On Fri, 2017-08-18 at 16:01 -0400, Waiman Long wrote: > [ ... ] > Instead of using bd_mutex, a new global blktrace mutex is now used > to protect against concurrent access, creation and destruction of the > blk_trace structure that is used only in the blktrace.c file. As > blktrace files will not be frequently accessed, using a global mutex > should not cause any performance problem. > [ ... ] > +/* > + * The bd_mutex was used previously for protecting blk_trace structure. > + * That could lead to deadlock with concurrent block device deletion and > + * sysfs access. So a global blktrace_mutex is now used instead for > + * protecting the blk_trace structure. > + * > + * The references to the opened sysfs or device files should prevent the > + * underlying block device from being removed. > + */ > +static DEFINE_MUTEX(blktrace_mutex); Hello Waiman, Thanks for having addressed my previous comment. Regarding this patch: sorry but I don't think it's a good idea to use a global mutex for serializing accesses to tracing data of a single block device. Global mutexes create unwanted lock dependencies between different block devices. Additionally, on multiprocessor systems global mutexes can cause cache line ping-pong between processors and hence can cause a severe slowdown. Please make blktrace_mutex per block device instead of global. Thanks, Bart.
