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Subject: Re: [PATCH] net: xfrm: use preempt-safe this_cpu_read() in
 ipcomp_alloc_tfms()
To:     Herbert Xu <herbert@gondor.apana.org.au>,
        Greg Hackmann <ghackmann@google.com>
Cc:     Steffen Klassert <steffen.klassert@secunet.com>,
        "David S. Miller" <davem@davemloft.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
References: <20180307192416.233202-1-ghackmann@google.com>
 <20180308073328.GA8111@gondor.apana.org.au>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <feddabcd-61c5-a1ef-8c1e-6e3f1d71a9b9@gmail.com>
Date:   Thu, 8 Mar 2018 08:31:47 -0800
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On 03/07/2018 11:33 PM, Herbert Xu wrote:
> On Wed, Mar 07, 2018 at 11:24:16AM -0800, Greg Hackmann wrote:
>> f7c83bcbfaf5 ("net: xfrm: use __this_cpu_read per-cpu helper") added a
>> __this_cpu_read() call inside ipcomp_alloc_tfms().  Since this call was
>> introduced, the rules around per-cpu accessors have been tightened and
>> __this_cpu_read() cannot be used in a preemptible context.
>>
>> syzkaller reported this leading to the following kernel BUG while
>> fuzzing sendmsg:
> 
> How about reverting f7c83bcbfaf5 instead?
> 
> Thanks,
> 

Because that would not be nice.

this_cpu_read() is faster than going through raw_smp_processor_id() and 
per_cpu_ptr()