linux-kernel - Re: [PATCH v8] ath9k: let sleep be interrupted when unregistering hwrng

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Date:   Mon, 11 Jul 2022 12:41:12 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Kalle Valo <kvalo@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        Gregory Erwin <gregerwin256@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Rui Salvaterra <rsalvaterra@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v8] ath9k: let sleep be interrupted when unregistering
 hwrng

On 07/07/22 19:26, Kalle Valo wrote:
> "Jason A. Donenfeld" <Jason@...c4.com> writes:
>
>> There are two deadlock scenarios that need addressing, which cause
>> problems when the computer goes to sleep, the interface is set down, and
>> hwrng_unregister() is called. When the deadlock is hit, sleep is delayed
>> for tens of seconds, causing it to fail. These scenarios are:
>>
>> 1) The hwrng kthread can't be stopped while it's sleeping, because it
>>    uses msleep_interruptible() instead of schedule_timeout_interruptible().
>>    The fix is a simple moving to the correct function. At the same time,
>>    we should cleanup a common and useless dmesg splat in the same area.
>>
>> 2) A normal user thread can't be interrupted by hwrng_unregister() while
>>    it's sleeping, because hwrng_unregister() is called from elsewhere.
>>    The solution here is to keep track of which thread is currently
>>    reading, and asleep, and signal that thread when it's time to
>>    unregister. There's a bit of book keeping required to prevent
>>    lifetime issues on current.
>>
>> Reported-by: Gregory Erwin <gregerwin256@...il.com>
>> Cc: Toke Høiland-Jørgensen <toke@...hat.com>
>> Cc: Kalle Valo <kvalo@...nel.org>
>> Cc: Rui Salvaterra <rsalvaterra@...il.com>
>> Cc: Herbert Xu <herbert@...dor.apana.org.au>
>> Cc: stable@...r.kernel.org
>> Fixes: fcd09c90c3c5 ("ath9k: use hw_random API instead of directly dumping into random.c")
>> Link: https://lore.kernel.org/all/CAO+Okf6ZJC5-nTE_EJUGQtd8JiCkiEHytGgDsFGTEjs0c00giw@mail.gmail.com/
>> Link: https://lore.kernel.org/lkml/CAO+Okf5k+C+SE6pMVfPf-d8MfVPVq4PO7EY8Hys_DVXtent3HA@mail.gmail.com/
>> Link: https://bugs.archlinux.org/task/75138
>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
>> ---
>> Changes v7->v8:
>> - Add a missing export_symbol.
>>
>>  drivers/char/hw_random/core.c        | 30 ++++++++++++++++++++++++----
>>  drivers/net/wireless/ath/ath9k/rng.c | 19 +++++++-----------
>>  kernel/sched/core.c                  |  1 +
>>  3 files changed, 34 insertions(+), 16 deletions(-)
>
> I don't see any acks for the hw_random and the scheduler change, adding more
> people to CC. Full patch here:
>
> https://patchwork.kernel.org/project/linux-wireless/patch/20220629114240.946411-1-Jason@zx2c4.com/
>
> Are everyone ok if I take this patch via wireless-next?
>

Thanks for the Cc.

I'm not hot on the export of wake_up_state(), IMO any wakeup with
!(state & TASK_NORMAL) should be reserved to kernel internals. Now, here
IIUC the problem is that the patch uses an inline invoking

  wake_up_state(p, TASK_INTERRUPTIBLE)

so this isn't playing with any 'exotic' task state, thus it shouldn't
actually need the export.

I've been trying to figure out if this could work with just a
wake_up_process(), but the sleeping pattern here is not very conforming
(cf. 'wait loop' pattern in sched/core.c), AFAICT the signal is used to
circumvent that :/