rcu-tasks: Add trc_inspect_reader() checks for exiting critical section
authorPaul E. McKenney <paulmck@kernel.org>
Wed, 28 Jul 2021 18:32:28 +0000 (11:32 -0700)
committerPaul E. McKenney <paulmck@kernel.org>
Wed, 15 Sep 2021 18:37:50 +0000 (11:37 -0700)
commit18f08e758f34e6dfe0668bee51bd2af7adacf381
tree82b2a416900287bb08cae7a357ab9dcb6cc26b0e
parent96017bf9039763a2e02dcc6adaa18592cd73a39d
rcu-tasks: Add trc_inspect_reader() checks for exiting critical section

Currently, trc_inspect_reader() treats a task exiting its RCU Tasks
Trace read-side critical section the same as being within that critical
section.  However, this can fail because that task might have already
checked its .need_qs field, which means that it might never decrement
the all-important trc_n_readers_need_end counter.  Of course, for that
to happen, the task would need to never again execute an RCU Tasks Trace
read-side critical section, but this really could happen if the system's
last trampoline was removed.  Note that exit from such a critical section
cannot be treated as a quiescent state due to the possibility of nested
critical sections.  This means that if trc_inspect_reader() sees a
negative nesting value, it must set up to try again later.

This commit therefore ignores tasks that are exiting their RCU Tasks
Trace read-side critical sections so that they will be rechecked later.

[ paulmck: Apply feedback from Neeraj Upadhyay and Boqun Feng. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
kernel/rcu/tasks.h