linux-kernel - Re: [PATCH v7] ath9k: let sleep be interrupted when unregistering hwrng

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Date:   Wed, 29 Jun 2022 11:24:49 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org
Cc:     Gregory Erwin <gregerwin256@...il.com>,
        Kalle Valo <kvalo@...nel.org>,
        Rui Salvaterra <rsalvaterra@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v7] ath9k: let sleep be interrupted when unregistering
 hwrng

"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>
> ---
>  drivers/char/hw_random/core.c        | 30 ++++++++++++++++++++++++----
>  drivers/net/wireless/ath/ath9k/rng.c | 19 +++++++-----------
>  2 files changed, 33 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 16f227b995e8..df45c265878e 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -38,6 +38,8 @@ static LIST_HEAD(rng_list);
>  static DEFINE_MUTEX(rng_mutex);
>  /* Protects rng read functions, data_avail, rng_buffer and rng_fillbuf */
>  static DEFINE_MUTEX(reading_mutex);
> +/* Keeps track of whoever is wait-reading it currently while holding reading_mutex. */
> +static struct task_struct *current_waiting_reader;
>  static int data_avail;
>  static u8 *rng_buffer, *rng_fillbuf;
>  static unsigned short current_quality;
> @@ -208,6 +210,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
>  	int err = 0;
>  	int bytes_read, len;
>  	struct hwrng *rng;
> +	bool wait;
>  
>  	while (size) {
>  		rng = get_current_rng();
> @@ -225,9 +228,15 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
>  			goto out_put;
>  		}
>  		if (!data_avail) {
> +			wait = !(filp->f_flags & O_NONBLOCK);
> +			if (wait && cmpxchg(&current_waiting_reader, NULL, current) != NULL) {
> +				err = -EINTR;
> +				goto out_unlock_reading;
> +			}
>  			bytes_read = rng_get_data(rng, rng_buffer,
> -				rng_buffer_size(),
> -				!(filp->f_flags & O_NONBLOCK));
> +				rng_buffer_size(), wait);
> +			if (wait && cmpxchg(&current_waiting_reader, current, NULL) != current)
> +				synchronize_rcu();

So this synchronize_rcu() is to ensure the hwrng_unregister() thread has
exited the rcu_read_lock() section below? Isn't that a bit... creative...
use of RCU? :)

Also, synchronize_rcu() can potentially take a while on a busy system,
is it OK to call it while holding the mutex?

-Toke