linux-kernel - Re: [PATCH v2] ath9k: sleep for less time when unregistering hwrng

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Date:   Fri, 24 Jun 2022 17:13:44 -0700
From:   Gregory Erwin <gregerwin256@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Kalle Valo <kvalo@...nel.org>,
        Rui Salvaterra <rsalvaterra@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] ath9k: sleep for less time when unregistering hwrng

On Fri, Jun 24, 2022 at 1:44 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Even though hwrng provides a `wait` parameter, it doesn't work very well
> when waiting for a long time. There are numerous deadlocks that emerge
> related to shutdown. Work around this API limitation by waiting for a
> shorter amount of time and erroring more frequently. This commit also
> prevents hwrng from splatting messages to dmesg when there's a timeout
> and prevents calling msleep_interruptible() for tons of time when a
> thread is supposed to be shutting down, since msleep_interruptible()
> isn't actually interrupted by kthread_stop().
>
> 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>
> ---
> I do not have an ath9k and therefore I can't test this myself. The
> analysis above was done completely statically, with no dynamic tracing
> and just a bug report of symptoms from Gregory. So it might be totally
> wrong. Thus, this patch very much requires Gregory's testing. Please
> don't apply it until we have his `Tested-by` line.
>
>  drivers/char/hw_random/core.c        | 10 ++++++++--
>  drivers/net/wireless/ath/ath9k/rng.c | 19 ++-----------------
>  2 files changed, 10 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 16f227b995e8..af1c1905bb7e 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -513,8 +513,13 @@ static int hwrng_fillfn(void *unused)
>                         break;
>
>                 if (rc <= 0) {
> -                       pr_warn("hwrng: no data available\n");
> -                       msleep_interruptible(10000);
> +                       int i;
> +
> +                       for (i = 0; i < 100; ++i) {
> +                               if (kthread_should_stop() ||
> +                                   msleep_interruptible(10000 / 100))
> +                                       goto out;
> +                       }
>                         continue;
>                 }
>
> @@ -529,6 +534,7 @@ static int hwrng_fillfn(void *unused)
>                 add_hwgenerator_randomness((void *)rng_fillbuf, rc,
>                                            entropy >> 10);
>         }
> +out:
>         hwrng_fill = NULL;
>         return 0;
>  }
> diff --git a/drivers/net/wireless/ath/ath9k/rng.c b/drivers/net/wireless/ath/ath9k/rng.c
> index cb5414265a9b..883110c66e5e 100644
> --- a/drivers/net/wireless/ath/ath9k/rng.c
> +++ b/drivers/net/wireless/ath/ath9k/rng.c
> @@ -52,20 +52,6 @@ static int ath9k_rng_data_read(struct ath_softc *sc, u32 *buf, u32 buf_size)
>         return j << 2;
>  }
>
> -static u32 ath9k_rng_delay_get(u32 fail_stats)
> -{
> -       u32 delay;
> -
> -       if (fail_stats < 100)
> -               delay = 10;
> -       else if (fail_stats < 105)
> -               delay = 1000;
> -       else
> -               delay = 10000;
> -
> -       return delay;
> -}
> -
>  static int ath9k_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
>  {
>         struct ath_softc *sc = container_of(rng, struct ath_softc, rng_ops);
> @@ -80,10 +66,9 @@ static int ath9k_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
>                         bytes_read += max & 3UL;
>                         memzero_explicit(&word, sizeof(word));
>                 }
> -               if (!wait || !max || likely(bytes_read) || fail_stats > 110)
> +               if (!wait || !max || likely(bytes_read) ||
> +                   ++fail_stats >= 100 || msleep_interruptible(5))
>                         break;
> -
> -               msleep_interruptible(ath9k_rng_delay_get(++fail_stats));
>         }
>
>         if (wait && !bytes_read && max)
> --
> 2.35.1
>

Jason,

This patch is working as you described. Trying to read from /dev/hwrng
consistently blocks for only 1.3s before returning an IO error. The longest
that I observed 'ip link set wlan0 down' to block was also about 1.3s,
and that was immediately after 'cat /dev/hwrng'. Additionally, the longest
duration that I observed for wiphy_suspend() to return was just under 100ms.

Tested-by: Gregory Erwin <gregerwin256@...il.com>