From: paulmck@kernel.org
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com,
mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
Neeraj Upadhyay <neeraju@codeaurora.org>,
"Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH tip/core/rcu 05/10] rcu: Fix missed wakeup of exp_wq waiters
Date: Mon, 9 Dec 2019 20:01:49 -0800 [thread overview]
Message-ID: <20191210040154.2498-5-paulmck@kernel.org> (raw)
In-Reply-To: <20191210040122.GA2419@paulmck-ThinkPad-P72>
From: Neeraj Upadhyay <neeraju@codeaurora.org>
Tasks waiting within exp_funnel_lock() for an expedited grace period to
elapse can be starved due to the following sequence of events:
1. Tasks A and B both attempt to start an expedited grace
period at about the same time. This grace period will have
completed when the lower four bits of the rcu_state structure's
->expedited_sequence field are 0b'0100', for example, when the
initial value of this counter is zero. Task A wins, and thus
does the actual work of starting the grace period, including
acquiring the rcu_state structure's .exp_mutex and sets the
counter to 0b'0001'.
2. Because task B lost the race to start the grace period, it
waits on ->expedited_sequence to reach 0b'0100' inside of
exp_funnel_lock(). This task therefore blocks on the rcu_node
structure's ->exp_wq[1] field, keeping in mind that the
end-of-grace-period value of ->expedited_sequence (0b'0100')
is shifted down two bits before indexing the ->exp_wq[] field.
3. Task C attempts to start another expedited grace period,
but blocks on ->exp_mutex, which is still held by Task A.
4. The aforementioned expedited grace period completes, so that
->expedited_sequence now has the value 0b'0100'. A kworker task
therefore acquires the rcu_state structure's ->exp_wake_mutex
and starts awakening any tasks waiting for this grace period.
5. One of the first tasks awakened happens to be Task A. Task A
therefore releases the rcu_state structure's ->exp_mutex,
which allows Task C to start the next expedited grace period,
which causes the lower four bits of the rcu_state structure's
->expedited_sequence field to become 0b'0101'.
6. Task C's expedited grace period completes, so that the lower four
bits of the rcu_state structure's ->expedited_sequence field now
become 0b'1000'.
7. The kworker task from step 4 above continues its wakeups.
Unfortunately, the wake_up_all() refetches the rcu_state
structure's .expedited_sequence field:
wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
This results in the wakeup being applied to the rcu_node
structure's ->exp_wq[2] field, which is unfortunate given that
Task B is instead waiting on ->exp_wq[1].
On a busy system, no harm is done (or at least no permanent harm is done).
Some later expedited grace period will redo the wakeup. But on a quiet
system, such as many embedded systems, it might be a good long time before
there was another expedited grace period. On such embedded systems,
this situation could therefore result in a system hang.
This issue manifested as DPM device timeout during suspend (which
usually qualifies as a quiet time) due to a SCSI device being stuck in
_synchronize_rcu_expedited(), with the following stack trace:
schedule()
synchronize_rcu_expedited()
synchronize_rcu()
scsi_device_quiesce()
scsi_bus_suspend()
dpm_run_callback()
__device_suspend()
This commit therefore prevents such delays, timeouts, and hangs by
making rcu_exp_wait_wake() use its "s" argument consistently instead of
refetching from rcu_state.expedited_sequence.
Fixes: 3b5f668e715b ("rcu: Overlap wakeups with next expedited grace period")
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree_exp.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 3b59c3e..fa143e4 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -557,7 +557,7 @@ static void rcu_exp_wait_wake(unsigned long s)
spin_unlock(&rnp->exp_lock);
}
smp_mb(); /* All above changes before wakeup. */
- wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
+ wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
}
trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
mutex_unlock(&rcu_state.exp_wake_mutex);
--
2.9.5
next prev parent reply other threads:[~2019-12-10 4:02 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-10 4:01 [PATCH tip/core/rcu 0/10] Expedited grace-period updates for v5.6 Paul E. McKenney
2019-12-10 4:01 ` [PATCH tip/core/rcu 01/10] rcu: Use *_ONCE() to protect lockless ->expmask accesses paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 02/10] rcu: Avoid modifying mask_ofl_ipi in sync_rcu_exp_select_node_cpus() paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 03/10] rcu: Fix data-race due to atomic_t copy-by-value paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 04/10] rcu: Substitute lookup for bit-twiddling in sync_rcu_exp_select_node_cpus() paulmck
2019-12-10 4:01 ` paulmck [this message]
2019-12-10 4:01 ` [PATCH tip/core/rcu 06/10] rcu: Allow only one expedited GP to run concurrently with wakeups paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 07/10] rcu: Rename sync_rcu_preempt_exp_done() to sync_rcu_exp_done() paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 08/10] rcu: Update tree_exp.h function-header comments paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 09/10] rcu: Replace synchronize_sched_expedited_wait() "_sched" with "_rcu" paulmck
2019-12-10 4:01 ` [PATCH tip/core/rcu 10/10] rcu: Enable tick for nohz_full CPUs slow to provide expedited QS paulmck
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