The following commit has been merged into the locking/urgent branch of tip: Commit-ID: b9a49520679e98700d3d89689cc91c08a1c88c1d Gitweb: https://git.kernel.org/tip/b9a49520679e98700d3d89689cc91c08a1c88c1d Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> AuthorDate: Sun, 19 Jan 2025 00:55:32 +01:00 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitterDate: Wed, 29 Jan 2025 15:21:31 +01:00 rcuref: Plug slowpath race in rcuref_put() 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: ee1ee6db07795 ("atomics: Provide rcuref - scalable reference counting") Reported-by: kernel test robot <oliver.sang@xxxxxxxxx> Debugged-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Closes: https://lore.kernel.org/oe-lkp/202412311453.9d7636a2-lkp@xxxxxxxxx --- include/linux/rcuref.h | 9 ++++++--- lib/rcuref.c | 5 ++--- 2 files changed, 8 insertions(+), 6 deletions(-) diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h index 2c8bfd0..6322d8c 100644 --- a/include/linux/rcuref.h +++ b/include/linux/rcuref.h @@ -71,27 +71,30 @@ static inline __must_check bool rcuref_get(rcuref_t *ref) return rcuref_get_slowpath(ref); } -extern __must_check bool rcuref_put_slowpath(rcuref_t *ref); +extern __must_check bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt); /* * Internal helper. Do not invoke directly. */ static __always_inline __must_check bool __rcuref_put(rcuref_t *ref) { + int cnt; + RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(), "suspicious rcuref_put_rcusafe() usage"); /* * Unconditionally decrease the reference count. The saturation and * dead zones provide enough tolerance for this. */ - if (likely(!atomic_add_negative_release(-1, &ref->refcnt))) + cnt = atomic_sub_return_release(1, &ref->refcnt); + if (likely(cnt >= 0)) return false; /* * Handle the last reference drop and cases inside the saturation * and dead zones. */ - return rcuref_put_slowpath(ref); + return rcuref_put_slowpath(ref, cnt); } /** diff --git a/lib/rcuref.c b/lib/rcuref.c index 97f300e..5bd726b 100644 --- a/lib/rcuref.c +++ b/lib/rcuref.c @@ -220,6 +220,7 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath); /** * rcuref_put_slowpath - Slowpath of __rcuref_put() * @ref: Pointer to the reference count + * @cnt: The resulting value of the fastpath decrement * * Invoked when the reference count is outside of the valid zone. * @@ -233,10 +234,8 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath); * with a concurrent get()/put() pair. Caller is not allowed to * deconstruct the protected object. */ -bool rcuref_put_slowpath(rcuref_t *ref) +bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt) { - unsigned int cnt = atomic_read(&ref->refcnt); - /* Did this drop the last reference? */ if (likely(cnt == RCUREF_NOREF)) { /*
