linux-kernel - Re: [PATCH] random: Don't freeze in add_hwgenerator

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

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@...iej.szmigiero.name>
> ---
>  drivers/char/random.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

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