On Wed, 8 Aug 2018 08:05:58 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> On Wed, Aug 08, 2018 at 10:49:10AM -0400, Steven Rostedt wrote: > > On Wed, 8 Aug 2018 07:33:10 -0700 > > "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote: > > > > > On Wed, Aug 08, 2018 at 09:07:24AM -0400, Steven Rostedt wrote: > > > > On Wed, 8 Aug 2018 06:03:02 -0700 > > > > "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote: > > > > > > > > > What's wrong with a this_cpu_inc()? It's atomic for the CPU. > > > > > Although > > > > > > it wont be atomic for the capture of the idx. But I also don't see > > > > > > interrupts being disabled, thus an NMI is no different than any > > > > > > interrupt doing the same thing, right? > > > > > > > > > > On architectures without increment-memory instructions, if you take > > > > > an NMI > > > > > between the load from sp->sda->srcu_lock_count and the later store, > > > > > you > > > > > lose a count. Note that both __srcu_read_lock() and > > > > > __srcu_read_unlock() > > > > > do increments of different locations, so you cannot rely on the usual > > > > > "NMI fixes up before exit" semantics you get when incrementing and > > > > > decrementing the same location. > > > > > > > > And how is this handled in the interrupt case? Interrupts are not > > > > disabled here. > > > > > > Actually, on most architectures interrupts are in fact disabled: > > > > > > #define this_cpu_generic_to_op(pcp, val, op) > > > \ > > > do { > > > \ > > > unsigned long __flags; \ > > > raw_local_irq_save(__flags); \ > > > raw_cpu_generic_to_op(pcp, val, op); \ > > > raw_local_irq_restore(__flags); \ > > > } while (0) > > > > > > NMIs, not so much. > > > > And do these archs have NMIs? > > It would appear so: Well the next question is, which of these archs that use it are in this list. > > $ find . -name 'Kconfig*' -exec grep -l 'select HAVE_NMI\>' {} \; > ./arch/sparc/Kconfig > ./arch/s390/Kconfig > ./arch/arm/Kconfig > ./arch/arm64/Kconfig > ./arch/mips/Kconfig > ./arch/sh/Kconfig > ./arch/powerpc/Kconfig Note, I know that powerpc "imitates" an NMI. It just sets the NMI as a priority higher than other interrupts. > ./arch/x86/Kconfig > And we get this: $ git grep this_cpu_add_4 arch/arm64/include/asm/percpu.h:#define this_cpu_add_4(pcp, val) _percpu_add(pcp, val) arch/s390/include/asm/percpu.h:#define this_cpu_add_4(pcp, val) arch_this_cpu_to_op_simple(pcp, val, +) arch/s390/include/asm/percpu.h:#define this_cpu_add_4(pcp, val) arch_this_cpu_add(pcp, val, "laa", "asi", int) arch/x86/include/asm/percpu.h:#define this_cpu_add_4(pcp, val) percpu_add_op((pcp), val) Which leaves us with sparc, arm, mips, sh and powerpc. sh is almost dead, and powerpc can be fixed, which I guess leaves us with sparc, arm and mips. -- Steve > Thanx, Paul