netdev - RE: [PATCH net] hv_netvsc: Fix race condition between netvsc_probe and netvsc

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Message-ID:
 <SA1PR21MB13357264DB2B49C20B8C375BBF7D2@SA1PR21MB1335.namprd21.prod.outlook.com>
Date: Tue, 30 Jan 2024 17:32:23 +0000
From: Dexuan Cui <decui@...rosoft.com>
To: Souradeep Chakrabarti <schakrabarti@...ux.microsoft.com>, KY Srinivasan
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	Long Li <longli@...rosoft.com>, "yury.norov@...il.com"
	<yury.norov@...il.com>, "leon@...nel.org" <leon@...nel.org>,
	"cai.huoqing@...ux.dev" <cai.huoqing@...ux.dev>,
	"ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-rdma@...r.kernel.org"
	<linux-rdma@...r.kernel.org>
CC: Souradeep Chakrabarti <schakrabarti@...rosoft.com>
Subject: RE: [PATCH net] hv_netvsc: Fix race condition between netvsc_probe
 and netvsc_remove

> From: Souradeep Chakrabarti <schakrabarti@...ux.microsoft.com>
> Sent: Tuesday, January 30, 2024 2:16 AM
> [...]
> In commit ac5047671758 ("hv_netvsc: Disable NAPI before closing the
> VMBus channel"), napi_disable was getting called for all channels,
> including all subchannels without confirming if they are enabled or not.

s/enabled/created/

> Which caused hv_netvsc getting hung at napi_disable, when
> netvsc_probe()
> and netvsc_remove() are happening simultaneously and netvsc_remove()

Technically, they are not happening simultaneously: netvsc_probe() itself has
finished, but the work item scheduled by it has not started yet.

> calls cancel_work_sync(&nvdev->subchan_work) before netvsc_sc_open()
> calls napi_enable for the sub channels. Which causes NAPIF_STATE_SCHED

Technically, nvdev->subchan_work has not started to run yet, i.e.
netvsc_subchan_work() -> rndis_set_subchannel() has not created the
sub-channels yet, so netvsc_sc_open() can't run.

It would be great if you could briefly explain how the NAPIF_STATE_SCHED bit
is set and cleared, e.g. it's pre-set in rndis_filter_device_add() -> netif_napi_add()
so if the sub-channels are not created, netvsc_sc_open() -> napi_enable() won't
clear the flag and the flag remains set for ever for the sub-channels. 

> bit not getting cleared for the subchannels.
> 
> Now during netvsc_device_remove(), when napi_disable is called for those
> subchannels, napi_disable gets stuck on infinite msleep.

The patch body looks good to me. Please post v2 with an updated changelog.

Reviewed-by: Dexuan Cui <decui@...rosoft.com>