Messages in this thread | | | Date | Sun, 26 Sep 2004 03:36:37 +0200 | From | Andrea Arcangeli <> | Subject | Re: ptep_establish/establish_pte needs set_pte_atomic and all set_pte must be written in asm |
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On Sun, Sep 26, 2004 at 10:59:43AM +1000, Benjamin Herrenschmidt wrote: > On Sun, 2004-09-26 at 10:46, Andrea Arcangeli wrote: > > > As far as the C language is concerned that *ptep = something can be > > implemented with 8 writes of 1 byte each (or alternatively with an > > assembler instruction that may make the written data visible not > > atomically to other cpus, despite it was written with a single opcode, > > similarly to what happens if you use incl without the lock prefix). I'm > > not saying such instruction exists in ppc64, but the compiler is > > definitely allowed to break the above. You can blame on the compiler to > > be inefficient, but you can't blame on the compiler for the security > > hazard it would generate. Only the kernel would be to blame if for > > whatever reason a gcc version would be underoptimized. > > BTW, for your reading pleasure :) > > #define atomic_set(v,i) (((v)->counter) = (i)) > > (asm-i386/atomic.h)
and then check this:
typedef struct { volatile int counter; } atomic_t; ^^^^^^^^
if the pte was at least volatile it would be a lot safer. C knows it must not mess with volatile.
> And that's really far from beeing the 2 only cases where the kernel _relies_ > on a write of a simple type like int or long to an aligned location to be > atomic. Almost all drivers manipulating DMA descriptors do that, jiffies > is a good example too afaik, and more and more and more ... so if the
jiffies, writel/readl all volatile incidentally.
> compiler is breaking that up, I think the set_pte race is the least of > our problems :)
the only one not volatile, or can you find more? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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