[PATCH v2 05/12] bcache: stop dc->writeback_rate_update properly

2018-01-13 Thread Coly Li 
struct delayed_work writeback_rate_update in struct cache_dev is a delayed
worker to call function update_writeback_rate() in period (the interval is
defined by dc->writeback_rate_update_seconds).

When a metadate I/O error happens on cache device, bcache error handling
routine bch_cache_set_error() will call bch_cache_set_unregister() to
retire whole cache set. On the unregister code path, this delayed work is
stopped by calling cancel_delayed_work_sync(>writeback_rate_update).

dc->writeback_rate_update is a special delayed work from others in bcache.
In its routine update_writeback_rate(), this delayed work is re-armed
itself. That means when cancel_delayed_work_sync() returns, this delayed
work can still be executed after several seconds defined by
dc->writeback_rate_update_seconds.

The problem is, after cancel_delayed_work_sync() returns, the cache set
unregister code path will continue and release memory of struct cache set.
Then the delayed work is scheduled to run, __update_writeback_rate()
will reference the already released cache_set memory, and trigger a NULL
pointer deference fault.

This patch introduces two more bcache device flags,
- BCACHE_DEV_WB_RUNNING
  bit set:  bcache device is in writeback mode and running, it is OK for
dc->writeback_rate_update to re-arm itself.
  bit clear:bcache device is trying to stop dc->writeback_rate_update,
this delayed work should not re-arm itself and quit.
- BCACHE_DEV_RATE_DW_RUNNING
  bit set:  routine update_writeback_rate() is executing.
  bit clear: routine update_writeback_rate() quits.

This patch also adds a function cancel_writeback_rate_update_dwork() to
wait for dc->writeback_rate_update quits before cancel it by calling
cancel_delayed_work_sync(). In order to avoid a deadlock by unexpected
quit dc->writeback_rate_update, after time_out seconds this function will
give up and continue to call cancel_delayed_work_sync().

And here I explain how this patch stops self re-armed delayed work properly
with the above stuffs.

update_writeback_rate() sets BCACHE_DEV_RATE_DW_RUNNING at its beginning
and clears BCACHE_DEV_RATE_DW_RUNNING at its end. Before calling
cancel_writeback_rate_update_dwork() clear flag BCACHE_DEV_WB_RUNNING.

Before calling cancel_delayed_work_sync() wait utill flag
BCACHE_DEV_RATE_DW_RUNNING is clear. So when calling
cancel_delayed_work_sync(), dc->writeback_rate_update must be already re-
armed, or quite by seeing BCACHE_DEV_WB_RUNNING cleared. In both cases
delayed work routine update_writeback_rate() won't be executed after
cancel_delayed_work_sync() returns.

Inside update_writeback_rate() before calling schedule_delayed_work(), flag
BCACHE_DEV_WB_RUNNING is checked before. If this flag is cleared, it means
someone is about to stop the delayed work. Because flag
BCACHE_DEV_RATE_DW_RUNNING is set already and cancel_delayed_work_sync()
has to wait for this flag to be cleared, we don't need to worry about race
condition here.

If update_writeback_rate() is scheduled to run after checking
BCACHE_DEV_RATE_DW_RUNNING and before calling cancel_delayed_work_sync()
in cancel_writeback_rate_update_dwork(), it is also safe. Because at this
moment BCACHE_DEV_WB_RUNNING is cleared with memory barrier. As I mentioned
previously, update_writeback_rate() will see BCACHE_DEV_WB_RUNNING is clear
and quit immediately.

Because there are more dependences inside update_writeback_rate() to struct
cache_set memory, dc->writeback_rate_update is not a simple self re-arm
delayed work. After trying many different methods (e.g. hold dc->count, or
use locks), this is the only way I can find which works to properly stop
dc->writeback_rate_update delayed work.

Changelog:
v2: Try to fix the race issue which is pointed out by Junhui.
v1: The initial version for review

Signed-off-by: Coly Li 
Cc: Michael Lyle 
Cc: Hannes Reinecke 
Cc: Junhui Tang 
---
 drivers/md/bcache/bcache.h|  9 +
 drivers/md/bcache/super.c | 39 +++
 drivers/md/bcache/sysfs.c |  3 ++-
 drivers/md/bcache/writeback.c | 29 -
 4 files changed, 70 insertions(+), 10 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 5e2d4e80198e..88d938c8d027 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -258,10 +258,11 @@ struct bcache_device {
struct gendisk  *disk;
 
unsigned long   flags;
-#define BCACHE_DEV_CLOSING 0
-#define BCACHE_DEV_DETACHING   1
-#define BCACHE_DEV_UNLINK_DONE 2
-
+#define BCACHE_DEV_CLOSING 0
+#define BCACHE_DEV_DETACHING   1
+#define BCACHE_DEV_UNLINK_DONE 2
+#define BCACHE_DEV_WB_RUNNING  4
+#define BCACHE_DEV_RATE_DW_RUNNING 8
unsignednr_stripes;
unsignedstripe_size;
atomic_t

[PATCH v2 05/12] bcache: stop dc->writeback_rate_update properly

2018-01-13 Thread Coly Li 
struct delayed_work writeback_rate_update in struct cache_dev is a delayed
worker to call function update_writeback_rate() in period (the interval is
defined by dc->writeback_rate_update_seconds).

When a metadate I/O error happens on cache device, bcache error handling
routine bch_cache_set_error() will call bch_cache_set_unregister() to
retire whole cache set. On the unregister code path, this delayed work is
stopped by calling cancel_delayed_work_sync(>writeback_rate_update).

dc->writeback_rate_update is a special delayed work from others in bcache.
In its routine update_writeback_rate(), this delayed work is re-armed
itself. That means when cancel_delayed_work_sync() returns, this delayed
work can still be executed after several seconds defined by
dc->writeback_rate_update_seconds.

The problem is, after cancel_delayed_work_sync() returns, the cache set
unregister code path will continue and release memory of struct cache set.
Then the delayed work is scheduled to run, __update_writeback_rate()
will reference the already released cache_set memory, and trigger a NULL
pointer deference fault.

This patch introduces two more bcache device flags,
- BCACHE_DEV_WB_RUNNING
  bit set:  bcache device is in writeback mode and running, it is OK for
dc->writeback_rate_update to re-arm itself.
  bit clear:bcache device is trying to stop dc->writeback_rate_update,
this delayed work should not re-arm itself and quit.
- BCACHE_DEV_RATE_DW_RUNNING
  bit set:  routine update_writeback_rate() is executing.
  bit clear: routine update_writeback_rate() quits.

This patch also adds a function cancel_writeback_rate_update_dwork() to
wait for dc->writeback_rate_update quits before cancel it by calling
cancel_delayed_work_sync(). In order to avoid a deadlock by unexpected
quit dc->writeback_rate_update, after time_out seconds this function will
give up and continue to call cancel_delayed_work_sync().

And here I explain how this patch stops self re-armed delayed work properly
with the above stuffs.

update_writeback_rate() sets BCACHE_DEV_RATE_DW_RUNNING at its beginning
and clears BCACHE_DEV_RATE_DW_RUNNING at its end. Before calling
cancel_writeback_rate_update_dwork() clear flag BCACHE_DEV_WB_RUNNING.

Before calling cancel_delayed_work_sync() wait utill flag
BCACHE_DEV_RATE_DW_RUNNING is clear. So when calling
cancel_delayed_work_sync(), dc->writeback_rate_update must be already re-
armed, or quite by seeing BCACHE_DEV_WB_RUNNING cleared. In both cases
delayed work routine update_writeback_rate() won't be executed after
cancel_delayed_work_sync() returns.

Inside update_writeback_rate() before calling schedule_delayed_work(), flag
BCACHE_DEV_WB_RUNNING is checked before. If this flag is cleared, it means
someone is about to stop the delayed work. Because flag
BCACHE_DEV_RATE_DW_RUNNING is set already and cancel_delayed_work_sync()
has to wait for this flag to be cleared, we don't need to worry about race
condition here.

If update_writeback_rate() is scheduled to run after checking
BCACHE_DEV_RATE_DW_RUNNING and before calling cancel_delayed_work_sync()
in cancel_writeback_rate_update_dwork(), it is also safe. Because at this
moment BCACHE_DEV_WB_RUNNING is cleared with memory barrier. As I mentioned
previously, update_writeback_rate() will see BCACHE_DEV_WB_RUNNING is clear
and quit immediately.

Because there are more dependences inside update_writeback_rate() to struct
cache_set memory, dc->writeback_rate_update is not a simple self re-arm
delayed work. After trying many different methods (e.g. hold dc->count, or
use locks), this is the only way I can find which works to properly stop
dc->writeback_rate_update delayed work.

Changelog:
v2: Try to fix the race issue which is pointed out by Junhui.
v1: The initial version for review

Signed-off-by: Coly Li 
Cc: Michael Lyle 
Cc: Hannes Reinecke 
Cc: Junhui Tang 
---
 drivers/md/bcache/bcache.h|  9 +
 drivers/md/bcache/super.c | 39 +++
 drivers/md/bcache/sysfs.c |  3 ++-
 drivers/md/bcache/writeback.c | 29 -
 4 files changed, 70 insertions(+), 10 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 5e2d4e80198e..88d938c8d027 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -258,10 +258,11 @@ struct bcache_device {
struct gendisk  *disk;
 
unsigned long   flags;
-#define BCACHE_DEV_CLOSING 0
-#define BCACHE_DEV_DETACHING   1
-#define BCACHE_DEV_UNLINK_DONE 2
-
+#define BCACHE_DEV_CLOSING 0
+#define BCACHE_DEV_DETACHING   1
+#define BCACHE_DEV_UNLINK_DONE 2
+#define BCACHE_DEV_WB_RUNNING  4
+#define BCACHE_DEV_RATE_DW_RUNNING 8
unsignednr_stripes;
unsignedstripe_size;
atomic_t