Commit 3375efed authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reported-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Tested-by: default avatarNaresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent b4b7914a

lib/percpu-refcount.c

+9 −3
Original line number Diff line number Diff line
@@ -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,

	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));

		      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);