Messages in this thread | | | Subject | Re: ptep_establish/establish_pte needs set_pte_atomic and all set_pte must be written in asm | From | Benjamin Herrenschmidt <> | Date | Sun, 26 Sep 2004 10:59:43 +1000 |
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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 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 compiler is breaking that up, I think the set_pte race is the least of our problems :)
Ben.
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