Messages in this thread | | | From | Willem de Bruijn <> | Date | Fri, 2 Aug 2019 10:52:28 -0400 | Subject | Re: [PATCH v2 1/2] cxgb4: sched: Use refcount_t for refcount |
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On Fri, Aug 2, 2019 at 10:27 AM Chuhong Yuan <hslester96@gmail.com> wrote: > > Willem de Bruijn <willemdebruijn.kernel@gmail.com> 于2019年8月2日周五 下午9:40写道: > > > > On Fri, Aug 2, 2019 at 4:36 AM Chuhong Yuan <hslester96@gmail.com> wrote: > > > > > > refcount_t is better for reference counters since its > > > implementation can prevent overflows. > > > So convert atomic_t ref counters to refcount_t. > > > > > > Signed-off-by: Chuhong Yuan <hslester96@gmail.com> > > > --- > > > Changes in v2: > > > - Convert refcount from 0-base to 1-base. > > > > This changes the initial value from 0 to 1, but does not change the > > release condition. So this introduces an accounting bug? > > I have noticed this problem and have checked other files which use refcount_t. > I find although the refcounts are 1-based, they still use > refcount_dec_and_test() > to check whether the resource should be released. > One example is drivers/char/mspec.c. > Therefore I think this is okay and do not change the release condition.
Indeed it is fine to use refcount_t with a model where the initial allocation already accounts for the first reference and thus initializes with refcount_set(.., 1).
But it is not correct to just change a previously zero initialization to one. As now an extra refcount_dec will be needed to release state. But the rest of the code has not changed, so this extra decrement will not happen.
For a correct conversion, see for instance commits
commit db5bce32fbe19f0c7482fb5a40a33178bbe7b11b net: prepare (struct ubuf_info)->refcnt conversion
and
commit c1d1b437816f0afa99202be3cb650c9d174667bc net: convert (struct ubuf_info)->refcnt to refcount_t
The second makes a search-and-replace style API change like your patches (though also notice the additional required #include).
But the other patch is needed first to change both the initial atomic_set *and* at least one atomic_inc, to maintain the same reference count over the object's lifetime.
That change requires understanding of the object's lifecycle, so I suggest only making those changes when aware of that whole data path.
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