The following commit has been merged into the core/rcu branch of tip: Commit-ID: 3375efeddf6972df47df26a5b5c643189bd3c02a Gitweb: https://git.kernel.org/tip/3375efeddf6972df47df26a5b5c643189bd3c02a Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Tue, 08 Dec 2020 14:43:43 -08:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Fri, 22 Jan 2021 15:24:23 -08:00 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. Cc: Ming Lei <ming.lei@xxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Reported-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Tested-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- 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);
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