On Tue, Sep 19, 2017 at 11:58:34AM -0400, Waiman Long wrote: > 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. So? Make the lock optional, too. > 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. If the functionally is orthogonal that is a reason not to share a lock, not to the contrary. > 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. Call it blk_trace mutex and move it right next to the blk_trace structure: ifdef CONFIG_BLK_DEV_IO_TRACE struct blk_trace *blk_trace; struct mutex blk_trace_mutex; #endif which makes it completely obvious to any read what you are protecting with it.