Re: [PATCH] Fix RCU race in access of nohz_cpu_mask

[Andrew Morton <akpm@osdl.org>]

"Paul E. McKenney" <paulmck@us.ibm.com> wrote:
>
> 1.	wmb() guarantees that any writes preceding the wmb() will
>  	be seen by the interconnect before any writes following the
>  	wmb().  But this applies -only- to the writes executed by
>  	the CPU doing the wmb().
> 
>  2.	rmb() guarantees that any changes seen by the interconnect
>  	preceding the rmb() will be seen by any reads following the
>  	rmb().  Again, this applies only to reads executed by the
>  	CPU doing the wmb().  However, the changes might be due to
>  	any CPU.
> 
>  3.	mb() combines the guarantees made by rmb() and wmb().

So foo_mb() in preemptible code is potentially buggy.

I guess we assume that a context switch accidentally did enough of the
right types of barriers for things to work OK.
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