----- On Jul 2, 2018, at 8:35 PM, Chris Lameter cl@xxxxxxxxx wrote: > On Mon, 2 Jul 2018, Mathieu Desnoyers wrote: > >> > >> > Platforms with 32 bit word size only guarantee atomicity of a 32 bit >> > write or RMV instruction. >> > >> > Special instructions may exist on a platform to perform 64 bit atomic >> > updates. We use cmpxchg64 f.e. on Intel 32 bit platforms to guarantee >> > atomicity8. >> > >> > So use the macros that we have to guarantee 64 bit ops and you should be >> > fine. See linux/arch/x86/include/asm/atomic64_32.h >> >> We are talking about user-space here. What we need is a single instruction >> atomic store, similar to what WRITE_ONCE() does in the kernel. The discussion >> is about whether doing the user-space equivalent of a WRITE_ONCE() to a u64 >> on a 32-bit architecture should be considered to provide single-copy atomicity >> on the low 32 bits. > > Right. You would need to make this work for userspace. atomic64_32.h is a > good reference as to which instructions provide 64 bit atomicity on 32 > bit platforms. We only need to update a pointer, so we don't need 64-bit atomicity on 32-bit processes. What we need is to ensure single-copy atomicity of the 32-bit pointer update on the 32-bit process in a field read from the kernel as a __u64. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
