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Date:   Tue, 24 Sep 2019 09:03:48 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>, kbuild@01.org,
        kbuild-all@01.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [rcu:dev.2019.09.23a 62/77] kernel/rcu/tree.c:2882
 kfree_call_rcu() warn: inconsistent returns 'irqsave:flags'.
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On Tue, Sep 24, 2019 at 08:15:12AM -0400, Joel Fernandes wrote:
> On Tue, Sep 24, 2019 at 4:05 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >
> > tree:   https://kernel.googlesource.com/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2019.09.23a
> > head:   97de53b94582c208ee239178b208b8e8b9472585
> > commit: 39676bb72323718a9e7bab1ba9c4a68319a96f62 [62/77] rcu: Add support for debug_objects debugging for kfree_rcu()
> >
> > If you fix the issue, kindly add following tag
> > Reported-by: kbuild test robot <lkp@intel.com>
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> >
> > smatch warnings:
> > kernel/rcu/tree.c:2882 kfree_call_rcu() warn: inconsistent returns 'irqsave:flags'.
> >   Locked on:   line 2867
> >   Unlocked on: line 2882
> >
> > git remote add rcu https://kernel.googlesource.com/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> > git remote update rcu
> > git checkout 39676bb72323718a9e7bab1ba9c4a68319a96f62
> > vim +2882 kernel/rcu/tree.c
> >
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2850) void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2851) {
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2852)        unsigned long flags;
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2853)        struct kfree_rcu_cpu *krcp;
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2854)
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2855)        head->func = func;
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2856)
> > 87970ccf5a9125 Paul E. McKenney        2019-09-19  2857         local_irq_save(flags);  // For safely calling this_cpu_ptr().
> >                                                                 ^^^^^^^^^^^^^^^^^^^^^
> > IRQs disabled.
> >
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2858)        krcp = this_cpu_ptr(&krc);
> > ff8db005e371cb Paul E. McKenney        2019-09-19  2859         if (krcp->initialized)
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2860)                spin_lock(&krcp->lock);
> > 5f5046cc15d514 Joel Fernandes (Google  2019-08-05  2861)
> > 87970ccf5a9125 Paul E. McKenney        2019-09-19  2862         // Queue the object but don't yet schedule the batch.
> > 39676bb7232371 Joel Fernandes (Google  2019-09-22  2863)        if (debug_rcu_head_queue(head)) {
> > 39676bb7232371 Joel Fernandes (Google  2019-09-22  2864)                // Probable double kfree_rcu(), just leak.
> > 39676bb7232371 Joel Fernandes (Google  2019-09-22  2865)                WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
> > 39676bb7232371 Joel Fernandes (Google  2019-09-22  2866)                          __func__, head);
> > 39676bb7232371 Joel Fernandes (Google  2019-09-22  2867)                return;
> >
> > Need to re-enable before returning.
> 
> Right. At this point the system has a bug anyway since it is a
> double-free. But yes, no reason to introduce more bugs. Will fix.
> Thank you, Dan!

What Joel said!

As long as I am doing several other reports anyway...

How does the to-be-squashed patch below look?

							Thanx, Paul

------------------------------------------------------------------------

commit 39af4c08038a8a6a7b1c9804fdd2c1921d583222
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Tue Sep 24 09:02:00 2019 -0700

    squash! rcu: Add support for debug_objects debugging for kfree_rcu()
    
    [ paulmck: Fix IRQ per kbuild test robot <lkp@intel.com> feedback. ]
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 921daa7..edb7539 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2870,7 +2870,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		// Probable double kfree_rcu(), just leak.
 		WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
 			  __func__, head);
-		return;
+		goto unlock_return;
 	}
 	head->func = func;
 	head->next = krcp->head;
@@ -2883,6 +2883,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
 	}
 
+unlock_return:
 	if (krcp->initialized)
 		spin_unlock(&krcp->lock);
 	local_irq_restore(flags);