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    Subject[PATCH sl-b 5/6] percpu_ref: Print allocator 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 prints the percpu_ref allocation site using the
    new kmem_last_alloc() and kmem_last_alloc_errstring() functions in order
    to provide a bit more information for the kernel-deployment case.

    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 | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
    index e59eda0..8c7b21a0 100644
    --- a/lib/percpu-refcount.c
    +++ b/lib/percpu-refcount.c
    @@ -169,6 +169,8 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
    struct percpu_ref *ref = data->ref;
    unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
    unsigned long count = 0;
    + void *allocaddr;
    + const char *allocerr;
    int cpu;

    for_each_possible_cpu(cpu)
    @@ -191,9 +193,16 @@ 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 (atomic_long_read(&data->count) <= 0) {
    + allocaddr = kmem_last_alloc(data);
    + allocerr = kmem_last_alloc_errstring(allocaddr);
    + if (allocerr)
    + WARN_ONCE(1, "percpu ref (%ps) <= 0 (%ld) after switching to atomic (%s)",
    + data->release, atomic_long_read(&data->count), allocerr);
    + else
    + WARN_ONCE(1, "percpu ref (%ps) <= 0 (%ld) after switching to atomic (allocated at %pS)",
    + data->release, atomic_long_read(&data->count), allocaddr);
    + }

    /* @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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