From: Neeraj Upadhyay neeraju@codeaurora.org
commit fd6bc19d7676a060a171d1cf3dcbf6fd797eb05f upstream.
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 Signed-off-by: David Chen david.chen@nutanix.com Acked-by: Neeraj Upadhyay neeraju@codeaurora.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/rcu/tree_exp.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -534,7 +534,7 @@ static void rcu_exp_wait_wake(struct rcu spin_unlock(&rnp->exp_lock); } smp_mb(); /* All above changes before wakeup. */ - wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]); + wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]); } trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake")); mutex_unlock(&rsp->exp_wake_mutex);