-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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@chello.nl> 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@chello.nl> Acked-by: Tejun Heo <tj@kernel.org> LKML-Reference: <20100917093009.252867712@chello.nl> Signed-off-by: Ingo Molnar <mingo@elte.hu> 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@chello.nl> 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@chello.nl> Cc: Tejun Heo <tj@kernel.org> LKML-Reference: <20100917093009.308453028@chello.nl> Signed-off-by: Ingo Molnar <mingo@elte.hu> Hope that helps. TJ. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://www.enigmail.net/ iEYEARECAAYFAlBovEUACgkQ7+w3pCnNYID+4ACgi4oUU3n/ac+eUHGevjqgbN85 e24AniE1ASr6nzSHSXoJFNRy0Y85H1ou =F8AK -----END PGP SIGNATURE-----