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    Subject[PATCH v2 sl-b 5/5] percpu_ref: Dump mem_dump_obj() info upon reference-count underflow
    Date
    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>
    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
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