Messages in this thread | | | Subject | Re: [patch] spinlocks: remove 'volatile' | From | Arjan van de Ven <> | Date | Thu, 06 Jul 2006 14:39:43 +0200 |
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On Thu, 2006-07-06 at 08:29 -0400, linux-os (Dick Johnson) wrote: > On Thu, 6 Jul 2006, Arjan van de Ven wrote: > > > On Thu, 2006-07-06 at 07:59 -0400, linux-os (Dick Johnson) wrote: > >> On Thu, 6 Jul 2006, Ingo Molnar wrote: > >> > >>> > >>> * Linus Torvalds <torvalds@osdl.org> wrote: > >>> > >>>> I wonder if we should remove the "volatile". There really isn't > >>>> anything _good_ that gcc can do with it, but we've seen gcc code > >>>> generation do stupid things before just because "volatile" seems to > >>>> just disable even proper normal working. > >> > >> Then GCC must be fixed. The keyword volatile is correct. It should > >> force the compiler to read the variable every time it's used. > > > > this is not really what the C standard says. > > > > > > > >> This is not pointless. If GCC generates bad code, tell the > >> GCC people. The volatile keyword is essential. > > > > no the "volatile" semantics are vague, trecherous and evil. It's a LOT > > better to insert the well defined "barrier()" in the right places. > > Look at: > > http://en.wikipedia.org/wiki/Volatile_variable > > This is just what is needed to prevent the compiler from making non working > code during optimization.
and an entry level document at wikipedia is more important than the C standard ;)
> > Also look at: > > http://en.wikipedia.org/wiki/Memory_barrier > > This is used to prevent out-of-order execution, not at all what is > necessary.
I did not talk about memory barriers. In fact, barrier() is NOT a memory barrier. It's a compiler optimization barrier!
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