Messages in this thread | | | Date | Wed, 9 Oct 2019 13:35:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 05/10] lib/refcount: Improve performance of generic REFCOUNT_FULL code |
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On Wed, Oct 09, 2019 at 11:25:08AM +0200, Peter Zijlstra wrote: > On Mon, Oct 07, 2019 at 04:46:58PM +0100, Will Deacon wrote: > > static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r) > > { > > + int old = atomic_fetch_sub_release(i, &r->refs); > > > > + if (old == i) { > > smp_acquire__after_ctrl_dep(); > > return true; > > } > > > > + if (unlikely(old - i < 0)) { > > + refcount_set(r, REFCOUNT_SATURATED); > > + WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n"); > > + } > > I'm failing to see how this preserves REFCOUNT_SATURATED for > non-underflow. AFAICT this should have: > > if (unlikely(old == REFCOUNT_SATURATED || old - i < 0))
Hmm, that is not sufficient, since you can be arbitrarily far away from it due to all the races (and add/sub really suck as a refcount interface). The same will make fixing the cmpxchg loops like dec_not_one() 'interesting'.
It is important though; to keep saturated, otherwise something that can do INT_MAX+n actual increments will get freed after INT_MAX decrements and still have n 'proper' references, *whoopsie*.
> > > + return false; > > } > > > > /**
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