on mb() then ... The other memory barrier in the function (prior to writing the read index) has been there forever and I am not sure why that needs to be a full barrier. I feel a read barrier should suffice. Regards, K. Y > > -- > Vitaly _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
