Re: [rcutorture] 82e310033d: WARNING:possible_recursive_locking_detected

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Dec 28, 2021 at 11:11:35PM +0800, kernel test robot wrote:
> 
> 
> Greeting,
> 
> FYI, we noticed the following commit (built with gcc-9):
> 
> commit: 82e310033d7c21a7a88427f14e0dad78d731a5cd ("rcutorture: Enable multiple concurrent callback-flood kthreads")
> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> 
> in testcase: rcutorture
> version: 
> with following parameters:
> 
> 	runtime: 300s
> 	test: default
> 	torture_type: rcu
> 
> test-description: rcutorture is rcutorture kernel module load/unload test.
> test-url: https://www.kernel.org/doc/Documentation/RCU/torture.txt
> 
> 
> on test machine: qemu-system-i386 -enable-kvm -cpu SandyBridge -smp 2 -m 4G
> 
> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> 
> 
> +-------------------------------------------------------------------------------+------------+------------+
> |                                                                               | 12e885433d | 82e310033d |
> +-------------------------------------------------------------------------------+------------+------------+
> | boot_successes                                                                | 95         | 47         |
> | boot_failures                                                                 | 31         | 25         |
> | invoked_oom-killer:gfp_mask=0x                                                | 5          | 4          |
> | Mem-Info                                                                      | 10         | 16         |
> | WARNING:at_kernel/rcu/rcutorture.c:#rcutorture_oom_notify[rcutorture]         | 24         | 15         |
> | EIP:rcutorture_oom_notify                                                     | 24         | 15         |
> | page_allocation_failure:order:#,mode:#(GFP_NOWAIT|__GFP_COMP),nodemask=(null) | 5          | 12         |
> | WARNING:possible_recursive_locking_detected                                   | 0          | 15         |
> | WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_fwd_prog.cold[rcutorture]     | 0          | 6          |
> | EIP:rcu_torture_fwd_prog.cold                                                 | 0          | 6          |
> +-------------------------------------------------------------------------------+------------+------------+
> 
> 
> If you fix the issue, kindly add following tag
> Reported-by: kernel test robot <oliver.sang@intel.com>

Good catch!  Does this following patch address it?

							Thanx, Paul

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

commit dd47cbdcc2f72ba3df1248fb7fe210acca18d09c
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Tue Dec 28 15:59:38 2021 -0800

    rcutorture: Fix rcu_fwd_mutex deadlock
    
    The rcu_torture_fwd_cb_hist() function acquires rcu_fwd_mutex, but is
    invoked from rcutorture_oom_notify() function, which hold this same
    mutex across this call.  This commit fixes the resulting deadlock.
    
    Reported-by: kernel test robot <oliver.sang@intel.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 918a2ea34ba13..9190dce686208 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2184,7 +2184,6 @@ static void rcu_torture_fwd_cb_hist(struct rcu_fwd *rfp)
 	for (i = ARRAY_SIZE(rfp->n_launders_hist) - 1; i > 0; i--)
 		if (rfp->n_launders_hist[i].n_launders > 0)
 			break;
-	mutex_lock(&rcu_fwd_mutex); // Serialize histograms.
 	pr_alert("%s: Callback-invocation histogram %d (duration %lu jiffies):",
 		 __func__, rfp->rcu_fwd_id, jiffies - rfp->rcu_fwd_startat);
 	gps_old = rfp->rcu_launder_gp_seq_start;
@@ -2197,7 +2196,6 @@ static void rcu_torture_fwd_cb_hist(struct rcu_fwd *rfp)
 		gps_old = gps;
 	}
 	pr_cont("\n");
-	mutex_unlock(&rcu_fwd_mutex);
 }
 
 /* Callback function for continuous-flood RCU callbacks. */
@@ -2435,7 +2433,9 @@ static void rcu_torture_fwd_prog_cr(struct rcu_fwd *rfp)
 			 n_launders, n_launders_sa,
 			 n_max_gps, n_max_cbs, cver, gps);
 		atomic_long_add(n_max_cbs, &rcu_fwd_max_cbs);
+		mutex_lock(&rcu_fwd_mutex); // Serialize histograms.
 		rcu_torture_fwd_cb_hist(rfp);
+		mutex_unlock(&rcu_fwd_mutex);
 	}
 	schedule_timeout_uninterruptible(HZ); /* Let CBs drain. */
 	tick_dep_clear_task(current, TICK_DEP_BIT_RCU);