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rcu-tasks: Add trc_inspect_reader() checks for exiting critical section

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commit 18f08e758f34e6dfe0668bee51bd2af7adacf381 upstream.

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>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Paul E. McKenney 2021-07-28 11:32:28 -07:00 committed by Greg Kroah-Hartman
parent 3e22624f8f
commit d93ba6e46e
1 changed files with 9 additions and 9 deletions
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18
kernel/rcu/tasks.h
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@ -874,7 +874,7 @@ reset_ipi:
static bool trc_inspect_reader(struct task_struct *t, void *arg)
{
int cpu = task_cpu(t);
bool in_qs = false;
int nesting;
bool ofl = cpu_is_offline(cpu);
if (task_curr(t)) {
@ -894,18 +894,18 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
n_heavy_reader_updates++;
if (ofl)
n_heavy_reader_ofl_updates++;
in_qs = true;
nesting = 0;
} else {
// The task is not running, so C-language access is safe.
in_qs = likely(!t->trc_reader_nesting);
nesting = t->trc_reader_nesting;
}
// Mark as checked so that the grace-period kthread will
// remove it from the holdout list.
t->trc_reader_checked = true;
if (in_qs)
return true; // Already in quiescent state, done!!!
// If not exiting a read-side critical section, mark as checked
// so that the grace-period kthread will remove it from the
// holdout list.
t->trc_reader_checked = nesting >= 0;
if (nesting <= 0)
return !nesting; // If in QS, done, otherwise try again later.
// The task is in a read-side critical section, so set up its
// state so that it will awaken the grace-period kthread upon exit