Messages in this thread |  | | Date | Wed, 04 Dec 2013 08:44:50 -0800 | From | "H. Peter Anvin" <> | Subject | Re: Regression due to 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() functions |
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On 12/04/2013 12:59 AM, Ingo Molnar wrote: > > * H. Peter Anvin <hpa@zytor.com> wrote: > >> Hi guys, >> >> 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test() >> functions >> >> causes a regression, because it incorrectly changed the constraints of >> bitops. >> >> Specifically, the GEN_BINARY_RMWcc() hardcodes a constraint as "er", but >> it needs to be "Ir" for the bitops themselves. "I" is correct (as >> opposed to "J" even on 64 bits, because we only generate the 64-bit >> version when we have a register operand. >> >> Unfortunately there isn't a way we can get gcc+gas to generate a version >> with an offset pointer. > > Does the regression manifest itself in any actual breakage - if yes, > how does it look like? (People experiencing similar symptoms will be > helped by seeing a fix matching their problems.) >
It was discovered because it caused a build failure in a not-yet-submitted driver patch. This happens when someone uses test_and_set_bit() or another similar operation on a fixed bit index above 255; the assembler throws an error at that point and the build fails.
*HOWEVER*, for bit indicies in the range 32-255, the current code will instead silently miscompile, as the CPU will truncate the argument to 5 bits. I don't know if there are any such instances in the current kernel, but it is entirely possible there is, with unknown but potentially disastrous results.
-hpa
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