linux-kernel - Faulty commit "watchdog: iTCO_wdt: Account for rebooting on second timeout"

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Message-ID: <20210803165108.4154cd52@endymion>
Date:   Tue, 3 Aug 2021 16:51:08 +0200
From:   Jean Delvare <jdelvare@...e.de>
To:     linux-watchdog@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Cc:     Jan Kiszka <jan.kiszka@...mens.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Michael Marley <michael@...haelmarley.com>
Subject: Faulty commit "watchdog: iTCO_wdt: Account for rebooting on second
 timeout"

Hi all,

Commit cb011044e34c ("watchdog: iTCO_wdt: Account for rebooting on
second timeout") causes a regression on several systems. Symptoms are:
system reboots automatically after a short period of time if watchdog
is enabled (by systemd for example). This has been reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=213809

Unfortunately this commit was backported to all stable kernel branches
(4.14, 4.19, 5.4, 5.10, 5.12 and 5.13). I'm not sure why that is the
case, BTW, as there is no Fixes tag and no Cc to stable@...r either.
And the fix is not trivial, has apparently not seen enough testing,
and addresses a problem that has a known and simple workaround. IMHO it
should never have been accepted as a stable patch in the first place.
Especially when the previous attempt to fix this issue already ended
with a regression and a revert.

Anyway... After a glance at the patch, I see what looks like a nice
thinko:

+	if (p->smi_res &&
+	    (SMI_EN(p) & (TCO_EN | GBL_SMI_EN)) != (TCO_EN | GBL_SMI_EN))

The author most certainly meant inl(SMI_EN(p)) (the register's value)
and not SMI_EN(p) (the register's address).

-- 
Jean Delvare
SUSE L3 Support