On 8/17/2021 3:32 AM, David Chen wrote:
-----Original Message----- From: Greg Kroah-Hartman gregkh@linuxfoundation.org Sent: Monday, August 16, 2021 12:31 PM To: David Chen david.chen@nutanix.com Cc: stable@vger.kernel.org; Paul E. McKenney paulmck@linux.vnet.ibm.com; neeraju@codeaurora.org Subject: Re: Request for backport fd6bc19d7676 to 4.14 and 4.19 branch
On Mon, Aug 16, 2021 at 07:19:34PM +0000, David Chen wrote:
Hi Greg,
We recently hit a hung task timeout issue in synchronize_rcu_expedited on
4.14 branch.
The issue seems to be identical to the one described in `fd6bc19d7676 rcu: Fix missed wakeup of exp_wq waiters` Can we backport it to 4.14 and
4.19 branch?
The patch doesn't apply cleanly, but it should be trivial to resolve, just do this
wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp-expedited_sequence) & 0x3]);
wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);I don't know if we should do it for 4.9, because the handling of sequence
number is a bit different.
Please provide a working backport, me hand-editing patches does not scale, and this way you get the proper credit for backporting it (after testing it).
Sure, appended at the end.
You have tested, this, right?
I don't have a good repro for the original issue, so I only ran rcutorture and some basic work load test to see if anything obvious went wrong.
thanks,
greg k-h
From 307a212335fe143027e3a9f7a9d548beead7ba33 Mon Sep 17 00:00:00 2001 From: Neeraj Upadhyay neeraju@codeaurora.org Date: Tue, 19 Nov 2019 03:17:07 +0000 Subject: [PATCH] rcu: Fix missed wakeup of exp_wq waiters
[ Upstream commit fd6bc19d7676a060a171d1cf3dcbf6fd797eb05f ]
Tasks waiting within exp_funnel_lock() for an expedited grace period to elapse can be starved due to the following sequence of events:
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'.
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.
Task C attempts to start another expedited grace period, but blocks on ->exp_mutex, which is still held by Task A.
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.
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'.
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'.
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]);
Minor: On these kernel versions, we had rsp pointer, per RCU flavor, whereas post 4.20 kernel versions, we have a single rcu_state. So, the commit message can be corrected here. The functionality is mostly unchanged and same fix is applicable.
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: Sasha Levin sashal@kernel.org Signed-off-by: David Chen david.chen@nutanix.com
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 46d61b597731..f90d10c1c3c8 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -534,7 +534,7 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s) 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]);
Acked-by: Neeraj Upadhyay neeraju@codeaurora.org