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Index: FreeBSD/sys/contrib/ck/src/ck_epoch.c
===================================================================
--- FreeBSD/sys/contrib/ck/src/ck_epoch.c
+++ FreeBSD/sys/contrib/ck/src/ck_epoch.c
@@ -137,6 +137,14 @@
CK_STACK_CONTAINER(struct ck_epoch_entry, stack_entry,
ck_epoch_entry_container)
+/*
+ * CK_EPOCH_LENGTH must be a power-of-2 (because (CK_EPOCH_LENGTH - 1) is used
+ * as a mask, and it must be at least 3 (see comments above).
+ */
+#if (CK_EPOCH_LENGTH < 3 || (CK_EPOCH_LENGTH & (CK_EPOCH_LENGTH - 1)) != 0)
+#error "CK_EPOCH_LENGTH must be a power of 2 and >= 3"
+#endif
+
#define CK_EPOCH_SENSE_MASK (CK_EPOCH_SENSE - 1)
bool
@@ -552,22 +560,36 @@
* There are cases where it will fail to reclaim as early as it could. If this
* becomes a problem, we could actually use a heap for epoch buckets but that
* is far from ideal too.
+ *
+ * Note that this code assumes that nothing will modify the record while the
+ * code is running.
*/
bool
ck_epoch_poll_deferred(struct ck_epoch_record *record, ck_stack_t *deferred)
{
bool active;
- unsigned int epoch;
+ unsigned int epoch, startpend;
struct ck_epoch_record *cr = NULL;
struct ck_epoch *global = record->global;
+ startpend = record->n_pending;
epoch = ck_pr_load_uint(&global->epoch);
/* Serialize epoch snapshots with respect to global epoch. */
ck_pr_fence_memory();
+
+ /*
+ * At this point, epoch is the current global epoch value.
+ * There may or may not be active threads which observed epoch - 1.
+ * (ck_epoch_scan() will tell us that.) However, there should be
+ * no active threads which observed epoch - 2. (Note that checking
+ * epoch - 2 is necessary, as race conditions can allow another
+ * thread to increment the global epoch before this thread runs.)
+ */
+ ck_epoch_dispatch(record, epoch - 2, deferred);
cr = ck_epoch_scan(global, cr, epoch, &active);
if (cr != NULL)
- return false;
+ goto done;
/* We are at a grace period if all threads are inactive. */
if (active == false) {
@@ -575,14 +597,21 @@
for (epoch = 0; epoch < CK_EPOCH_LENGTH; epoch++)
ck_epoch_dispatch(record, epoch, deferred);
- return true;
+ goto done;
}
- /* If an active thread exists, rely on epoch observation. */
+ /*
+ * If an active thread exists, rely on epoch observation.
+ *
+ * All the active threads entered the epoch section during
+ * the current epoch. Therefore, we can now run the handlers
+ * for the immediately preceding epoch and advance the epoch.
+ */
(void)ck_pr_cas_uint(&global->epoch, epoch, epoch + 1);
+ ck_epoch_dispatch(record, epoch - 1, deferred);
- ck_epoch_dispatch(record, epoch + 1, deferred);
- return true;
+done:
+ return (record->n_pending != startpend);
}
bool

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