Hi
This changes the timeout to a sequence count. And adds a comment.
Mikulas
---
Avoit the timeout.
Use a sequence count to resolve the race. The count increases each time
an exception reallocation finishes. Use wait_event() to wait until the count
changes.
The chunk-reallocation logic is explained in the comment in the patch.
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
---
drivers/md/dm-snap.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 42 insertions(+), 2 deletions(-)
Index: linux-2.6.32-devel/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.32-devel.orig/drivers/md/dm-snap.c 2009-12-07 12:40:15.000000000 +0100
+++ linux-2.6.32-devel/drivers/md/dm-snap.c 2009-12-07 14:18:48.000000000 +0100
@@ -257,6 +257,8 @@ static struct list_head *_origins;
static struct rw_semaphore _origins_lock;
static DECLARE_WAIT_QUEUE_HEAD(_pending_exception_done);
+static DEFINE_SPINLOCK(_pending_exception_done_spinlock);
+static u64 _pending_exception_done_count = 0;
static int init_origin_hash(void)
{
@@ -756,6 +758,17 @@ static int __origin_write(struct list_he
static void merge_callback(int read_err, unsigned long write_err,
void *context);
+static u64 read_pending_exception_done_count(void)
+{
+ u64 current_count;
+
+ spin_lock(&_pending_exception_done_spinlock);
+ current_count = _pending_exception_done_count;
+ spin_unlock(&_pending_exception_done_spinlock);
+
+ return current_count;
+}
+
static void snapshot_merge_process(struct dm_snapshot *s)
{
int r;
@@ -763,6 +776,7 @@ static void snapshot_merge_process(struc
struct origin *o;
int must_wait;
struct dm_io_region src, dest;
+ u64 previous_count;
BUG_ON(!test_bit(MERGE_RUNNING, &s->bits));
if (unlikely(test_bit(SHUTDOWN_MERGE, &s->bits)))
@@ -792,8 +806,31 @@ static void snapshot_merge_process(struc
src.sector = chunk_to_sector(s->store, new_chunk);
src.count = dest.count;
+ /*
+ * Reallocate the other snapshots:
+ *
+ * The chunk size of the merging snapshot may be larger than the chunk
+ * size of some other snapshot. So we may need to reallocate multiple
+ * chunks in a snapshot.
+ *
+ * We don't do linking of pending exceptions and waiting for the last
+ * one --- that would complicate code too much and it would also be
+ * bug-prone.
+ *
+ * Instead, we try to scan all the overlapping exceptions in all
+ * non-merging snapshots and if something was reallocated then wait
+ * for any pending exception to complete. Retry after the wait, until
+ * all exceptions are done.
+ *
+ * This may seem ineffective, but in practice, people hardly use more
+ * than one or two snapshots. In case of two snapshots (one merging and
+ * one non-merging) with the same chunksize, wait and wakeup is done
+ * only once.
+ */
+
test_again:
- /* Reallocate other snapshots */
+ previous_count = read_pending_exception_done_count();
+
must_wait = 0;
/*
* Merging snapshot already has the origin's __minimum_chunk_size()
@@ -809,7 +846,7 @@ test_again:
}
up_read(&_origins_lock);
if (must_wait) {
- sleep_on_timeout(&_pending_exception_done, HZ / 100 + 1);
+ wait_event(_pending_exception_done, read_pending_exception_done_count() != previous_count);
goto test_again;
}
@@ -1345,6 +1382,9 @@ static void pending_complete(struct dm_s
origin_bios = bio_list_get(&pe->origin_bios);
free_pending_exception(pe);
+ spin_lock(&_pending_exception_done_spinlock);
+ _pending_exception_done_count++;
+ spin_unlock(&_pending_exception_done_spinlock);
wake_up_all(&_pending_exception_done);
up_write(&s->lock);
--
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