Messages in this thread | | | Subject | Re: [patch] spinlocks: remove 'volatile' | From | Steven Rostedt <> | Date | Thu, 13 Jul 2006 23:30:03 -0400 |
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On Sat, 2006-07-08 at 08:46 -0500, Chase Venters wrote: > On Saturday 08 July 2006 01:44, trajce nedev wrote: > > On Sat, 8 Jul 2006, Chase Venters wrote: > > >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. > > > > Ok, let's port a spinlock macro that spins instead of context switches > > instead of using the pthread garbage on IA64 or AMD64: > > > > #if ((defined (_M_IA64) || defined (_M_AMD64)) && !defined(NT_INTEREX)) > > #include <windows.h> > > #pragma intrinsic (_InterlockedExchange) > > > > typedef volatile LONG lock_t[1]; > > > > #define LockInit(v) ((v)[0] = 0) > > #define LockFree(v) ((v)[0] = 0) > > #define Unlock(v) ((v)[0] = 0) > > > > __forceinline void Lock(volatile LONG *hPtr) > > { > > int iValue; > > > > for (;;) { > > iValue = _InterlockedExchange((LPLONG)hPtr, 1); > > if (iValue == 0) > > return; > > while (*hPtr); > > } > > } > > > > Please show me how I can write this to spinlock without using volatile. > > Please show me how that lock is safe without a compiler memory barrier! What's > to stop your compiler from moving loads and stores across your inlined lock > code? > > When you add the missing compiler memory barrier, the "volatile" classifier > becomes unnecessary. > > Actually, please just read the thread. We've been over this already. It's > starting to get really old.
Actually it was good that he posted. It just proved Linus's assumption that those that use volatile usually don't understand exactly what is going on. I certainly learned a lot in this thread ;)
[ /me goes off to fix some of my broken "volatile" code ... ]
-- Steve
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