On some 32-bit architectures, 64bit accesses are atomic when certain conditions are satisfied. For example, on LPAE (Large Physical Address Extension) enabled ARM architecture, 'ldrd/strd' (load/store doublewords) instructions are 64bit atomic as long as the address is 64-bit aligned. This feature is to guarantee atomic accesses on newly introduced 64bit wide descriptors in the translation tables, and 's/u64' variables can be accessed atomically when they are aligned(8) on LPAE enabled ARM architecture machines. Introducing 64BIT_ATOMIC_ACCESS and 64BIT_ATOMIC_ALIGNED_ACCESS, which can be true for the 32bit architectures as well as 64bit architectures, we can optimize some kernel codes using seqlock (timekeeping) or mimics of it (like in sched/cfq) simply to read or write 64bit variables. The existing codes depend on CONFIG_64BIT to determine whether the 64bit variables can be directly accessed or need additional synchronization primitives like seqlock. CONFIG_64BIT_ATOMIC_ACCESS can be used instead of CONFIG_64BIT in the cases. 64BIT_ATOMIC_ALIGNED_ACCESS can be used in the variable declaration to indicate the alignment requirement to the compiler (__attribute__((aligned(8)))) in the way of #ifdef. Signed-off-by: Hoeun Ryu <hoeun.ryu@xxxxxxxxx> --- arch/Kconfig | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 21d0089..1def331 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -115,6 +115,26 @@ config UPROBES managed by the kernel and kept transparent to the probed application. ) +config 64BIT_ATOMIC_ACCESS + def_bool 64BIT + help + On some 32bit architectures as well as 64bit architectures, + 64bit accesses are atomic when certain conditions are satisfied. + + This symbol should be selected by an architecture if 64 bit + accesses can be atomic. + +config 64BIT_ATOMIC_ALIGNED_ACCESS + def_bool n + depends on 64BIT_ATOMIC_ACCESS + help + On 64BIT_ATOMIC_ACCESS enabled system, the address should be + aligned by 8 to guarantee the accesses are atomic. + + This symbol should be selected by an architecture if 64 bit + accesses are required to be 64 bit aligned to guarantee that + the 64bit accesses are atomic. + config HAVE_64BIT_ALIGNED_ACCESS def_bool 64BIT && !HAVE_EFFICIENT_UNALIGNED_ACCESS help -- 2.7.4
