[PATCH tip/core/rcu 6/8] rcu-tasks: Fix grace-period/unlock race in RCU Tasks Trace

From: paulmck
Date: Thu Sep 17 2020 - 17:08:27 EST

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

The more intense grace-period processing resulting from the 50x RCU
Tasks Trace grace-period speedups exposed the following race condition:

o Task A running on CPU 0 executes rcu_read_lock_trace(),
entering a read-side critical section.

o When Task A eventually invokes rcu_read_unlock_trace()
to exit its read-side critical section, this function
notes that the ->trc_reader_special.s flag is zero and
and therefore invoke wil set ->trc_reader_nesting to zero
using WRITE_ONCE(). But before that happens...

o The RCU Tasks Trace grace-period kthread running on some other
CPU interrogates Task A, but this fails because this task is
currently running. This kthread therefore sends an IPI to CPU 0.

o CPU 0 receives the IPI, and thus invokes trc_read_check_handler().
Because Task A has not yet cleared its ->trc_reader_nesting
counter, this function sees that Task A is still within its
read-side critical section. This function therefore sets the
->trc_reader_nesting.b.need_qs flag, AKA the .need_qs flag.

Except that Task A has already checked the .need_qs flag, which
is part of the ->trc_reader_special.s flag. The .need_qs flag
therefore remains set until Task A's next rcu_read_unlock_trace().

o Task A now invokes synchronize_rcu_tasks_trace(), which cannot
start a new grace period until the current grace period completes.
And thus cannot return until after that time.

But Task A's .need_qs flag is still set, which prevents the current
grace period from completing. And because Task A is blocked, it
will never execute rcu_read_unlock_trace() until its call to
synchronize_rcu_tasks_trace() returns.

We are therefore deadlocked.

This race is improbable, but 80 hours of rcutorture made it happen twice.
The race was possible before the grace-period speedup, but roughly 50x
less probable. Several thousand hours of rcutorture would have been
necessary to have a reasonable chance of making this happen before this
50x speedup.

This commit therefore eliminates this deadlock by setting
->trc_reader_nesting to a large negative number before checking the
.need_qs and zeroing (or decrementing with respect to its initial
value) ->trc_reader_nesting. For its part, the IPI handler's
trc_read_check_handler() function adds a check for negative values,
deferring evaluation of the task in this case. Taken together, these
changes avoid this deadlock scenario.

Fixes: 276c410448db ("rcu-tasks: Split ->trc_reader_need_end")
Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: <bpf@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.7.x
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
include/linux/rcupdate_trace.h | 4 ++++
kernel/rcu/tasks.h | 6 ++++++
2 files changed, 10 insertions(+)

diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index d9015aa..a6a6a3a 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -50,6 +50,7 @@ static inline void rcu_read_lock_trace(void)
struct task_struct *t = current;

WRITE_ONCE(t->trc_reader_nesting, READ_ONCE(t->trc_reader_nesting) + 1);
+ barrier();
smp_mb(); // Pairs with update-side barriers
@@ -72,6 +73,9 @@ static inline void rcu_read_unlock_trace(void)

nesting = READ_ONCE(t->trc_reader_nesting) - 1;
+ barrier(); // Critical section before disabling.
+ // Disable IPI-based setting of .need_qs.
+ WRITE_ONCE(t->trc_reader_nesting, INT_MIN);
if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
WRITE_ONCE(t->trc_reader_nesting, nesting);
return; // We assume shallow reader nesting.
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index a0eaed5..e583a2d 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -830,6 +830,12 @@ static void trc_read_check_handler(void *t_in)
WRITE_ONCE(t->trc_reader_checked, true);
goto reset_ipi;
+ // If we are racing with an rcu_read_unlock_trace(), try again later.
+ if (unlikely(t->trc_reader_nesting < 0)) {
+ if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
+ wake_up(&trc_wait);
+ goto reset_ipi;
+ }
WRITE_ONCE(t->trc_reader_checked, true);

// Get here if the task is in a read-side critical section. Set