netdev - Re: [PATCH net] igb: fix deadlock caused by taking RTNL in RPM resume path

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Message-ID: <20211130165110.291af62a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 30 Nov 2021 16:51:10 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH net] igb: fix deadlock caused by taking RTNL in RPM
 resume path

On Tue, 30 Nov 2021 22:35:27 +0100 Heiner Kallweit wrote:
> On 30.11.2021 18:12, Jakub Kicinski wrote:
> >> Not sure whether igb uses RPM the same way as r8169. There the device
> >> is runtime-suspended (D3hot) w/o link. Once cable is plugged in the PHY
> >> triggers a PME, and PCI core runtime-resumes the device (MAC).
> >> In this case RTNL isn't held by the caller. Therefore I don't think
> >> it's safe to assume that all callers hold RTNL.  
> > 
> > No, no - I meant to leave the locking in but add ASSERT_RTNL() to catch
> > if rpm == true && rtnl_held() == false.
> >   
> This is a valid case. Maybe it's not my day today, I still don't get
> how we would benefit from adding an ASSERT_RTNL().
> 
> Based on the following I think that RPM resume and device open()
> can't collide, because RPM resume is finished before open()
> starts its actual work.
> 
> static int __igb_open(struct net_device *netdev, bool resuming)
> {
> ...
> if (!resuming)
> 		pm_runtime_get_sync(&pdev->dev);

Ah, thanks, gotta start looking at the code before I say things..