Messages in this thread | | | Date | Thu, 5 Dec 2013 10:05:47 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt: Document ACCESS_ONCE() |
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On Thu, Dec 05, 2013 at 10:50:42AM +0100, Ingo Molnar wrote: > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > + (*) The compiler is within its rights to reload a variable, for example, > > + in cases where high register pressure prevents the compiler from > > + keeping all data of interest in registers. The compiler might > > + therefore optimize the variable tmp out of our previous example: > > + > > + while (tmp = a) > > + do_something_with(tmp); > > + > > + This could result in the following code, which is perfectly safe in > > + single-threaded code, but can be fatal in concurrent code: > > + > > + while (a) > > + do_something_with(a); > > + > > + For example, the optimized version of this code could result in > > + passing a zero to do_something_with() in the case where the variable > > + a was modified by some other CPU between the "while" statement and > > + the call to do_something_with(). > > Nit: I guess references to variable names such as 'a' should be quoted > (same for 'tmp', 'b', etc): > > For example, the optimized version of this code could result in > passing a zero to do_something_with() in the case where the variable > 'a' was modified by some other CPU between the "while" statement and > the call to do_something_with(). > > which makes reading it less ambiguous and more fluid IMO. This > observation applies to the whole document as 'a' is used in many > places.
Good point, fixed.
Thanx, Paul
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