Boris Brezillon <boris.brezil...@free-electrons.com> writes: > On Wed, 22 Nov 2017 13:16:08 -0800 > Eric Anholt <e...@anholt.net> wrote: > >> Boris Brezillon <boris.brezil...@free-electrons.com> 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. This seems fine to me as a bugfix.
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