Am 16.01.2015 um 00:09 schrieb Michael Ellerman: > On Thu, 2015-01-15 at 09:58 +0100, Christian Borntraeger wrote: >> ACCESS_ONCE does not work reliably on non-scalar types. For >> example gcc 4.6 and 4.7 might remove the volatile tag for such >> accesses during the SRA (scalar replacement of aggregates) step >> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) >> >> Change the ppc/kvm code to replace ACCESS_ONCE with READ_ONCE. >> >> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> >> --- >> arch/powerpc/kvm/book3s_hv_rm_xics.c | 8 ++++---- >> arch/powerpc/kvm/book3s_xics.c | 16 ++++++++-------- >> 2 files changed, 12 insertions(+), 12 deletions(-) >> >> diff --git a/arch/powerpc/kvm/book3s_hv_rm_xics.c b/arch/powerpc/kvm/book3s_hv_rm_xics.c >> index 7b066f6..7c22997 100644 >> --- a/arch/powerpc/kvm/book3s_hv_rm_xics.c >> +++ b/arch/powerpc/kvm/book3s_hv_rm_xics.c >> @@ -152,7 +152,7 @@ static void icp_rm_down_cppr(struct kvmppc_xics *xics, struct kvmppc_icp *icp, >> * in virtual mode. >> */ >> do { >> - old_state = new_state = ACCESS_ONCE(icp->state); >> + old_state = new_state = READ_ONCE(icp->state); > > These are all icp->state. > > Which is a union, but it's only the size of unsigned long. So in practice there > shouldn't be a bug here right? This bug was that gcc lost the volatile tag when propagating aggregates to scalar types. So in theory a union could be affected. See the original problem ( http://marc.info/?i=54611D86.4040306%40de.ibm.com ) which happened on union ipte_control { unsigned long val; struct { unsigned long k : 1; unsigned long kh : 31; unsigned long kg : 32; }; }; Christian -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>