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percpu_ref: Update doc to dissuade users from depending on internal RCU grace periods

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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 <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Linus Torvalds <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
This commit is contained in:
Tejun Heo 2018-03-14 12:45:12 -07:00
commit b3a5d11199
2 changed files with 14 additions and 6 deletions
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2
lib/percpu-refcount.c
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@ -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)