// SPDX-License-Identifier: GPL-2.0-only /* * rcuref - A scalable reference count implementation for RCU managed objects * * rcuref is provided to replace open coded reference count implementations * based on atomic_t. It protects explicitely RCU managed objects which can * be visible even after the last reference has been dropped and the object * is heading towards destruction. * * A common usage pattern is: * * get() * rcu_read_lock(); * p = get_ptr(); * if (p && !atomic_inc_not_zero(&p->refcnt)) * p = NULL; * rcu_read_unlock(); * return p; * * put() * if (!atomic_dec_return(&->refcnt)) { * remove_ptr(p); * kfree_rcu((p, rcu); * } * * atomic_inc_not_zero() is implemented with a try_cmpxchg() loop which has * O(N^2) behaviour under contention with N concurrent operations. * * rcuref uses atomic_add_negative_relaxed() for the fast path, which scales * better under contention. * * Why not refcount? * ================= * * In principle it should be possible to make refcount use the rcuref * scheme, but the destruction race described below cannot be prevented * unless the protected object is RCU managed. * * Theory of operation * =================== * * rcuref uses an unsigned integer reference counter. As long as the * counter value is greater than or equal to RCUREF_ONEREF and not larger * than RCUREF_MAXREF the reference is alive: * * ONEREF MAXREF SATURATED RELEASED DEAD NOREF * 0 0x7FFFFFFF 0x8000000 0xA0000000 0xBFFFFFFF 0xC0000000 0xE0000000 0xFFFFFFFF * <---valid --------> <-------saturation zone-------> <-----dead zone-----> * * The get() and put() operations do unconditional increments and * decrements. The result is checked after the operation. This optimizes * for the fast path. * * If the reference count is saturated or dead, then the increments and * decrements are not harmful as the reference count still stays in the * respective zones and is always set back to STATURATED resp. DEAD. The * zones have room for 2^28 racing operations in each direction, which * makes it practically impossible to escape the zones. * * Once the last reference is dropped the reference count becomes * RCUREF_NOREF which forces rcuref_put() into the slowpath operation. The * slowpath then tries to set the reference count from RCUREF_NOREF to * RCUREF_DEAD via a cmpxchg(). This opens a small window where a * concurrent rcuref_get() can acquire the reference count and bring it * back to RCUREF_ONEREF or even drop the reference again and mark it DEAD. * * If the cmpxchg() succeeds then a concurrent rcuref_get() will result in * DEAD + 1, which is inside the dead zone. If that happens the reference * count is put back to DEAD. * * The actual race is possible due to the unconditional increment and * decrements in rcuref_get() and rcuref_put(): * * T1 T2 * get() put() * if (atomic_add_negative(-1, &ref->refcnt)) * succeeds-> atomic_cmpxchg(&ref->refcnt, NOREF, DEAD); * * atomic_add_negative(1, &ref->refcnt); <- Elevates refcount to DEAD + 1 * * As the result of T1's add is negative, the get() goes into the slow path * and observes refcnt being in the dead zone which makes the operation fail. * * Possible critical states: * * Context Counter References Operation * T1 0 1 init() * T2 1 2 get() * T1 0 1 put() * T2 -1 0 put() tries to mark dead * T1 0 1 get() * T2 0 1 put() mark dead fails * T1 -1 0 put() tries to mark dead * T1 DEAD 0 put() mark dead succeeds * T2 DEAD+1 0 get() fails and puts it back to DEAD * * Of course there are more complex scenarios, but the above illustrates * the working principle. The rest is left to the imagination of the * reader. * * Deconstruction race * =================== * * The release operation must be protected by prohibiting a grace period in * order to prevent a possible use after free: * * T1 T2 * put() get() * // ref->refcnt = ONEREF * if (!atomic_add_negative(-1, &ref->refcnt)) * return false; <- Not taken * * // ref->refcnt == NOREF * --> preemption * // Elevates ref->refcnt to ONEREF * if (!atomic_add_negative(1, &ref->refcnt)) * return true; <- taken * * if (put(&p->ref)) { <-- Succeeds * remove_pointer(p); * kfree_rcu(p, rcu); * } * * RCU grace period ends, object is freed * * atomic_cmpxchg(&ref->refcnt, NOREF, DEAD); <- UAF * * This is prevented by disabling preemption around the put() operation as * that's in most kernel configurations cheaper than a rcu_read_lock() / * rcu_read_unlock() pair and in many cases even a NOOP. In any case it * prevents the grace period which keeps the object alive until all put() * operations complete. * * Saturation protection * ===================== * * The reference count has a saturation limit RCUREF_MAXREF (INT_MAX). * Once this is exceedded the reference count becomes stale by setting it * to RCUREF_SATURATED, which will cause a memory leak, but it prevents * wrap arounds which obviously cause worse problems than a memory * leak. When saturation is reached a warning is emitted. * * Race conditions * =============== * * All reference count increment/decrement operations are unconditional and * only verified after the fact. This optimizes for the good case and takes * the occasional race vs. a dead or already saturated refcount into * account. The saturation and dead zones are large enough to accomodate * for that. * * Memory ordering * =============== * * Memory ordering rules are slightly relaxed wrt regular atomic_t functions * and provide only what is strictly required for refcounts. * * The increments are fully relaxed; these will not provide ordering. The * rationale is that whatever is used to obtain the object to increase the * reference count on will provide the ordering. For locked data * structures, its the lock acquire, for RCU/lockless data structures its * the dependent load. * * rcuref_get() provides a control dependency ordering future stores which * ensures that the object is not modified when acquiring a reference * fails. * * rcuref_put() provides release order, i.e. all prior loads and stores * will be issued before. It also provides a control dependency ordering * against the subsequent destruction of the object. * * If rcuref_put() successfully dropped the last reference and marked the * object DEAD it also provides acquire ordering. */ #include <linux/export.h> #include <linux/rcuref.h> /** * rcuref_get_slowpath - Slowpath of rcuref_get() * @ref: Pointer to the reference count * * Invoked when the reference count is outside of the valid zone. * * Return: * False if the reference count was already marked dead * * True if the reference count is saturated, which prevents the * object from being deconstructed ever. */ bool rcuref_get_slowpath(rcuref_t *ref) { unsigned int cnt = atomic_read(&ref->refcnt); /* * If the reference count was already marked dead, undo the * increment so it stays in the middle of the dead zone and return * fail. */ if (cnt >= RCUREF_RELEASED) { atomic_set(&ref->refcnt, RCUREF_DEAD); return false; } /* * If it was saturated, warn and mark it so. In case the increment * was already on a saturated value restore the saturation * marker. This keeps it in the middle of the saturation zone and * prevents the reference count from overflowing. This leaks the * object memory, but prevents the obvious reference count overflow * damage. */ if (WARN_ONCE(cnt > RCUREF_MAXREF, "rcuref saturated - leaking memory")) atomic_set(&ref->refcnt, RCUREF_SATURATED); return true; } 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. * * Return: * True if this was the last reference with no future references * possible. This signals the caller that it can safely schedule the * object, which is protected by the reference counter, for * deconstruction. * * False if there are still active references or the put() raced * with a concurrent get()/put() pair. Caller is not allowed to * deconstruct the protected object. */ bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt) { /* Did this drop the last reference? */ if (likely(cnt == RCUREF_NOREF)) { /* * Carefully try to set the reference count to RCUREF_DEAD. * * This can fail if a concurrent get() operation has * elevated it again or the corresponding put() even marked * it dead already. Both are valid situations and do not * require a retry. If this fails the caller is not * allowed to deconstruct the object. */ if (!atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD)) return false; /* * The caller can safely schedule the object for * deconstruction. Provide acquire ordering. */ smp_acquire__after_ctrl_dep(); return true; } /* * If the reference count was already in the dead zone, then this * put() operation is imbalanced. Warn, put the reference count back to * DEAD and tell the caller to not deconstruct the object. */ if (WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")) { atomic_set(&ref->refcnt, RCUREF_DEAD); return false; } /* * This is a put() operation on a saturated refcount. Restore the * mean saturation value and tell the caller to not deconstruct the * object. */ if (cnt > RCUREF_MAXREF) atomic_set(&ref->refcnt, RCUREF_SATURATED); return false; } EXPORT_SYMBOL_GPL(rcuref_put_slowpath);
