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SubjectRe: [PATCH 1/8] ppc/kvm: Replace ACCESS_ONCE with READ_ONCE
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@de.ibm.com>
>> ---
>> 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




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