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rcuref: Plug slowpath race in rcuref_put()

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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: ee1ee6db07 ("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
This commit is contained in:
Thomas Gleixner 2025-01-19 00:55:32 +01:00
parent 5e0e02f0d7
commit b9a4952067
2 changed files with 8 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
include/linux/rcuref.h
View file

@ -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);
}
/**

5
lib/rcuref.c
View file

@ -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)) {
/*