Messages in this thread | | | From | Sergei Organov <> | Subject | Re: [PATCH] Document Linux's memory barriers [try #4] | Date | Wed, 15 Mar 2006 12:09:43 +0300 |
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"linux-os \(Dick Johnson\)" <linux-os@analogic.com> writes:
> On Tue, 14 Mar 2006, David Howells wrote: > >> Sergei Organov <osv@javad.com> wrote: >> >>> "You can prevent an `asm' instruction from being deleted by writing the >>> keyword `volatile' after the `asm'. [...] >>> The `volatile' keyword indicates that the instruction has important >>> side-effects. GCC will not delete a volatile `asm' if it is reachable. >>> (The instruction can still be deleted if GCC can prove that >>> control-flow will never reach the location of the instruction.) *Note >>> that even a volatile `asm' instruction can be moved relative to other >>> code, including across jump instructions.*" >> >> Ummm... If "asm volatile" statements don't form compiler barriers, then how do >> you specify a compiler barrier? Or is that what the "memory" bit in: >> >> #define barrier() __asm__ __volatile__("": : :"memory") >> >> does? >> >> David > > Yeh. This is the problem (restated) that I mentioned the other > day when you must do a dummy read of the PCI/Bus to flush all > the writes, to some variable that gcc can't decide isn't > important. That's why (void)readl(PCI_STATUS) won't work > (with gcc 3.3.3 anyway).
If it indeed doesn't, then it's a bug in GCC. GCC shouldn't throw away volatile accesses. I've already quoted the GCC manual for you:
" Less obvious expressions are where something which looks like an access is used in a void context. An example would be,
volatile int *src = SOMEVALUE; *src;
With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to. "
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