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SubjectRe: [PATCH] Fix RCU race in access of nohz_cpu_mask
"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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