On Thu, Nov 23, 2017 at 09:24:11AM +0100, Boris Brezillon wrote: > On Wed, 22 Nov 2017 16:13:31 -0800 > Eric Anholt <eric@xxxxxxxxxx> wrote: > > > Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> writes: > > > > > On Wed, 22 Nov 2017 13:16:08 -0800 > > > Eric Anholt <eric@xxxxxxxxxx> wrote: > > > > > >> Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> writes: > > >> > > >> > With CONFIG_REFCOUNT_FULL enabled, refcount_inc() complains when it's > > >> > passed a refcount object that has its counter set to 0. In this driver, > > >> > this is a valid use case since we want to increment ->usecnt only when > > >> > the BO object starts to be used by real HW components and this is > > >> > definitely not the case when the BO is created. > > >> > > > >> > Fix the problem by using refcount_inc_not_zero() instead of > > >> > refcount_inc() and fallback to refcount_set(1) when > > >> > refcount_inc_not_zero() returns false. Note that this 2-steps operation > > >> > is not racy here because the whole section is protected by a mutex > > >> > which guarantees that the counter does not change between the > > >> > refcount_inc_not_zero() and refcount_set() calls. > > >> > > >> If we're not following the model, and protecting the refcount by a > > >> mutex, shouldn't we just be using addition and subtraction instead of > > >> refcount's atomics? > > > > > > Actually, this mutex is protecting the bo->madv value which has to be > > > checked when the counter reaches 0 (when decrementing) or 1 (when > > > incrementing). We just benefit from this protection here, but ideally > > > it would be better to have an refcount_inc_allow_zero() as suggested by > > > Daniel. > > > > Let me restate this to see if it makes sense: The refcount is always >= > > 0, this is is the only path that increases the refcount from 0 to 1, and > > it's (incidentally) protected by a mutex, so there's no race between the > > attempted increase from nonzero and the set from nonzero to 1. > > Yep. > > > > > This seems fine to me as a bugfix. > > The discussion is going on in the other thread, let's wait a bit > before taking a decision. Let's not block the bugfix on reaching perfection imo. I'd merge this as the minimal fix, and then apply the pretty paint once we have a clear idea for that. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
