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Subject[PATCHv2] lib/ratelimit: Lockless ratelimiting
Date
Currently ratelimit_state is protected with spin_lock. If the .lock is
taken at the moment of ___ratelimit() call, the message is suppressed to
make ratelimiting robust.

That results in the following issue issue:
CPU0 CPU1
------------------ ------------------
printk_ratelimit() printk_ratelimit()
| |
try_spin_lock() try_spin_lock()
| |
time_is_before_jiffies() return 0; // suppress

So, concurrent call of ___ratelimit() results in silently suppressing
one of the messages, regardless if the limit is reached or not.
And rc->missed is not increased in such case so the issue is covered
from user.

Convert ratelimiting to use atomic counters and drop spin_lock.

Note: That might be unexpected, but with the first interval of messages
storm one can print up to burst*2 messages. So, it doesn't guarantee
that in *any* interval amount of messages is lesser than burst.
But, that differs lightly from previous behavior where one can start
burst=5 interval and print 4 messages on the last milliseconds of
interval + new 5 messages from new interval (totally 9 messages in
lesser than interval value):

msg0 msg1-msg4 msg0-msg4
| | |
| | |
|--o---------------------o-|-----o--------------------|--> (t)
<------->
Lesser than burst

Dropped dev/random patch since v1 version:
lkml.kernel.org/r/<20180510125211.12583-1-dima@arista.com>

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
drivers/char/random.c | 16 ++++-------
drivers/gpu/drm/i915/i915_perf.c | 4 +--
include/linux/ratelimit.h | 24 +++++++++-------
lib/ratelimit.c | 59 +++++++++++++++++++---------------------
4 files changed, 50 insertions(+), 53 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a8fb0020ba5c..ba67ea0dc568 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -936,24 +936,20 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
crng->init_time = jiffies;
spin_unlock_irqrestore(&crng->lock, flags);
if (crng == &primary_crng && crng_init < 2) {
+ int unseeded_miss, urandom_miss;
+
invalidate_batched_entropy();
numa_crng_init();
crng_init = 2;
process_random_ready_list();
wake_up_interruptible(&crng_init_wait);
pr_notice("random: crng init done\n");
- if (unseeded_warning.missed) {
+ if ((unseeded_miss = atomic_xchg(&unseeded_warning.missed, 0)))
pr_notice("random: %d get_random_xx warning(s) missed "
- "due to ratelimiting\n",
- unseeded_warning.missed);
- unseeded_warning.missed = 0;
- }
- if (urandom_warning.missed) {
+ "due to ratelimiting\n", unseeded_miss);
+ if ((urandom_miss = atomic_xchg(&urandom_warning.missed, 0)))
pr_notice("random: %d urandom warning(s) missed "
- "due to ratelimiting\n",
- urandom_warning.missed);
- urandom_warning.missed = 0;
- }
+ "due to ratelimiting\n", urandom_miss);
}
}

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index 019bd2d073ad..75f6203f6e8e 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -1317,9 +1317,9 @@ static void i915_oa_stream_destroy(struct i915_perf_stream *stream)

put_oa_config(dev_priv, stream->oa_config);

- if (dev_priv->perf.oa.spurious_report_rs.missed) {
+ if (atomic_read(&dev_priv->perf.oa.spurious_report_rs.missed)) {
DRM_NOTE("%d spurious OA report notices suppressed due to ratelimiting\n",
- dev_priv->perf.oa.spurious_report_rs.missed);
+ atomic_read(&dev_priv->perf.oa.spurious_report_rs.missed));
}
}

diff --git a/include/linux/ratelimit.h b/include/linux/ratelimit.h
index 8ddf79e9207a..7b5914e12d5b 100644
--- a/include/linux/ratelimit.h
+++ b/include/linux/ratelimit.h
@@ -4,7 +4,6 @@

#include <linux/param.h>
#include <linux/sched.h>
-#include <linux/spinlock.h>

#define DEFAULT_RATELIMIT_INTERVAL (5 * HZ)
#define DEFAULT_RATELIMIT_BURST 10
@@ -13,20 +12,20 @@
#define RATELIMIT_MSG_ON_RELEASE BIT(0)

struct ratelimit_state {
- raw_spinlock_t lock; /* protect the state */
+ atomic_t printed;
+ atomic_t missed;

int interval;
int burst;
- int printed;
- int missed;
unsigned long begin;
unsigned long flags;
};

#define RATELIMIT_STATE_INIT(name, interval_init, burst_init) { \
- .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
.interval = interval_init, \
.burst = burst_init, \
+ .printed = ATOMIC_INIT(0), \
+ .missed = ATOMIC_INIT(0), \
}

#define RATELIMIT_STATE_INIT_DISABLED \
@@ -42,9 +41,10 @@ static inline void ratelimit_state_init(struct ratelimit_state *rs,
{
memset(rs, 0, sizeof(*rs));

- raw_spin_lock_init(&rs->lock);
rs->interval = interval;
rs->burst = burst;
+ atomic_set(&rs->printed, 0);
+ atomic_set(&rs->missed, 0);
}

static inline void ratelimit_default_init(struct ratelimit_state *rs)
@@ -53,16 +53,20 @@ static inline void ratelimit_default_init(struct ratelimit_state *rs)
DEFAULT_RATELIMIT_BURST);
}

+/*
+ * Keeping it simple: not reenterable and not safe for concurrent
+ * ___ratelimit() call as used only by devkmsg_release().
+ */
static inline void ratelimit_state_exit(struct ratelimit_state *rs)
{
+ int missed;
+
if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE))
return;

- if (rs->missed) {
+ if ((missed = atomic_xchg(&rs->missed, 0)))
pr_warn("%s: %d output lines suppressed due to ratelimiting\n",
- current->comm, rs->missed);
- rs->missed = 0;
- }
+ current->comm, missed);
}

static inline void
diff --git a/lib/ratelimit.c b/lib/ratelimit.c
index d01f47135239..c99305e9239f 100644
--- a/lib/ratelimit.c
+++ b/lib/ratelimit.c
@@ -13,6 +13,18 @@
#include <linux/jiffies.h>
#include <linux/export.h>

+static void ratelimit_end_interval(struct ratelimit_state *rs, const char *func)
+{
+ rs->begin = jiffies;
+
+ if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE)) {
+ unsigned missed = (unsigned)atomic_xchg(&rs->missed, 0);
+
+ if (missed)
+ pr_warn("%s: %u callbacks suppressed\n", func, missed);
+ }
+}
+
/*
* __ratelimit - rate limiting
* @rs: ratelimit_state data
@@ -27,45 +39,30 @@
*/
int ___ratelimit(struct ratelimit_state *rs, const char *func)
{
- unsigned long flags;
- int ret;
-
if (!rs->interval)
return 1;

- /*
- * If we contend on this state's lock then almost
- * by definition we are too busy to print a message,
- * in addition to the one that will be printed by
- * the entity that is holding the lock already:
- */
- if (!raw_spin_trylock_irqsave(&rs->lock, flags))
+ if (unlikely(!rs->burst)) {
+ atomic_add_unless(&rs->missed, 1, -1);
+ if (time_is_before_jiffies(rs->begin + rs->interval))
+ ratelimit_end_interval(rs, func);
+
return 0;
+ }

- if (!rs->begin)
- rs->begin = jiffies;
+ if (atomic_add_unless(&rs->printed, 1, rs->burst))
+ return 1;

if (time_is_before_jiffies(rs->begin + rs->interval)) {
- if (rs->missed) {
- if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE)) {
- printk_deferred(KERN_WARNING
- "%s: %d callbacks suppressed\n",
- func, rs->missed);
- rs->missed = 0;
- }
- }
- rs->begin = jiffies;
- rs->printed = 0;
- }
- if (rs->burst && rs->burst > rs->printed) {
- rs->printed++;
- ret = 1;
- } else {
- rs->missed++;
- ret = 0;
+ if (atomic_cmpxchg(&rs->printed, rs->burst, 0))
+ ratelimit_end_interval(rs, func);
}
- raw_spin_unlock_irqrestore(&rs->lock, flags);

- return ret;
+ if (atomic_add_unless(&rs->printed, 1, rs->burst))
+ return 1;
+
+ atomic_add_unless(&rs->missed, 1, -1);
+
+ return 0;
}
EXPORT_SYMBOL(___ratelimit);
--
2.13.6
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