netdev - Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in phy_stop

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Message-ID: <f74f1aad-3990-ae54-316f-751c3b15de41@gmail.com>
Date:   Thu, 31 Aug 2017 10:53:37 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Mason <slash.tmp@...e.fr>, David Daney <ddaney.cavm@...il.com>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        netdev <netdev@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        David Miller <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>, Mans Rullgard <mans@...sr.com>
Subject: Re: [PATCH net] Revert "net: phy: Correctly process PHY_HALTED in
 phy_stop_machine()"

On 08/31/2017 10:49 AM, Mason wrote:
> On 31/08/2017 18:57, Florian Fainelli wrote:
>> And the race is between phy_detach() setting phydev->attached_dev = NULL
>> and phy_state_machine() running in PHY_HALTED state and calling
>> netif_carrier_off().
> 
> I must be missing something.
> (Since a thread cannot race against itself.)
> 
> phy_disconnect calls phy_stop_machine which
> 1) stops the work queue from running in a separate thread
> 2) calls phy_state_machine *synchronously*
>      which runs the PHY_HALTED case with everything well-defined
> end of phy_stop_machine
> 
> phy_disconnect only then calls phy_detach()
> which makes future calls of phy_state_machine perilous.
> 
> This all happens in the same thread, so I'm not yet
> seeing where the race happens?

The race is as described in David's earlier email, so let's recap:

Thread 1			Thread 2
phy_disconnect()
phy_stop_interrupts()
phy_stop_machine()
phy_state_machine()
 -> queue_delayed_work()
phy_detach()
				phy_state_machine()
				-> netif_carrier_off()

If phy_detach() finishes earlier than the workqueue had a chance to be
scheduled and process PHY_HALTED again, then we trigger the NULL pointer
de-reference.

workqueues are not tasklets, the CPU scheduling them gets no guarantee
they will run on the same CPU.
-- 
Florian