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rcuref: Plug slowpath race in rcuref_put()
stable inclusion from stable-v6.6.81 commit 1d26aaa86124e2622cfe38d6fdb9c4da3b49449d category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/IBYZED Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d26aaa86124e2622cfe38d6fdb9c4da3b49449d -------------------------------- commit b9a49520679e98700d3d89689cc91c08a1c88c1d upstream. Kernel test robot reported an "imbalanced put" in the rcuref_put() slow path, which turned out to be a false positive. Consider the following race: ref = 0 (via rcuref_init(ref, 1)) T1 T2 rcuref_put(ref) -> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff -> rcuref_put_slowpath(ref) rcuref_get(ref) -> atomic_add_negative_relaxed(1, &ref->refcnt) -> return true; # ref -> 0 rcuref_put(ref) -> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff -> rcuref_put_slowpath() -> cnt = atomic_read(&ref->refcnt); # cnt -> 0xffffffff / RCUREF_NOREF -> atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD)) # ref -> 0xe0000000 / RCUREF_DEAD -> return true -> cnt = atomic_read(&ref->refcnt); # cnt -> 0xe0000000 / RCUREF_DEAD -> if (cnt > RCUREF_RELEASED) # 0xe0000000 > 0xc0000000 -> WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()") The problem is the additional read in the slow path (after it decremented to RCUREF_NOREF) which can happen after the counter has been marked RCUREF_DEAD. Prevent this by reusing the return value of the decrement. Now every "final" put uses RCUREF_NOREF in the slow path and attempts the final cmpxchg() to RCUREF_DEAD. [ bigeasy: Add changelog ] Fixes: ee1ee6db ("atomics: Provide rcuref - scalable reference counting") Reported-by:kernel test robot <oliver.sang@intel.com> Debugged-by: