linux-kernel - RE: [PATCH V2 02/10] Drivers: hv: utils: run polling callback always in interrupt context

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Date:	Thu, 10 Dec 2015 23:23:06 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Dexuan Cui <decui@...rosoft.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: RE: [PATCH V2 02/10] Drivers: hv: utils: run polling callback always
 in interrupt context



> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, December 10, 2015 2:19 AM
> To: KY Srinivasan <kys@...rosoft.com>; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; vkuznets@...hat.com; jasowang@...hat.com
> Subject: RE: [PATCH V2 02/10] Drivers: hv: utils: run polling callback always in
> interrupt context
> 
> > -----Original Message-----
> > From: devel [mailto:driverdev-devel-bounces@...uxdriverproject.org] On
> Behalf
> > Of K. Y. Srinivasan
> > Sent: Friday, October 30, 2015 9:13
> > To: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> > devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> > vkuznets@...hat.com; jasowang@...hat.com
> > Subject: [PATCH V2 02/10] Drivers: hv: utils: run polling callback always in
> > interrupt context
> >
> > From: Olaf Hering <olaf@...fle.de>
> >
> > All channel interrupts are bound to specific VCPUs in the guest
> > at the point channel is created. While currently, we invoke the
> > polling function on the correct CPU (the CPU to which the channel
> > is bound to) in some cases we may run the polling function in
> > a non-interrupt context. This  potentially can cause an issue as the
> > polling function can be interrupted by the channel callback function.
> > Fix the issue by running the polling function on the appropriate CPU
> > at interrupt level. Additional details of the issue being addressed by
> > this patch are given below:
> >
> > Currently hv_fcopy_onchannelcallback is called from interrupts and also
> > via the ->write function of hv_utils. Since the used global variables to
> > maintain state are not thread safe the state can get out of sync.
> > This affects the variable state as well as the channel inbound buffer.
> >
> > As suggested by KY adjust hv_poll_channel to always run the given
> > callback on the cpu which the channel is bound to. This avoids the need
> > for locking because all the util services are single threaded and only
> > one transaction is active at any given point in time.
> >
> > Additionally, remove the context variable, they will always be the same as
> > recv_channel.
> >
> > Signed-off-by: Olaf Hering <olaf@...fle.de>
> > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > ---
> > 	V2: Added the check to catch unsolicited daemon writes - Vitaly
> >
> >  drivers/hv/hv_fcopy.c     |   34 +++++++++++++---------------------
> >  drivers/hv/hv_kvp.c       |   28 ++++++++++------------------
> >  drivers/hv/hv_snapshot.c  |   29 +++++++++++------------------
> >  drivers/hv/hyperv_vmbus.h |    6 +-----
> >  4 files changed, 35 insertions(+), 62 deletions(-)
> 
> (Sorry for not joining the discussion when the patch was firstly made)
> 
> It looks the patch has not been Greg's tree yet.
> 
> I have 2 questions about the patch:
> 
> 1. hv_poll_channel() is invoked in fcopy_handle_handshake(), but not in
> vss_handle_handshake() and kvp_handle_handshake().
> Why -- I guess we missed the vss/kvp cases somehow?
I will fix this.

> 
> 2.  With the patch, hv_fcopy_onchannelcallback() can be invoked in the
> tasklet (i.e., vmbus_on_event(). NB: local irq is enabled), and in the
> hard irq handler(the IPI handler, e.g.,
> fcopy_poll_wrapper() -> fcopy_poll_wrapper()).
> 
> Can the former be interrupted by the latter?
> e.g., when the callback is running in the tasklet on vCPU0,
> fcopy_timeout_func() or fcopy_on_msg() could send the IPI to
> vCPU0 from another vCPU.

Keep in mind that when the poll function is run, the state will not be
HVUTIL_READY. The state will be set to HVUTIL_READY in the IPI
handler. So, it is ok if the tasklet is interrupted by the IPI handler.

Regards,

K. Y

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