*/
#define SYNCHRONIZE_SRCU_READER_DELAY 5
-/*
- * Flip the readers' index by incrementing ->completed, then wait
- * until there are no more readers using the counters referenced by
- * the old index value. (Recall that the index is the bottom bit
- * of ->completed.)
- *
- * 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.
- */
-static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
+static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
{
- int idx;
int trycount = 0;
- idx = sp->completed++ & 0x1;
-
/*
- * If a reader fetches the index before the above increment,
+ * If a reader fetches the index before the ->completed increment,
* but increments its counter after srcu_readers_active_idx_check()
* sums it, then smp_mb() D will pair with __srcu_read_lock()'s
* smp_mb() B to ensure that the SRCU read-side critical section
* 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
+ * the old index value. (Recall that the index is the bottom bit
+ * of ->completed.)
+ *
+ * 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 the next __synchronize_srcu() invoking wait_idx() for such readers
+ * before starting a new grace period.
+ */
+static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
+{
+ int idx;
+
+ idx = sp->completed++ & 0x1;
+ wait_idx(sp, idx, expedited);
+}
+
/*
* Helper function for synchronize_srcu() and synchronize_srcu_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) &&
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.
+ * Suppose that during the previous grace period, a reader
+ * picked up the old value of the index, but did not increment
+ * its counter until after the previous instance of
+ * __synchronize_srcu() did the counter summation and recheck.
+ * That previous grace period was OK because the reader did
+ * not start until after the grace period started, so the grace
+ * period was not obligated to wait for that reader.
+ *
+ * However, the current SRCU grace period does have to wait for
+ * that reader. This is handled by invoking wait_idx() on the
+ * non-active set of counters (hence sp->completed - 1). Once
+ * wait_idx() returns, we know that all readers that picked up
+ * the old value of ->completed and that already incremented their
+ * counter will have completed.
*
- * The second flip is required when a new 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
- * grace period would be OK because the SRCU read-side critical
- * section started after this SRCU grace period started, so the
- * grace period is not required to wait for the reader.
+ * But what about readers that picked up the old value of
+ * ->completed, but -still- have not managed to increment their
+ * counter? We do not need to wait for those readers, because
+ * they will have started their SRCU read-side critical section
+ * after the current grace period starts.
*
- * 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.
+ * Because it is unlikely that readers will be preempted between
+ * fetching ->completed and incrementing their counter, wait_idx()
+ * will normally not need to wait.
*/
- for (; idx < 2; idx++)
- flip_idx_and_wait(sp, expedited);
+ wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
+
+ /*
+ * Now that wait_idx() has waited for the really old readers,
+ * invoke flip_idx_and_wait() to flip the counter and wait
+ * for current SRCU readers.
+ */
+ flip_idx_and_wait(sp, expedited);
+
mutex_unlock(&sp->mutex);
}
projects / linux-2.6-block.git / blobdiff