+ percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch added to -mm tree

+ percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch added to -mm tree

[akpm]

The patch titled
     Subject: percpu_ref: dump mem_dump_obj() info upon reference-count underflow
has been added to the -mm tree.  Its filename is
     percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch

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------------------------------------------------------
From: "Paul E. McKenney" <paulmck@kernel.org>
Subject: percpu_ref: dump mem_dump_obj() info upon reference-count underflow

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.

Link: https://lkml.kernel.org/r/20210106011750.13709-6-paulmck@kernel.org
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/percpu-refcount.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- a/lib/percpu-refcount.c~percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow
+++ a/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_
 			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_
 	 */
 	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);
_

Patches currently in -mm which might be from paulmck@kernel.org are

mm-add-mem_dump_obj-to-print-source-of-memory-block.patch
mm-make-mem_dump_obj-handle-null-and-zero-sized-pointers.patch
mm-make-mem_dump_obj-handle-vmalloc-memory.patch
mm-make-mem_obj_dump-vmalloc-dumps-include-start-and-length.patch
rcu-make-call_rcu-print-mem_dump_obj-info-for-double-freed-callback.patch
percpu_ref-dump-mem_dump_obj-info-upon-reference-count-underflow.patch