Hi Herbert,
On Thu, 16 Aug 2007, Herbert Xu wrote: > On Thu, Aug 16, 2007 at 06:28:42AM +0530, Satyam Sharma wrote: > > > > > The udelay itself certainly should have some form of cpu_relax in it. > > > > Yes, a form of barrier() must be present in mdelay() or udelay() itself > > as you say, having it in __const_udelay() is *not* enough (superflous > > actually, considering it is already a separate translation unit and > > invisible to the compiler). > > As long as __const_udelay does something which has the same > effect as barrier it is enough even if it's in the same unit. Only if __const_udelay() is inlined. But as I said, __const_udelay() -- although marked "inline" -- will never be inlined anywhere in the kernel in reality. It's an exported symbol, and never inlined from modules. Even from built-in targets, the definition of __const_udelay is invisible when gcc is compiling the compilation units of those callsites. The compiler has no idea that that function has barriers or not, so we're saved here _only_ by the lucky fact that __const_udelay() is in a different compilation unit. > As a matter of fact it does on i386 where __delay either uses > rep_nop or asm/volatile. __delay() can be either delay_tsc() or delay_loop() on i386. delay_tsc() uses the rep_nop() there for it's own little busy loop, actually. But for a call site that inlines __const_udelay() -- if it were ever moved to a .h file and marked inline -- the call to __delay() will _still_ be across compilation units. So, again for this case, it does not matter if the callee function has compiler barriers or not (it would've been a different story if we were discussing real/CPU barriers, I think), what saves us here is just the fact that a call is made to a function from a different compilation unit, which is invisible to the compiler when compiling the callsite, and hence acting as the compiler barrier. Regarding delay_loop(), it uses "volatile" for the "asm" which has quite different semantics from the C language "volatile" type-qualifier keyword and does not imply any compiler barrier at all. Satyam - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
