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From: paulmck@kernel.org
To: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-mm@kvack.org
Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com,
	iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
	ming.lei@redhat.com, axboe@kernel.dk, kernel-team@fb.com,
	"Paul E. McKenney" <paulmck@kernel.org>
Subject: [PATCH mm,percpu_ref,rcu 6/6] percpu_ref: Dump mem_dump_obj() info upon reference-count underflow
Date: Tue,  5 Jan 2021 17:17:50 -0800
Message-ID: <20210106011750.13709-6-paulmck@kernel.org> (raw)
In-Reply-To: <20210106011603.GA13180@paulmck-ThinkPad-P72>

From: "Paul E. McKenney" <paulmck@kernel.org>

Reference-count underflow for percpu_ref is detected in the RCU callback
percpu_ref_switch_to_atomic_rcu(), and the resulting warning does not
print anything allowing easy identification of which percpu_ref use
case is underflowing.  This is of course not normally a problem when
developing a new percpu_ref use case because it is most likely that
the problem resides in this new use case.  However, when deploying a
new kernel to a large set of servers, the underflow might well be a new
corner case in any of the old percpu_ref use cases.

This commit therefore calls mem_dump_obj() to dump out any additional
available information on the underflowing percpu_ref instance.

Cc: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 lib/percpu-refcount.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index e59eda0..a1071cd 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -5,6 +5,7 @@
 #include <linux/sched.h>
 #include <linux/wait.h>
 #include <linux/slab.h>
+#include <linux/mm.h>
 #include <linux/percpu-refcount.h>
 
 /*
@@ -168,6 +169,7 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
 			struct percpu_ref_data, rcu);
 	struct percpu_ref *ref = data->ref;
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
+	static atomic_t underflows;
 	unsigned long count = 0;
 	int cpu;
 
@@ -191,9 +193,13 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
 	 */
 	atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);
 
-	WARN_ONCE(atomic_long_read(&data->count) <= 0,
-		  "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
-		  data->release, atomic_long_read(&data->count));
+	if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
+		      "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
+		      data->release, atomic_long_read(&data->count)) &&
+	    atomic_inc_return(&underflows) < 4) {
+		pr_err("%s(): percpu_ref underflow", __func__);
+		mem_dump_obj(data);
+	}
 
 	/* @ref is viewed as dead on all CPUs, send out switch confirmation */
 	percpu_ref_call_confirm_rcu(rcu);
-- 
2.9.5


  parent reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-06  1:16 [PATCH v4 sl-b 0/6] Export return addresses etc. for better diagnostics Paul E. McKenney
2021-01-06  1:17 ` [PATCH mm,percpu_ref,rcu 1/6] mm: Add mem_dump_obj() to print source of memory block paulmck
2021-01-08 13:50   ` Vlastimil Babka
2021-01-08 19:01     ` Paul E. McKenney
2021-01-08 19:41       ` Vlastimil Babka
2021-01-06  1:17 ` [PATCH mm,percpu_ref,rcu 2/6] mm: Make mem_dump_obj() handle NULL and zero-sized pointers paulmck
2021-01-06  1:17 ` [PATCH mm,percpu_ref,rcu 3/6] mm: Make mem_dump_obj() handle vmalloc() memory paulmck
2021-01-08 15:30   ` Vlastimil Babka
2021-01-06  1:17 ` [PATCH mm,percpu_ref,rcu 4/6] mm: Make mem_obj_dump() vmalloc() dumps include start and length paulmck
2021-01-08 15:30   ` Vlastimil Babka
2021-01-06  1:17 ` [PATCH mm,percpu_ref,rcu 5/6] rcu: Make call_rcu() print mem_dump_obj() info for double-freed callback paulmck
2021-01-06  1:17 ` paulmck [this message]
2021-01-06 21:48 ` [PATCH v4 sl-b 0/6] Export return addresses etc. for better diagnostics Andrew Morton
2021-01-06 23:42   ` Paul E. McKenney
2021-01-08  0:26     ` Paul E. McKenney
2021-01-08 15:35       ` Vlastimil Babka
2021-01-08 17:36         ` Paul E. McKenney

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