On Fri, 30 Aug 2013, Will Deacon wrote: > This is the flavour we have for ARM's hw_breakpoint code, where we have an > array of perf_event * instead of int... > > > Index: linux/arch/arm/kernel/hw_breakpoint.c > > =================================================================== > > --- linux.orig/arch/arm/kernel/hw_breakpoint.c 2013-08-26 13:48:40.956794980 -0500 > > +++ linux/arch/arm/kernel/hw_breakpoint.c 2013-08-26 13:48:40.952795024 -0500 > > @@ -344,13 +344,13 @@ int arch_install_hw_breakpoint(struct pe > > /* Breakpoint */ > > ctrl_base = ARM_BASE_BCR; > > val_base = ARM_BASE_BVR; > > - slots = (struct perf_event **)__get_cpu_var(bp_on_reg); > > + slots = (struct perf_event **)__this_cpu_read(bp_on_reg); > > ...so I don't think this is quite right, and indeed, we get a bunch of errors > from GCC: > > arch/arm/kernel/hw_breakpoint.c: In function ‘arch_install_hw_breakpoint’: > arch/arm/kernel/hw_breakpoint.c:347:33: error: incompatible types when assigning to type ‘struct perf_event *[16]’ from type ‘struct perf_event **’ > arch/arm/kernel/hw_breakpoint.c:347:1: error: incompatible types when assigning to type ‘struct perf_event *[16]’ from type ‘struct perf_event **’ > arch/arm/kernel/hw_breakpoint.c:347:1: error: incompatible types when assigning to type ‘struct perf_event *[16]’ from type ‘struct perf_event **’ > arch/arm/kernel/hw_breakpoint.c:347:1: error: incompatible types when assigning to type ‘struct perf_event *[16]’ from type ‘struct perf_event **’ Did you apply the first patch of this series which is a bug fix? > changing to match your recipe still doesn't work, however: > > arch/arm/kernel/hw_breakpoint.c: In function ‘arch_install_hw_breakpoint’: > arch/arm/kernel/hw_breakpoint.c:347:33: error: cast specifies array type Yep that is the macro bug that was fixed in the first patch. > > > > WARN_ON(preemptible()); > > > > - if (local_inc_return(&__get_cpu_var(mde_ref_count)) == 1) > > + if (this_cpu_inc_return(mde_ref_count) == 1) > > enable = DBG_MDSCR_MDE; > > I'm not sure that this is safe. We rely on local_inc_return to be atomic > with respect to the current CPU, which will end up being a wrapper around > atomic64_inc_return. However, this_cpu_inc_return simply uses a lock, so > other people accessing the count in a different manner (local_dec_and_test > below) may break local atomicity unless we start disabling interrupts or > something horrible like that. I do not see any special code for ARM for this_cpu_inc_return. The fallback solution in the core code is to disable interrupts for the inc_return and arch/arm/include/asm/percpu.h includes asm-generic/percpu.h. Where did you see it using a lock?
