Hi Mike, On 05/18/2010 10:46 PM +0900, Mike Snitzer wrote: > Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> wrote: >> On 05/18/2010 02:27 AM +0900, Mike Snitzer wrote: >>> Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> wrote: >>>> As far as I understand, the current model of device-mapper is: >>>> - a table (precisely, a target) has various attributes, >>>> bio-based/request-based is one of such attributes >>>> - a table and its attributes are bound to the block device on resume >>>> If we want to fix a problem, I think we should either work based on >>>> this model or change the model. >>>> >>>> Your patch makes that loading table affects the block device, so you >>>> are changing the model. >>>> >>>> If you change the model, it should be done carefully. >>>> For example, the current model allows most of the table loading code >>>> to run without exclusive lock on the device because it doesn't affect >>>> the device itself. If you change this model, table loading needs to >>>> be serialized with appropriate locking. >>> >>> Nice catch, yes md->queue needs protection (see patch below). >> >> Not enough. (See drivers/md/dm-ioctl.c:table_load().) >> Table load sequence is: >> 1. populate table >> 2. set the table to ->new_map of the hash_cell for the mapped_device >> in protection by _hash_lock. >> >> Since your fix only serializes the step 1, concurrent table loading >> could end up with inconsistent status; e.g. request-based table is >> bound to the mapped_device while the queue is initialized as bio-based. >> With your new model, those 2 steps above must be atomic. > > Ah, yes.. I looked at the possibility of serializing the entirety of > table_load but determined that would be too excessive (would reduce > parallelism of table_load). But I clearly missed the fact that there > could be a race to the _hash_lock protected critical section in > table_load() -- leading to queue inconsistency. > > I'll post v5 of the overall patch which will revert the mapped_device > 'queue_lock' serialization that I proposed in v4. v5 will contain > the following patch to localize all table load related queue > manipulation to the _hash_lock protected critical section in > table_load(). So it sets the queue up _after_ the table's type is > established with dm_table_set_type(). dm_table_setup_md_queue() may allocate memory with blocking mode. Blocking allocation inside exclusive _hash_lock can cause deadlock; e.g. when it has to wait for other dm devices to resume to free some memory. Also, your patch changes the queue configuration even when a table is already active and used. (e.g. Loading bio-based table to a mapped_device which is already active/used as request-based sets q->requst_fn in NULL.) That could cause some critical problems. Thanks, Kiyoshi Ueda -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
