On Thu, May 24 2018 at 10:10am -0400, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Wed, May 23, 2018 at 02:10:46PM -0400, Mike Snitzer wrote: > > [Peter, in this v2 I switched to using _GPL for the exports and updated > > the patch header. As covered in previous mail, please let me know if > > you're OK with me staging this change for 4.18 via linux-dm.git with > > your Ack, thanks] > > > > From: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > Subject: [PATCH] swait: export symbols __prepare_to_swait and __finish_swait > > > > __prepare_to_swait and __finish_swait are declared in > > include/linux/swait.h but they are not exported; so they are not useable > > from kernel modules. > > > > A new consumer of swait (in dm-writecache) reduces its locking overhead > > by using the spinlock in swait_queue_head to protect not only the wait > > queue, but also the list of events. Consequently, this swait consuming > > kernel module needs to use these unlocked functions. > > > > Peter Zijlstra explained: > > "The reason swait exists is to be deterministic (for RT) -- something > > that regular wait code cannot be. > > And by (ab)using / exporting the wait internal lock you risk losing > > that. So I don't think the proposed [dm-writecache] usage is bad, it > > is possible to create badness. > > So if we're going to export them; someone needs to keep an eye on things > > and ensure the lock isn't abused." > > > > So while this new use of the wait internal lock doesn't jeopardize the > > realtime requirements of swait, these exports do open swait's internal > > locking up to being abused. As such, EXPORT_SYMBOL_GPL is used because > > any future consumers of __prepare_to_swait and __finish_swait must > > always be thoroughly scrutinized. > > > > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Thanks Peter. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
