linux-kernel - Re: [PATCH] Revert "seqlock: lockdep assert non-preemptibility on seqcount

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Message-ID: <a873e584-ddf3-dacd-4901-514bd961841e@roeck-us.net>
Date:   Wed, 19 Aug 2020 09:15:14 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Mark Starovoytov <mstarovoitov@...vell.com>,
        Igor Russkikh <irusskikh@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        linux-kernel@...r.kernel.org, mingo@...hat.com, paulmck@...nel.org,
        peterz@...radead.org, rostedt@...dmis.org, tglx@...utronix.de,
        will@...nel.org
Subject: Re: [PATCH] Revert "seqlock: lockdep assert non-preemptibility on
 seqcount_t write"

On 8/19/20 12:34 AM, Valdis Klētnieks wrote:
> On Wed, 19 Aug 2020 09:00:22 +0200, Sebastian Andrzej Siewior said:
>> On 2020-08-18 17:56:49 [-0700], Guenter Roeck wrote:
>>> Nice catch. FWIW, there is no obvious reason why this would need to be atomic.
>>> The calling code does not set a lock, meaning there can be two (or more)
>>> callers entering this code. Weird, especially since the code looks like it
>>> would actually need a mutex to work correctly. It might be interesting to
>>> see what happens if there are, say, half a dozen scripts/processes trying
>>> to read the hwmon attribute introduced by this patch at the same time.
>>
>> => https://lkml.kernel.org/r/20200818161439.3dkf6jzp3vuwmvvh@linutronix.de
> 
> Looks reasonable to me, though I've not verified that it's preemptible at that
> point...
> 

hw_atl_b0_get_mac_temp is called through the .hw_get_mac_temp callback.
This callback is executed from aq_hwmon_read(), which in turn is called
from the hwmon core, more specifically from hwmon_attr_show().
Calls from the hwmon core are unlocked.

Guenter



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