On Thu, Dec 05, 2013 at 10:33:34AM +0100, Ingo Molnar wrote:
>
> * Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
>
> > + (*) The compiler is within its rights to reorder memory accesses unless
> > + you tell it not to. For example, consider the following interaction
> > + between process-level code and an interrupt handler:
> > +
> > + void process_level(void)
> > + {
> > + msg = get_message();
> > + flag = true;
> > + }
> > +
> > + void interrupt_handler(void)
> > + {
> > + if (flag)
> > + process_message(msg);
> > + }
> > +
> > + There is nothing to prevent the the compiler from transforming
> > + process_level() to the following, in fact, this might well be a
> > + win for single-threaded code:
> > +
> > + void process_level(void)
> > + {
> > + flag = true;
> > + msg = get_message();
> > + }
> > +
> > + If the interrupt occurs between these two statement, then
> > + interrupt_handler() might be passed a garbled msg. Use ACCESS_ONCE()
> > + to prevent this as follows:
> > +
> > + void process_level(void)
> > + {
> > + ACCESS_ONCE(msg) = get_message();
> > + ACCESS_ONCE(flag) = true;
> > + }
> > +
> > + void interrupt_handler(void)
> > + {
> > + if (ACCESS_ONCE(flag))
> > + process_message(ACCESS_ONCE(msg));
> > + }
>
> Technically, if the interrupt handler is the innermost context, the
> ACCESS_ONCE() is not needed in the interrupt_handler() code.
>
> Since for the vast majority of Linux code IRQ handlers are the most
> atomic contexts (very few drivers deal with NMIs) I suspect we should
> either remove that ACCESS_ONCE() from the example or add a comment
> explaining that in many cases those are superfluous?
How about the following additional paragraph?
Note that the ACCESS_ONCE() wrappers in interrupt_handler()
are needed if this interrupt handler can itself be interrupted
by something that also accesses 'flag' and 'msg', for example,
a nested interrupt or an NMI. Otherwise, ACCESS_ONCE() is not
needed in interrupt_handler() other than for documentation purposes.
> > + (*) For aligned memory locations whose size allows them to be accessed
> > + with a single memory-reference instruction, prevents "load tearing"
> > + and "store tearing," in which a single large access is replaced by
> > + multiple smaller accesses. For example, given an architecture having
> > + 16-bit store instructions with 7-bit immediate fields, the compiler
> > + might be tempted to use two 16-bit store-immediate instructions to
> > + implement the following 32-bit store:
> > +
> > + p = 0x00010002;
> > +
> > + Please note that GCC really does use this sort of optimization,
> > + which is not surprising given that it would likely take more
> > + than two instructions to build the constant and then store it.
> > + This optimization can therefore be a win in single-threaded code.
> > + In fact, a recent bug (since fixed) caused GCC to incorrectly use
> > + this optimization in a volatile store. In the absence of such bugs,
> > + use of ACCESS_ONCE() prevents store tearing:
> > +
> > + ACCESS_ONCE(p) = 0x00010002;
>
> I suspect the last sentence should read:
>
> > + In the absence of such bugs,
> > + use of ACCESS_ONCE() prevents store tearing in this example:
> > +
> > + ACCESS_ONCE(p) = 0x00010002;
>
> Otherwise it could be read as a more generic statement (leaving out
> 'load tearing')?
Good point, fixed.
Indeed, I don't have a good example for load tearing. I do have some -bad-
examples, like the following:
struct __attribute__((__packed__)) foo {
short a;
int b;
short c;
};
struct foo foov;
short aa;
int bb;
short cc;
...
aa = foov.a;
bb = foov.b;
cc = foov.c;
A clever compiler might choose to pack aa, bb, and cc in memory, then
implement the three assignments using two 32-bit loads and two 32-bit
stores, which would result in load tearing of foov.b.
Hmmm... Maybe I should give this example anyway, just to show that
load tearing really could occur in practice... If nothing else, it
should be a cautionary tale for those tempted to pack their structures.
And there are quite a number of packed structures in the Linux kernel.
Sold! I have added this example, but using a pair of struct foo variables
in order to forestall maidenly protests from those who believe that no
production-quality compiler would ever misalign variable bb. ;-)
Thanx, Paul
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