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Date: Wed, 22 Feb 2012 17:29:36 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: paulmck@...ux.vnet.ibm.com
CC: linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
fweisbec@...il.com, patches@...aro.org
Subject: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace
period
>From 4ddf62aaf2c4ebe6b9d4a1c596e8b43a678f1f0d Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@...fujitsu.com>
Date: Wed, 22 Feb 2012 14:12:02 +0800
Subject: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace period
flip_idx_and_wait() is not changed, and is split as two functions
and only a short comments is added for smp_mb() E.
__synchronize_srcu() use a different algorithm for "leak" readers.
detail is in the comments of the patch.
Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
kernel/srcu.c | 105 ++++++++++++++++++++++++++++++++++----------------------
1 files changed, 64 insertions(+), 41 deletions(-)
diff --git a/kernel/srcu.c b/kernel/srcu.c
index a51ac48..346f9d7 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -249,6 +249,37 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
*/
#define SYNCHRONIZE_SRCU_READER_DELAY 5
+static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
+{
+ int trycount = 0;
+
+ /*
+ * SRCU read-side critical sections are normally short, so wait
+ * a small amount of time before possibly blocking.
+ */
+ if (!srcu_readers_active_idx_check(sp, idx)) {
+ udelay(SYNCHRONIZE_SRCU_READER_DELAY);
+ while (!srcu_readers_active_idx_check(sp, idx)) {
+ if (expedited && ++ trycount < 10)
+ udelay(SYNCHRONIZE_SRCU_READER_DELAY);
+ else
+ schedule_timeout_interruptible(1);
+ }
+ }
+
+ /*
+ * The following smp_mb() E pairs with srcu_read_unlock()'s
+ * smp_mb C to ensure that if srcu_readers_active_idx_check()
+ * sees srcu_read_unlock()'s counter decrement, then any
+ * of the current task's subsequent code will happen after
+ * that SRCU read-side critical section.
+ *
+ * It also ensures the order between the above waiting and
+ * the next flipping.
+ */
+ smp_mb(); /* E */
+}
+
/*
* Flip the readers' index by incrementing ->completed, then wait
* until there are no more readers using the counters referenced by
@@ -258,12 +289,12 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
* Of course, it is possible that a reader might be delayed for the
* full duration of flip_idx_and_wait() between fetching the
* index and incrementing its counter. This possibility is handled
- * by __synchronize_srcu() invoking flip_idx_and_wait() twice.
+ * by the next __synchronize_srcu() invoking wait_idx() for such readers
+ * before start a new grace perioad.
*/
static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
{
int idx;
- int trycount = 0;
idx = sp->completed++ & 0x1;
@@ -278,28 +309,7 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
*/
smp_mb(); /* D */
- /*
- * SRCU read-side critical sections are normally short, so wait
- * a small amount of time before possibly blocking.
- */
- if (!srcu_readers_active_idx_check(sp, idx)) {
- udelay(SYNCHRONIZE_SRCU_READER_DELAY);
- while (!srcu_readers_active_idx_check(sp, idx)) {
- if (expedited && ++ trycount < 10)
- udelay(SYNCHRONIZE_SRCU_READER_DELAY);
- else
- schedule_timeout_interruptible(1);
- }
- }
-
- /*
- * The following smp_mb() E pairs with srcu_read_unlock()'s
- * smp_mb C to ensure that if srcu_readers_active_idx_check()
- * sees srcu_read_unlock()'s counter decrement, then any
- * of the current task's subsequent code will happen after
- * that SRCU read-side critical section.
- */
- smp_mb(); /* E */
+ wait_idx(sp, idx, expedited);
}
/*
@@ -307,8 +317,6 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
*/
static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
{
- int idx = 0;
-
rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
!lock_is_held(&rcu_bh_lock_map) &&
!lock_is_held(&rcu_lock_map) &&
@@ -318,27 +326,42 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
mutex_lock(&sp->mutex);
/*
- * If there were no helpers, then we need to do two flips of
- * the index. The first flip is required if there are any
- * outstanding SRCU readers even if there are no new readers
- * running concurrently with the first counter flip.
- *
- * The second flip is required when a new reader picks up
+ * When in the previous grace perioad, if a reader picks up
* the old value of the index, but does not increment its
* counter until after its counters is summed/rechecked by
- * srcu_readers_active_idx_check(). In this case, the current SRCU
+ * srcu_readers_active_idx_check(). In this case, the previous SRCU
* grace period would be OK because the SRCU read-side critical
- * section started after this SRCU grace period started, so the
+ * section started after the SRCU grace period started, so the
* grace period is not required to wait for the reader.
*
- * However, the next SRCU grace period would be waiting for the
- * other set of counters to go to zero, and therefore would not
- * wait for the reader, which would be very bad. To avoid this
- * bad scenario, we flip and wait twice, clearing out both sets
- * of counters.
+ * However, such leftover readers affect this new SRCU grace period.
+ * So we have to wait for such readers. This wait_idx() should be
+ * considerred as the wait_idx() in the flip_idx_and_wait() of
+ * the previous grace perioad except that it is for leftover readers
+ * started before this synchronize_srcu(). So when it returns,
+ * there is no leftover readers that starts before this grace period.
+ *
+ * If there are some leftover readers that do not increment its
+ * counter until after its counters is summed/rechecked by
+ * srcu_readers_active_idx_check(), In this case, this SRCU
+ * grace period would be OK as above comments says. We defines
+ * such readers as leftover-leftover readers, we consider these
+ * readers fteched index of (sp->completed + 1), it means they
+ * are considerred as exactly the same as the readers after this
+ * grace period.
+ *
+ * wait_idx() is expected very fast, because leftover readers
+ * are unlikely produced.
*/
- for (; idx < 2; idx++)
- flip_idx_and_wait(sp, expedited);
+ wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
+
+ /*
+ * Starts a new grace period, this flip is required if there are
+ * any outstanding SRCU readers even if there are no new readers
+ * running concurrently with the counter flip.
+ */
+ flip_idx_and_wait(sp, expedited);
+
mutex_unlock(&sp->mutex);
}
--
1.7.4.4
--
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