As discussed on LKML http://marc.info/?i=54611D86.4040306%40de.ibm.com ACCESS_ONCE might fail with specific compiler for non-scalar accesses. Here is a set of patches to tackle that problem. The first patch is already in kvm/next and this series is against rc3 (as kvm/next so that we can avoid a merge conflict as soon as this series has stabilized). The 2nd patch introduces READ_ONCE/ASSIGN_ONCE as suggested by Linus. The 2nd to last patch will force ACCESS_ONCE to error-out if it is used on non-scalar accesses. I have cross-compiled the resulting kernel with defconfig for microblaze, m68k, alpha, s390,x86_64, i686, sparc, sparc64, mips, ia64, arm and arm64. Will Deacon pointed me to the right defconfig for arm32 to also trigger a finding here. Runtime testing was only done on s390x. There is a small problem left with sparc (32bit) and m68k: mm/rmap.c: In function 'mm_find_pmd': include/linux/compiler.h:220:72: warning: '__val' may be used uninitialized in this function [-Wmaybe-uninitialized] ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; }) ^ include/linux/compiler.h:220:20: note: '__val' was declared here ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; }) ^ mm/rmap.c:584:9: note: in expansion of macro 'READ_ONCE' pmde = READ_ONCE(*pmd); Reason is that for both architectures pmd_t is long[16]. WTF? So the next spin will either fix m68k/sparc or use a barrier + ACCESS_ONCE. Comments? Christian Borntraeger (10): KVM: s390: Fix ipte locking kernel: Provide READ_ONCE and ASSIGN_ONCE mm: replace ACCESS_ONCE with READ_ONCE x86/spinlock: Replace ACCESS_ONCE with READ_ONCE/ASSIGN_ONCE x86: Replace ACCESS_ONCE in gup with READ_ONCE mips: Replace ACCESS_ONCE in gup with READ_ONCE arm64: Replace ACCESS_ONCE for spinlock code with READ_ONCE arm: Replace ACCESS_ONCE for spinlock code with READ_ONCE tighten rules for ACCESS ONCE KVM: s390: change ipte lock from barrier to READ_ONCE arch/arm/include/asm/spinlock.h | 4 ++-- arch/arm64/include/asm/spinlock.h | 4 ++-- arch/mips/mm/gup.c | 2 +- arch/s390/kvm/gaccess.c | 14 +++++++------ arch/x86/include/asm/spinlock.h | 8 +++---- arch/x86/mm/gup.c | 2 +- include/linux/compiler.h | 44 ++++++++++++++++++++++++++++++++++++++- mm/gup.c | 2 +- mm/memory.c | 2 +- mm/rmap.c | 2 +- 10 files changed, 64 insertions(+), 20 deletions(-)
