Re: [patch] spinlocks: remove 'volatile'

[Chase Venters]

On Saturday 08 July 2006 01:12, trajce nedev wrote:
> On Fri, 7 Jul 2006, Linus Torvalds wrote:
> >No.
> >
> >"volatile" simply CANNOT get the job done. It fundamentally does _nothing_
> >for all the issues that are fundamental today: CPU memory ordering in SMP,
> >special IO synchronization requirements for memory-mapped IO registers etc
> >etc.
> >
> >It's not that "volatile" is the "portable way". It's that "volatile" is
> >fundamentally not sufficient for the job.
>
> Incorrect.  I haven't been following this thread very closely but your
> assault on volatile is inappropriate.  There are several present day uses
> in assember instructions.

Perhaps you should have followed this thread closely before composing your 
assault on Linus. We're not talking about "asm volatile". We're talking about 
the "volatile" keyword as applied to variables. 'volatile' as applied to 
inline ASM is of course necessary in many cases -- no one is disputing that.

> For example, if you're going to clobber hard registers specifically,
> volatile is required because you cannot write a clobber that overlaps input
> or output operands:
> 	asm volatile ("movc3 %0,%1,%2"
>
> 		: /* none */
> 		: "g" (from), "g" (to), "g" (count)
> 		: "r0", "r1", "r2", "r3", "r4", "r5");
>
> You cannot have an operand that describes a register class with a single
> member if that register is in the clobber list; there is simply no way to
> specify that an input operand is modified without explicitly specifying it
> as output.  If all the output operands are for that purpose, they are
> considered unused.  volatile is _necessary_ for the asm to prevent the
> compiler from deleting it for this reason.
>
> Furthermore, if the asm modifies memory unpredictably, you must add
> `memory` to the list of clobbers.  The compiler will not keep the memory
> value cached in registers any longer across asm instructions; volatile is
> added if the memory you're touching is not listed in the inputs or outputs
> since the `memory` clobber does not count as a side-effect of issuing the
> asm. volatile indicates the asm has side effects that are _important_ and
> will not delete it (if it's reachable, otherwise it is fair game).
>
> If your asm has output operands, gcc currently assumes that the instruction
> has no side effects except changing these output operands (for
> optimization).  The compiler is free to eliminate or move out of loops any
> instructions with side effects if the output operands aren't used.  Even
> more dangerously, if the asm has a side effect on a variable that doesn't
> otherwise appear to change, the old value _may_ be reused later if it's
> found in a register.  volatile is used to prevent the asm from being
> deleted in this case (or more dangerously, combined):
> 	#define get_and_set_priority(new)		\
> 	({ 	int __old;				\
> 		asm volatile ("get_and_set_priority %0, %1"	\
>
> 			: "=g" (__old) : "g" (new));	\
>
> 		__old; })
> The alternative is to write an asm with no outputs so that the compiler
> knows the instruction has side effects and won't eliminate or move it.
>
> Also, gcc will not reschedule instructions across volatile asm's which is
> often necessary:
> 	*(volatile int *)addr = foo;
> 	asm volatile ("eieio" : : );
> If addr contains the address of a memory-mapped device register, then the
> PowerPC eieio instruction informs the processor that it is necessary to
> store to that device register before issuing any other I/O.
>
> 		Trajce Nedev
> 		tnedev@xxxxxxx
>

Thanks,
Chase
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