linux-kernel - Re: read_barrier_depends() usage in vhost.c

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Date:   Wed, 18 Dec 2019 17:19:07 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Will Deacon <will@...nel.org>
Cc:     mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, paulmck@...nel.org,
        peterz@...radead.org, stern@...land.harvard.edu
Subject: Re: read_barrier_depends() usage in vhost.c

Will Deacon <will@...nel.org> wrote:
>
>> --->8
>> 
>> // drivers/vhost/vhost.c
>> static int get_indirect(struct vhost_virtqueue *vq,
>>                       struct iovec iov[], unsigned int iov_size,
>>                       unsigned int *out_num, unsigned int *in_num,
>>                       struct vhost_log *log, unsigned int *log_num,
>>                       struct vring_desc *indirect)
>> {
>>       [...]
>> 
>>       /* We will use the result as an address to read from, so most
>>        * architectures only need a compiler barrier here. */
>>       read_barrier_depends();
>> 
>> --->8
>> 
>> Unfortunately, although the barrier is commented (hurrah!), it's not
>> particularly enlightening about the accesses making up the dependency
>> chain, and I don't understand the supposed need for a compiler barrier
>> either (read_barrier_depends() doesn't generally provide this).
>> 
>> Does anybody know which accesses are being ordered here? Usually you'd need
>> a READ_ONCE()/rcu_dereference() beginning the chain, but I haven't managed
>> to find one...

I think what it's trying to separate is using indirect->addr as a
base and then reading from that through copy_from_iter.

Cheers,
-- 
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