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| author | Tejun Heo <tj@kernel.org> | 2018-03-14 20:45:12 +0100 |
|---|---|---|
| committer | Tejun Heo <tj@kernel.org> | 2018-03-19 18:09:44 +0100 |
| commit | b3a5d111994450909158929560906f2c1c6c1d85 (patch) | |
| tree | 2c6c4c0a295c3522ed41e47b63ba6b84356bfe28 /lib/percpu-refcount.c | |
| parent | mm: Allow to kill tasks doing pcpu_alloc() and waiting for pcpu_balance_workfn() (diff) | |
| download | linux-b3a5d111994450909158929560906f2c1c6c1d85.tar.xz linux-b3a5d111994450909158929560906f2c1c6c1d85.zip | |
percpu_ref: Update doc to dissuade users from depending on internal RCU grace periods
percpu_ref internally uses sched-RCU to implement the percpu -> atomic
mode switching and the documentation suggested that this could be
depended upon. This doesn't seem like a good idea.
* percpu_ref uses sched-RCU which has different grace periods regular
RCU. Users may combine percpu_ref with regular RCU usage and
incorrectly believe that regular RCU grace periods are performed by
percpu_ref. This can lead to, for example, use-after-free due to
premature freeing.
* percpu_ref has a grace period when switching from percpu to atomic
mode. It doesn't have one between the last put and release. This
distinction is subtle and can lead to surprising bugs.
* percpu_ref allows starting in and switching to atomic mode manually
for debugging and other purposes. This means that there may not be
any grace periods from kill to release.
This patch makes it clear that the grace periods are percpu_ref's
internal implementation detail and can't be depended upon by the
users.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Diffstat (limited to 'lib/percpu-refcount.c')
| -rw-r--r-- | lib/percpu-refcount.c | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index 30e7dd88148b..9f96fa7bc000 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -322,6 +322,8 @@ EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu); * This function normally doesn't block and can be called from any context * but it may block if @confirm_kill is specified and @ref is in the * process of switching to atomic mode by percpu_ref_switch_to_atomic(). + * + * There are no implied RCU grace periods between kill and release. */ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, percpu_ref_func_t *confirm_kill) |