Messages in this thread | | | Date | Fri, 22 Nov 2013 10:13:13 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2 RFC 1/3] documentation: Add needed ACCESS_ONCE() calls to memory-barriers.txt |
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On Fri, Nov 22, 2013 at 04:39:08PM +0100, Peter Zijlstra wrote: > On Thu, Nov 21, 2013 at 01:31:27PM -0800, Paul E. McKenney wrote: > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > > > The Documentation/memory-barriers.txt file was written before the need > > for ACCESS_ONCE() was fully appreciated. It therefore contains no > > ACCESS_ONCE() calls, which can be a problem when people lift examples > > from it. This commit therefore adds ACCESS_ONCE() calls. > > > > Under the 'COMPILER BARRIER' section we state that: > > "This is a general barrier - lesser varieties of compiler barrier do not > exist." > > One could argue ACCESS_ONCE() is such a lesser barrier.
Fair point -- I should have updated this section when adding ACCESS_ONCE().
How about the following?
Thanx, Paul
------------------------------------------------------------------------
COMPILER BARRIER ----------------
The Linux kernel has an explicit compiler barrier function that prevents the compiler from moving the memory accesses either side of it to the other side:
barrier();
This is a general barrier -- there are no read-read or write-write variants of barrier(). Howevever, ACCESS_ONCE() can be thought of as a weak form for barrier() that affects only the specific accesses flagged by the ACCESS_ONCE().
The compiler barrier has no direct effect on the CPU, which may then reorder things however it wishes.
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