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On 30/09/12 15:35, Robert P. J. Day wrote:>
> but toward the end, you can read where i'm a bit puzzled by two macros i ran across just this morning that i had never seen before, and defined in <linux/percpu.h>:
>
> #define get_cpu_ptr(var) ({ \ preempt_disable(); \ this_cpu_ptr(var); })
>
> #define put_cpu_ptr(var) do { \ (void)(var); \ preempt_enable(); \ } while (0)
You may be aware of this already, but if not, it's a very useful technique to find out *why* some code was added. Comments in the kernel are usually kept sparse and the 'why' is often in the commit
history.
$ git blame include/linux/percpu.h | grep get_cpu_ptr
8b8e2ec1e (Peter Zijlstra 2010-09-16 19:21:28 +0200 42) #define get_cpu_ptr(var) ({
$ git show 8b8e2ec1e
commit 8b8e2ec1eeca7f6941bc81cefc9663018d6ceb57
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Thu Sep 16 19:21:28 2010 +0200
percpu: Add {get,put}_cpu_ptr
These are similar to {get,put}_cpu_var() except for dynamically
allocated per-cpu memory.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
LKML-Reference: <20100917093009.252867712@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
LKML (Linux Kernel Mailing List): https://lkml.org/lkml/2010/9/17/74
$ git blame kernel/events/core.c | grep 'cpuctx = get_cpu_ptr'
41945f6cc kernel/perf_event.c (Peter Zijlstra 2010-09-16 19:17:24 +0200 4229) cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
41945f6cc kernel/perf_event.c (Peter Zijlstra 2010-09-16 19:17:24 +0200 4375) cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
41945f6cc kernel/perf_event.c (Peter Zijlstra 2010-09-16 19:17:24 +0200 4571) cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
$ git show 41945f6cc
commit 41945f6ccf1e86f87fddf6b32db9cf431c05fb54
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Thu Sep 16 19:17:24 2010 +0200
perf: Avoid RCU vs preemption assumptions
The per-pmu per-cpu context patch converted things from
get_cpu_var() to this_cpu_ptr(), but that only works if
rcu_read_lock() actually disables preemption, and since
there is no such guarantee, we need to fix that.
Use the newly introduced {get,put}_cpu_ptr().
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
LKML-Reference: <20100917093009.308453028@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Hope that helps.
TJ.
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