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From: Herbert Xu <herbert@gondor.apana.org.au>
To: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc: Theodore Ts'o <tytso@mit.edu>, Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Keerthy <j-keerthy@ti.com>, Stephen Boyd <swboyd@chromium.org>,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] random: Don't freeze in add_hwgenerator_randomness() if stopping kthread
Date: Fri, 15 Nov 2019 14:08:33 +0800
Message-ID: <20191115060833.xze5kkq64pxci2la@gondor.apana.org.au> (raw)
In-Reply-To: <20191110135543.3476097-1-mail@maciej.szmigiero.name>

On Sun, Nov 10, 2019 at 02:55:42PM +0100, Maciej S. Szmigiero wrote:
> Since commit 59b569480dc8
> ("random: Use wait_event_freezable() in add_hwgenerator_randomness()")
> there is a race in add_hwgenerator_randomness() between freezing and
> stopping the calling kthread.
> 
> This commit changed wait_event_interruptible() call with
> kthread_freezable_should_stop() as a condition into wait_event_freezable()
> with just kthread_should_stop() as a condition to fix a warning that
> kthread_freezable_should_stop() might sleep inside the wait.
> 
> wait_event_freezable() ultimately calls __refrigerator() with its
> check_kthr_stop argument set to false, which causes it to keep the kthread
> frozen even if somebody calls kthread_stop() on it.
> 
> Calling wait_event_freezable() with kthread_should_stop() as a condition
> is racy because it doesn't take into account the situation where this
> condition becomes true on a kthread marked for freezing only after this
> condition has already been checked.
> 
> Calling freezing() should avoid the issue that the commit 59b569480dc8 has
> fixed, as it is only a checking function, it doesn't actually do the
> freezing.
> 
> add_hwgenerator_randomness() has two post-boot users: in khwrng the
> kthread will be frozen anyway by call to kthread_freezable_should_stop()
> in its main loop, while its second user (ath9k-hwrng) is not freezable at
> all.
> 
> This change allows a VM with virtio-rng loaded to write s2disk image
> successfully.
> 
> Fixes: 59b569480dc8 ("random: Use wait_event_freezable() in add_hwgenerator_randomness()")
> Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
> ---
>  drivers/char/random.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-10 13:55 Maciej S. Szmigiero
2019-11-15  6:08 ` Herbert Xu [this message]
     [not found] ` <5dcee409.1c69fb81.f5027.48ad@mx.google.com>
2019-11-16 23:01   ` Maciej S. Szmigiero
2019-11-17  0:56     ` Herbert Xu
2019-11-17  4:09       ` Theodore Y. Ts'o

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