From: "Paul E. McKenney" <[email protected]> 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 <[email protected]> Cc: Jens Axboe <[email protected]> Reported-by: Andrii Nakryiko <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> --- 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
