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Date:   Tue, 5 Oct 2021 11:17:16 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     linux-crypto@vger.kernel.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] crypto: testmgr - Only disable migration in
 crypto_disable_simd_for_test()
Message-ID: <YVyWrKUk81e10zM7@gmail.com>
References: <20210928115401.441339-1-bigeasy@linutronix.de>
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Hi Sebastian,

On Tue, Sep 28, 2021 at 01:54:01PM +0200, Sebastian Andrzej Siewior wrote:
> crypto_disable_simd_for_test() disables preemption in order to receive a
> stable per-CPU variable which it needs to modify in order to alter
> crypto_simd_usable() results.
> 
> This can also be achived by migrate_disable() which forbidds CPU
> migrations but allows the task to be preempted. The latter is important
> for PREEMPT_RT since operation like skcipher_walk_first() may allocate
> memory which must not happen with disabled preemption on PREEMPT_RT.
> 
> Use migrate_disable() in crypto_disable_simd_for_test() to achieve a
> stable per-CPU pointer.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

The comment above the definition of migrate_disable() in include/linux/preempt.h
claims that it is a temporary workaround.

Is there a better way to do this that should be used instead?

- Eric