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| author | Paul E. McKenney <paulmck@kernel.org> | 2025-01-08 13:37:17 -0800 |
|---|---|---|
| committer | Boqun Feng <boqun.feng@gmail.com> | 2025-02-04 21:50:06 -0800 |
| commit | df0cee43114bd212e3fc9add2a18639605049b95 (patch) | |
| tree | 9cbddffbf13c16c310dc9eb0aec4d44d66b6e4d9 | |
| parent | bea1d19f03644b0eb7ac0b7d96f79a3af13f196b (diff) | |
| download | linux-stable-df0cee43114bd212e3fc9add2a18639605049b95.tar.gz linux-stable-df0cee43114bd212e3fc9add2a18639605049b95.tar.bz2 linux-stable-df0cee43114bd212e3fc9add2a18639605049b95.zip | |
docs: Improve discussion of this_cpu_ptr(), add raw_cpu_ptr()
Most of the this_cpu_*() operations may be used in preemptible code,
but not this_cpu_ptr(), and for good reasons. Therefore, better explain
the reasons and call out raw_cpu_ptr() as an alternative in certain very
special cases.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
| -rw-r--r-- | Documentation/core-api/this_cpu_ops.rst | 22 |
1 files changed, 16 insertions, 6 deletions
diff --git a/Documentation/core-api/this_cpu_ops.rst b/Documentation/core-api/this_cpu_ops.rst index 91acbcf30e9b..533ac5dd5750 100644 --- a/Documentation/core-api/this_cpu_ops.rst +++ b/Documentation/core-api/this_cpu_ops.rst @@ -138,12 +138,22 @@ get_cpu/put_cpu sequence requires. No processor number is available. Instead, the offset of the local per cpu area is simply added to the per cpu offset. -Note that this operation is usually used in a code segment when -preemption has been disabled. The pointer is then used to -access local per cpu data in a critical section. When preemption -is re-enabled this pointer is usually no longer useful since it may -no longer point to per cpu data of the current processor. - +Note that this operation can only be used in code segments where +smp_processor_id() may be used, for example, where preemption has been +disabled. The pointer is then used to access local per cpu data in a +critical section. When preemption is re-enabled this pointer is usually +no longer useful since it may no longer point to per cpu data of the +current processor. + +The special cases where it makes sense to obtain a per-CPU pointer in +preemptible code are addressed by raw_cpu_ptr(), but such use cases need +to handle cases where two different CPUs are accessing the same per cpu +variable, which might well be that of a third CPU. These use cases are +typically performance optimizations. For example, SRCU implements a pair +of counters as a pair of per-CPU variables, and rcu_read_lock_nmisafe() +uses raw_cpu_ptr() to get a pointer to some CPU's counter, and uses +atomic_inc_long() to handle migration between the raw_cpu_ptr() and +the atomic_inc_long(). Per cpu variables and offsets ----------------------------- |