Re: "[PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt" apply to 3.18-stable tree

On Thu, Mar 21, 2019 at 04:03:04PM +0000, He, Bo wrote:

The rcu_gp_kthread_wake() function is invoked when it might be necessary to wake the RCU grace-period kthread. Because self-wakeups are normally a useless waste of CPU cycles, if rcu_gp_kthread_wake() is invoked from this kthread, it naturally refuses to do the wakeup.

Unfortunately, natural though it might be, this heuristic fails when rcu_gp_kthread_wake() is invoked from an interrupt or softirq handler that interrupted the grace-period kthread just after the final check of the wait-event condition but just before the schedule() call. In this case, a wakeup is required, even though the call to rcu_gp_kthread_wake() is within the RCU grace-period kthread's context. Failing to provide this wakeup can result in grace periods failing to start, which in turn results in out-of-memory conditions.

This race window is quite narrow, but it actually did happen during real testing. It would of course need to be fixed even if it was strictly theoretical in nature.

[ backport for 3.18 commit 1d1f898df6586c5ea9aeaf349f13089c6fa37903 upstream. ]

Fixes: 48a7639ce80c ("rcu: Make callers awaken grace-period kthread") Reported-by: "He, Bo" bo.he@intel.com Co-developed-by: "Zhang, Jun" jun.zhang@intel.com Co-developed-by: "He, Bo" bo.he@intel.com Co-developed-by: "xiao, jin" jin.xiao@intel.com Co-developed-by: Bai, Jie A jie.a.bai@intel.com Signed-off: "Zhang, Jun" jun.zhang@intel.com Signed-off: "He, Bo" bo.he@intel.com Signed-off: "xiao, jin" jin.xiao@intel.com Signed-off: Bai, Jie A jie.a.bai@intel.com Signed-off-by: "Zhang, Jun" jun.zhang@intel.com [ paulmck: Switch from !in_softirq() to "!in_interrupt() && !in_serving_softirq() to avoid redundant wakeups and to also handle the interrupt-handler scenario as well as the softirq-handler scenario that actually occurred in testing. ]

They all look good, thank you!

I subjected all of the others to light rcutorture testing, which they passed. This v3.18 patch hung, however. Trying it again with stock v3.18 got the same hang, so I believe we can exonerate the patch and give it a good firm "maybe" on 3.18.

Worth paying special attention to further test results from 3.18.x, though!

Thanx, Paul

Signed-off-by: Paul E. McKenney paulmck@linux.ibm.com Link: https://lkml.kernel.org/r/CD6925E8781EFD4D8E11882D20FC406D52A11F61@SHSMSX104...

---

kernel/rcu/tree.c | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 9815447d22e0..f9fb34e1aa71 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1399,15 +1399,23 @@ static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp) } /*

• • Awaken the grace-period kthread for the specified flavor of RCU.
• • Don't do a self-awaken, and don't bother awakening when there is
• • nothing for the grace-period kthread to do (as in several CPUs
• • raced to awaken, and we lost), and finally don't try to awaken
• • a kthread that has not yet been created.

• • Awaken the grace-period kthread. Don't do a self-awaken (unless in
• • an interrupt or softirq handler), and don't bother awakening when there
• • is nothing for the grace-period kthread to do (as in several CPUs raced
• • to awaken, and we lost), and finally don't try to awaken a kthread that
• • has not yet been created. If all those checks are passed, track some
• • debug information and awaken.
• • So why do the self-wakeup when in an interrupt or softirq handler
• • in the grace-period kthread's context? Because the kthread might have
• • been interrupted just as it was going to sleep, and just after the final
• • pre-sleep check of the awaken condition. In this case, a wakeup really
• • is required, and is therefore supplied.
  */

static void rcu_gp_kthread_wake(struct rcu_state *rsp) {

• if (current == rsp->gp_kthread ||

• if ((current == rsp->gp_kthread &&

•     !in_interrupt() && !in_serving_softirq()) ||
   !ACCESS_ONCE(rsp->gp_flags) ||
   !rsp->gp_kthread)
  

  return;

-- 2.20.1

-----Original Message----- From: gregkh@linuxfoundation.org gregkh@linuxfoundation.org Sent: Thursday, March 21, 2019 1:43 AM To: Zhang, Jun jun.zhang@intel.com; He, Bo bo.he@intel.com; Bai, Jie A jie.a.bai@intel.com; Xiao, Jin jin.xiao@intel.com; paulmck@linux.ibm.com Cc: stable@vger.kernel.org Subject: FAILED: patch "[PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt" failed to apply to 3.18-stable tree

The patch below does not apply to the 3.18-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

...

From 1d1f898df6586c5ea9aeaf349f13089c6fa37903 Mon Sep 17 00:00:00 2001

From: "Zhang, Jun" jun.zhang@intel.com Date: Tue, 18 Dec 2018 06:55:01 -0800 Subject: [PATCH] rcu: Do RCU GP kthread self-wakeup from softirq and interrupt

The rcu_gp_kthread_wake() function is invoked when it might be necessary to wake the RCU grace-period kthread. Because self-wakeups are normally a useless waste of CPU cycles, if rcu_gp_kthread_wake() is invoked from this kthread, it naturally refuses to do the wakeup.

Unfortunately, natural though it might be, this heuristic fails when rcu_gp_kthread_wake() is invoked from an interrupt or softirq handler that interrupted the grace-period kthread just after the final check of the wait-event condition but just before the schedule() call. In this case, a wakeup is required, even though the call to rcu_gp_kthread_wake() is within the RCU grace-period kthread's context. Failing to provide this wakeup can result in grace periods failing to start, which in turn results in out-of-memory conditions.

This race window is quite narrow, but it actually did happen during real testing. It would of course need to be fixed even if it was strictly theoretical in nature.

This patch does not Cc stable because it does not apply cleanly to earlier kernel versions.

Fixes: 48a7639ce80c ("rcu: Make callers awaken grace-period kthread") Reported-by: "He, Bo" bo.he@intel.com Co-developed-by: "Zhang, Jun" jun.zhang@intel.com Co-developed-by: "He, Bo" bo.he@intel.com Co-developed-by: "xiao, jin" jin.xiao@intel.com Co-developed-by: Bai, Jie A jie.a.bai@intel.com Signed-off: "Zhang, Jun" jun.zhang@intel.com Signed-off: "He, Bo" bo.he@intel.com Signed-off: "xiao, jin" jin.xiao@intel.com Signed-off: Bai, Jie A jie.a.bai@intel.com Signed-off-by: "Zhang, Jun" jun.zhang@intel.com [ paulmck: Switch from !in_softirq() to "!in_interrupt() && !in_serving_softirq() to avoid redundant wakeups and to also handle the interrupt-handler scenario as well as the softirq-handler scenario that actually occurred in testing. ] Signed-off-by: Paul E. McKenney paulmck@linux.ibm.com Link: https://lkml.kernel.org/r/CD6925E8781EFD4D8E11882D20FC406D52A11F61@SHSMSX104...

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 9ceb93f848cd..21775eebb8f0 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1593,15 +1593,23 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp) } /*

• • Awaken the grace-period kthread. Don't do a self-awaken, and don't
• • bother awakening when there is nothing for the grace-period kthread
• • to do (as in several CPUs raced to awaken, and we lost), and finally
• • don't try to awaken a kthread that has not yet been created. If
• • all those checks are passed, track some debug information and awaken.

• • Awaken the grace-period kthread. Don't do a self-awaken (unless in
• • an interrupt or softirq handler), and don't bother awakening when
• there
• • is nothing for the grace-period kthread to do (as in several CPUs
• raced
• • to awaken, and we lost), and finally don't try to awaken a kthread
• that
• • has not yet been created. If all those checks are passed, track
• some
• • debug information and awaken.
• • So why do the self-wakeup when in an interrupt or softirq handler
• • in the grace-period kthread's context? Because the kthread might
• have
• • been interrupted just as it was going to sleep, and just after the
• final
• • pre-sleep check of the awaken condition. In this case, a wakeup
• really
• • is required, and is therefore supplied.
  */

static void rcu_gp_kthread_wake(void) {

• if (current == rcu_state.gp_kthread ||

• if ((current == rcu_state.gp_kthread &&

•     !in_interrupt() && !in_serving_softirq()) ||
   !READ_ONCE(rcu_state.gp_flags) ||
   !rcu_state.gp_kthread)
  

  return;