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@xxxxxxxxxx>
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@xxxxxxxxxx
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reported-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxx 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
