[PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

[Boqun Feng]

On Mon, Oct 26, 2015 at 11:20:01AM +0900, Michael Ellerman wrote:
> 
> Sorry guys, these threads are so long I tend not to read them very actively :}
> 
> Looking at the system call path, the straight line path does not include any
> barriers. I can't see any hidden in macros either.
> 
> We also have an explicit sync in the switch_to() path, which suggests that we
> know system call is not a full barrier.
> 
> Also looking at the architecture, section 1.5 which talks about the
> synchronisation that occurs on system calls, defines nothing in terms of
> memory ordering, and includes a programming note which says "Unlike the
> Synchronize instruction, a context synchronizing operation does not affect the
> order in which storage accesses are performed.".
> 

Thank you, Michael. So IIUC, "sc" and "rfid" just imply an execution
barrier like "isync" rather than a memory barrier. So memory barriers
are needed if a system call need a memory ordering guarantee.

Regards,
Boqun

> Whether that's actually how it's implemented I don't know, I'll see if I can
> find out.
> 
> cheers
> 
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