Messages in this thread | | | From | Joel Fernandes <> | Date | Mon, 24 Jun 2019 16:43:21 -0400 | Subject | Re: [PATCH RFC v2] Convert struct pid count to refcount_t |
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On Mon, Jun 24, 2019 at 3:10 PM Jann Horn <jannh@google.com> wrote: > > On Mon, Jun 24, 2019 at 8:52 PM Joel Fernandes <joel@joelfernandes.org> wrote: > > On Mon, Jun 24, 2019 at 02:45:34PM -0400, Joel Fernandes (Google) wrote: > > > struct pid's count is an atomic_t field used as a refcount. Use > > > refcount_t for it which is basically atomic_t but does additional > > > checking to prevent use-after-free bugs. > > > > > > For memory ordering, the only change is with the following: > > > - if ((atomic_read(&pid->count) == 1) || > > > - atomic_dec_and_test(&pid->count)) { > > > + if (refcount_dec_and_test(&pid->count)) { > > > kmem_cache_free(ns->pid_cachep, pid); > > > > > > Here the change is from: > > > Fully ordered --> RELEASE + ACQUIRE (as per refcount-vs-atomic.rst) > > > This ACQUIRE should take care of making sure the free happens after the > > > refcount_dec_and_test(). > > > > > > The above hunk also removes atomic_read() since it is not needed for the > > > code to work and it is unclear how beneficial it is. The removal lets > > > refcount_dec_and_test() check for cases where get_pid() happened before > > > the object was freed. > [...] > > I had a question about refcount_inc(). > > > > As per Documentation/core-api/refcount-vs-atomic.rst , it says: > > > > A control dependency (on success) for refcounters guarantees that > > if a reference for an object was successfully obtained (reference > > counter increment or addition happened, function returned true), > > then further stores are ordered against this operation. > > > > However, in refcount_inc() I don't see any memory barriers (in the case where > > CONFIG_REFCOUNT_FULL=n). Is the documentation wrong? > > That part of the documentation only talks about cases where you have a > control dependency on the return value of the refcount operation. But > refcount_inc() does not return a value, so this isn't relevant for > refcount_inc(). > > Also, AFAIU, the control dependency mentioned in the documentation has > to exist *in the caller* - it's just pointing out that if you write > code like the following, you have a control dependency between the > refcount operation and the write: > > if (refcount_inc_not_zero(&obj->refcount)) { > WRITE_ONCE(obj->x, y); > } > > For more information on the details of this stuff, try reading the > section "CONTROL DEPENDENCIES" of Documentation/memory-barriers.txt.
Makes sense now, thank you Jann!
- Joel
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