diff options
| author | Thomas Gleixner <tglx@linutronix.de> | 2025-01-19 00:55:32 +0100 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2025-01-29 15:21:31 +0100 |
| commit | b9a49520679e98700d3d89689cc91c08a1c88c1d (patch) | |
| tree | 7591e6d0c813f180f4277640562467df0cd970c7 | |
| parent | 5e0e02f0d7e52cfc8b1adfc778dd02181d8b47b4 (diff) | |
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@intel.com>
Debugged-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/oe-lkp/202412311453.9d7636a2-lkp@intel.com
| -rw-r--r-- | include/linux/rcuref.h | 9 | ||||
| -rw-r--r-- | lib/rcuref.c | 5 |
2 files changed, 8 insertions, 6 deletions
diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h index 2c8bfd0f1b6b..6322d8c1c6b4 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 97f300eca927..5bd726b71e39 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)) { /* |