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SubjectRe: [PATCH 1/4] rcu: introduce noref debug
On Fri,  6 Oct 2017 14:57:46 +0200
Paolo Abeni <pabeni@redhat.com> wrote:

> We currently lack a debugging infrastructure to ensure that
> long-lived noref dst are properly handled - namely dropped
> or converted to a refcounted version before escaping the current
> RCU section.
>
> This changeset implements such infra, tracking the noref pointer
> on a per CPU store and checking that all noref tracked at any
> given RCU nesting level are cleared before leaving such RCU
> section.
>
> Each noref entity is identified using the entity pointer and an
> additional, optional, key/opaque pointer. This is needed to cope
> with usage case scenario where the same noref entity is stored
> in different places (e.g. noref dst on skb_clone()).
>
> To keep the patch/implementation simple RCU_NOREF_DEBUG depends
> on PREEMPT_RCU=n in Kconfig.
>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> include/linux/rcupdate.h | 11 ++++++
> kernel/rcu/Kconfig.debug | 15 ++++++++
> kernel/rcu/Makefile | 1 +
> kernel/rcu/noref_debug.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 116 insertions(+)
> create mode 100644 kernel/rcu/noref_debug.c
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de50d8a4cf41..20c1ce08e3eb 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -60,6 +60,15 @@ void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
> void synchronize_sched(void);
> void rcu_barrier_tasks(void);
>
> +#ifdef CONFIG_RCU_NOREF_DEBUG
> +void rcu_track_noref(void *key, void *noref, bool track);
> +void __rcu_check_noref(void);
> +
> +#else
> +static inline void rcu_track_noref(void *key, void *noref, bool add) { }
> +static inline void __rcu_check_noref(void) { }
> +#endif
> +
> #ifdef CONFIG_PREEMPT_RCU
>
> void __rcu_read_lock(void);
> @@ -85,6 +94,7 @@ static inline void __rcu_read_lock(void)
>
> static inline void __rcu_read_unlock(void)
> {
> + __rcu_check_noref();
> if (IS_ENABLED(CONFIG_PREEMPT_COUNT))
> preempt_enable();
> }
> @@ -723,6 +733,7 @@ static inline void rcu_read_unlock_bh(void)
> "rcu_read_unlock_bh() used illegally while idle");
> rcu_lock_release(&rcu_bh_lock_map);
> __release(RCU_BH);
> + __rcu_check_noref();
> local_bh_enable();
> }
>
> diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
> index 0ec7d1d33a14..6c7f52a3e809 100644
> --- a/kernel/rcu/Kconfig.debug
> +++ b/kernel/rcu/Kconfig.debug
> @@ -68,6 +68,21 @@ config RCU_TRACE
> Say Y here if you want to enable RCU tracing
> Say N if you are unsure.
>
> +config RCU_NOREF_DEBUG
> + bool "Debugging for RCU-protected noref entries"
> + depends on PREEMPT_RCU=n
> + select PREEMPT_COUNT
> + default n
> + help
> + In case of long lasting rcu_read_lock sections this debug
> + feature enables one to ensure that no rcu managed dereferenced
> + data leaves the locked section. One use case is the tracking
> + of dst_entries in struct sk_buff ->dst, which is used to pass
> + the dst_entry around in the whole stack while under RCU lock.
> +
> + Say Y here if you want to enable noref RCU debugging
> + Say N if you are unsure.
> +
> config RCU_EQS_DEBUG
> bool "Provide debugging asserts for adding NO_HZ support to an arch"
> depends on DEBUG_KERNEL
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 13c0fc852767..c67d7c65c582 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -11,3 +11,4 @@ obj-$(CONFIG_TREE_RCU) += tree.o
> obj-$(CONFIG_PREEMPT_RCU) += tree.o
> obj-$(CONFIG_TINY_RCU) += tiny.o
> obj-$(CONFIG_RCU_NEED_SEGCBLIST) += rcu_segcblist.o
> +obj-$(CONFIG_RCU_NOREF_DEBUG) += noref_debug.o
> \ No newline at end of file
> diff --git a/kernel/rcu/noref_debug.c b/kernel/rcu/noref_debug.c
> new file mode 100644
> index 000000000000..ae2e104b11d6
> --- /dev/null
> +++ b/kernel/rcu/noref_debug.c
> @@ -0,0 +1,89 @@
> +#include <linux/bug.h>
> +#include <linux/percpu.h>
> +#include <linux/skbuff.h>
> +#include <linux/bitops.h>
> +
> +#define NOREF_MAX 7
> +struct noref_entry {
> + void *noref;
> + void *key;
> + int nesting;
> +};
> +
> +struct noref_cache {
> + struct noref_entry store[NOREF_MAX];
> +};
> +
> +static DEFINE_PER_CPU(struct noref_cache, per_cpu_noref_cache);
> +
> +static int noref_cache_lookup(struct noref_cache *cache, void *noref, void *key)
> +{
> + int i;
> +
> + for (i = 0; i < NOREF_MAX; ++i)
> + if (cache->store[i].noref == noref &&
> + cache->store[i].key == key)
> + return i;
> +
> + return -1;
> +}
> +

Please add a comment above this function on how to use it.

-- Steve

> +void rcu_track_noref(void *key, void *noref, bool track)
> +{
> + struct noref_cache *cache = this_cpu_ptr(&per_cpu_noref_cache);
> + int i;
> +
> + if (track) {
> + /* find the first empty slot */
> + i = noref_cache_lookup(cache, NULL, 0);
> + if (i < 0) {
> + WARN_ONCE(1, "can't find empty an slot to track a noref"
> + " noref tracking will be inaccurate");
> + return;
> + }
> +
> + cache->store[i].noref = noref;
> + cache->store[i].key = key;
> + cache->store[i].nesting = preempt_count();
> + return;
> + }
> +
> + i = noref_cache_lookup(cache, noref, key);
> + if (i == -1) {
> + WARN_ONCE(1, "to-be-untracked noref entity %p not found in "
> + "cache\n", noref);
> + return;
> + }
> +
> + cache->store[i].noref = NULL;
> + cache->store[i].key = NULL;
> + cache->store[i].nesting = 0;
> +}
> +EXPORT_SYMBOL_GPL(rcu_track_noref);
> +
> +void __rcu_check_noref(void)
> +{
> + struct noref_cache *cache = this_cpu_ptr(&per_cpu_noref_cache);
> + char *cur, buf[strlen("0xffffffffffffffff") * NOREF_MAX + 1];
> + int nesting = preempt_count();
> + int i, ret, cnt = 0;
> +
> + cur = buf;
> + for (i = 0; i < NOREF_MAX; ++i) {
> + if (!cache->store[i].noref ||
> + cache->store[i].nesting != nesting)
> + continue;
> +
> + cnt++;
> + ret = sprintf(cur, " %p", cache->store[i].noref);
> + if (ret >= 0)
> + cur += ret;
> + cache->store[i].noref = NULL;
> + cache->store[i].key = NULL;
> + cache->store[i].nesting = 0;
> + }
> +
> + WARN_ONCE(cnt, "%d noref entities escaped an RCU section, "
> + "nesting %d, leaked noref list %s", cnt, nesting, buf);
> +}
> +EXPORT_SYMBOL_GPL(__rcu_check_noref);

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