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 10:18:33 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	KY Srinivasan <kys@...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: 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?

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.

Thanks,
-- Dexuan
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