On Wed, 2013-12-04 at 08:44 -0800, H. Peter Anvin wrote: > On 12/04/2013 12:59 AM, Ingo Molnar wrote: > > > > * H. Peter Anvin <[email protected]> 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.
I'm seeing reports from folks using vfio for PCI device assignment through QEMU that hugepages aren't being released on VM shutdown and reverting 0c44c2d0f459 resolves the problem. I haven't confirmed this myself nor do I pretend to understand the changes this commit makes. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
