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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 [off-list ref]
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 [off-list ref]
    Acked-by: Tejun Heo [off-list ref]
    LKML-Reference: [off-list ref]
    Signed-off-by: Ingo Molnar [off-list ref]

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 [off-list ref]
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 [off-list ref]
    Cc: Tejun Heo [off-list ref]
    LKML-Reference: [off-list ref]
    Signed-off-by: Ingo Molnar [off-list ref]


Hope that helps.

TJ.



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