On 09/19/2017 10:38 AM, Christoph Hellwig wrote: > On Tue, Sep 19, 2017 at 08:49:12AM -0400, Waiman Long wrote: >> On 09/18/2017 08:01 PM, Christoph Hellwig wrote: >>> Taking a look at this it seems like using a lock in struct block_device >>> isn't the right thing to do anyway - all the action is on fields in >>> struct blk_trace, so having a lock inside that would make a lot more >>> sense. >>> >>> It would also help to document what exactly we're actually protecting. >> I think I documented in the patch that the lock has to protect changes >> in the blktrace structure as well as the allocation and destruction of >> it. Because of that, it can't be put inside the blktrace structure. The >> original code use the bd_mutex of the block_device structure. I just >> change the code to use another bd_fsfreeze_mutex in the same structure. > Either way it has absolutely nothing to do with struct block_device, > given that struct blk_trace hangs off the request_queue. > > Reusing a mutex just because it happens to live in a structure also > generally is a bad idea if the protected data is in no way related. I was trying not to add a new mutex to a structure just for blktrace as it is an optional feature that is enabled only if the CONFIG_BLK_DEV_IO_TRACE config option is defined and it will only need to be taken occasionally. As filesystem freeze looks orthogonal to blktrace and the mutex also looks to be used sparingly, I think it is a good match to overload it to control blktrace as well. I could modify the patch to use a mutex in the request_queue structure. The current sysfs_lock mutex has about 74 references. So I am not totally sure if it is safe to reuse. So the only option is to add something like #ifdef CONFIG_BLK_DEV_IO_TRACE struct mutex blktrace_mutex; #endif to the request_queue structure. That structure is large enough that adding a mutex won't increase the size much percentage-wise. I would like to keep the current patch as is as I don't see any problem with it. However, I can revise the patch as discussed above if you guys prefer that alternative. Cheers, Longman
