Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcumacro

From: Eric Dumazet
Date: Wed May 22 2013 - 08:30:47 EST

On Wed, 2013-05-22 at 15:58 +0400, Roman Gushchin wrote:

> +/*
> + * Same as ACCESS_ONCE(), but used for accessing field of a structure.
> + * The main goal is preventing compiler to store &ptr->field in a register.

But &ptr->field is a constant during the whole duration of
udp4_lib_lookup2() and could be in a register, in my case field is at
offset 0, and ptr is a parameter (so could be in a 'register')

The bug you found is that compiler caches the indirection (ptr->field)
into a register, not that compiler stores &ptr->field into a register.

> + */
> +#define ACCESS_FIELD_ONCE(PTR, FIELD) (((volatile typeof(*PTR) *)PTR)->FIELD)
> +

Here we force the compiler to consider ptr as volatile, but semantically
it is not required in rcu_dereference(ptr->field)

We want field to be reloaded, not ptr.

So yes, the patch appears to fix the bug, but it sounds not logical to

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