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atomics: Provide rcuref - scalable reference counting

atomic_t based reference counting, including refcount_t, uses atomic_inc_not_zero() for acquiring a reference. atomic_inc_not_zero() is implemented with a atomic_try_cmpxchg() loop. High contention of the reference count leads to retry loops and scales badly. There is nothing to improve on this implementation as the semantics have to be preserved. Provide rcuref as a scalable alternative solution which is suitable for RCU managed objects. Similar to refcount_t it comes with overflow and underflow detection and mitigation. rcuref treats the underlying atomic_t as an unsigned integer and partitions this space into zones: 0x00000000 - 0x7FFFFFFF valid zone (1 .. (INT_MAX + 1) references) 0x80000000 - 0xBFFFFFFF saturation zone 0xC0000000 - 0xFFFFFFFE dead zone 0xFFFFFFFF no reference rcuref_get() unconditionally increments the reference count with atomic_add_negative_relaxed(). rcuref_put() unconditionally decrements the reference count with atomic_add_negative_release(). This unconditional increment avoids the inc_not_zero() problem, but requires a more complex implementation on the put() side when the count drops from 0 to -1. When this transition is detected then it is attempted to mark the reference count dead, by setting it to the midpoint of the dead zone with a single atomic_cmpxchg_release() operation. This operation can fail due to a concurrent rcuref_get() elevating the reference count from -1 to 0 again. If the unconditional increment in rcuref_get() hits a reference count which is marked dead (or saturated) it will detect it after the fact and bring back the reference count to the midpoint of the respective zone. The zones provide enough tolerance which makes it practically impossible to escape from a zone. The racy implementation of rcuref_put() requires to protect rcuref_put() against a grace period ending in order to prevent a subtle use after free. As RCU is the only mechanism which allows to protect against that, it is not possible to fully replace the atomic_inc_not_zero() based implementation of refcount_t with this scheme. The final drop is slightly more expensive than the atomic_dec_return() counterpart, but that's not the case which this is optimized for. The optimization is on the high frequeunt get()/put() pairs and their scalability. The performance of an uncontended rcuref_get()/put() pair where the put() is not dropping the last reference is still on par with the plain atomic operations, while at the same time providing overflow and underflow detection and mitigation. The performance of rcuref compared to plain atomic_inc_not_zero() and atomic_dec_return() based reference counting under contention: - Micro benchmark: All CPUs running a increment/decrement loop on an elevated reference count, which means the 0 to -1 transition never happens. The performance gain depends on microarchitecture and the number of CPUs and has been observed in the range of 1.3X to 4.7X - Conversion of dst_entry::__refcnt to rcuref and testing with the localhost memtier/memcached benchmark. That benchmark shows the reference count contention prominently. The performance gain depends on microarchitecture and the number of CPUs and has been observed in the range of 1.1X to 2.6X over the previous fix for the false sharing issue vs. struct dst_entry::__refcnt. When memtier is run over a real 1Gb network connection, there is a small gain on top of the false sharing fix. The two changes combined result in a 2%-5% total gain for that networked test.

Thomas Gleixner committed 3 years ago

Commit ee1ee6d1 parent e5ab9ef4 files · +443 −1Browse files

Unified · Split

4 files
  1. include/linux/rcuref.h +155 −0
  2. include/linux/types.h +6 −0
  3. lib/Makefile +1 −1
  4. lib/rcuref.c +281 −0
include/linux/rcuref.h+155−0
@@ -0,0 +1,155 @@
1+/* SPDX-License-Identifier: GPL-2.0-only */
2+#ifndef _LINUX_RCUREF_H
3+#define _LINUX_RCUREF_H
4+
5+#include <linux/atomic.h>
6+#include <linux/bug.h>
7+#include <linux/limits.h>
8+#include <linux/lockdep.h>
9+#include <linux/preempt.h>
10+#include <linux/rcupdate.h>
11+
12+#define RCUREF_ONEREF 0x00000000U
13+#define RCUREF_MAXREF 0x7FFFFFFFU
14+#define RCUREF_SATURATED 0xA0000000U
15+#define RCUREF_RELEASED 0xC0000000U
16+#define RCUREF_DEAD 0xE0000000U
17+#define RCUREF_NOREF 0xFFFFFFFFU
18+
19+/**
20+ * rcuref_init - Initialize a rcuref reference count with the given reference count
21+ * @ref: Pointer to the reference count
22+ * @cnt: The initial reference count typically '1'
23+ */
24+static inline void rcuref_init(rcuref_t *ref, unsigned int cnt)
25+{
26+ atomic_set(&ref->refcnt, cnt - 1);
27+}
28+
29+/**
30+ * rcuref_read - Read the number of held reference counts of a rcuref
31+ * @ref: Pointer to the reference count
32+ *
33+ * Return: The number of held references (0 ... N)
34+ */
35+static inline unsigned int rcuref_read(rcuref_t *ref)
36+{
37+ unsigned int c = atomic_read(&ref->refcnt);
38+
39+ /* Return 0 if within the DEAD zone. */
40+ return c >= RCUREF_RELEASED ? 0 : c + 1;
41+}
42+
43+extern __must_check bool rcuref_get_slowpath(rcuref_t *ref);
44+
45+/**
46+ * rcuref_get - Acquire one reference on a rcuref reference count
47+ * @ref: Pointer to the reference count
48+ *
49+ * Similar to atomic_inc_not_zero() but saturates at RCUREF_MAXREF.
50+ *
51+ * Provides no memory ordering, it is assumed the caller has guaranteed the
52+ * object memory to be stable (RCU, etc.). It does provide a control dependency
53+ * and thereby orders future stores. See documentation in lib/rcuref.c
54+ *
55+ * Return:
56+ * False if the attempt to acquire a reference failed. This happens
57+ * when the last reference has been put already
58+ *
59+ * True if a reference was successfully acquired
60+ */
61+static inline __must_check bool rcuref_get(rcuref_t *ref)
62+{
63+ /*
64+ * Unconditionally increase the reference count. The saturation and
65+ * dead zones provide enough tolerance for this.
66+ */
67+ if (likely(!atomic_add_negative_relaxed(1, &ref->refcnt)))
68+ return true;
69+
70+ /* Handle the cases inside the saturation and dead zones */
71+ return rcuref_get_slowpath(ref);
72+}
73+
74+extern __must_check bool rcuref_put_slowpath(rcuref_t *ref);
75+
76+/*
77+ * Internal helper. Do not invoke directly.
78+ */
79+static __always_inline __must_check bool __rcuref_put(rcuref_t *ref)
80+{
81+ RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(),
82+ "suspicious rcuref_put_rcusafe() usage");
83+ /*
84+ * Unconditionally decrease the reference count. The saturation and
85+ * dead zones provide enough tolerance for this.
86+ */
87+ if (likely(!atomic_add_negative_release(-1, &ref->refcnt)))
88+ return false;
89+
90+ /*
91+ * Handle the last reference drop and cases inside the saturation
92+ * and dead zones.
93+ */
94+ return rcuref_put_slowpath(ref);
95+}
96+
97+/**
98+ * rcuref_put_rcusafe -- Release one reference for a rcuref reference count RCU safe
99+ * @ref: Pointer to the reference count
100+ *
101+ * Provides release memory ordering, such that prior loads and stores are done
102+ * before, and provides an acquire ordering on success such that free()
103+ * must come after.
104+ *
105+ * Can be invoked from contexts, which guarantee that no grace period can
106+ * happen which would free the object concurrently if the decrement drops
107+ * the last reference and the slowpath races against a concurrent get() and
108+ * put() pair. rcu_read_lock()'ed and atomic contexts qualify.
109+ *
110+ * Return:
111+ * True if this was the last reference with no future references
112+ * possible. This signals the caller that it can safely release the
113+ * object which is protected by the reference counter.
114+ *
115+ * False if there are still active references or the put() raced
116+ * with a concurrent get()/put() pair. Caller is not allowed to
117+ * release the protected object.
118+ */
119+static inline __must_check bool rcuref_put_rcusafe(rcuref_t *ref)
120+{
121+ return __rcuref_put(ref);
122+}
123+
124+/**
125+ * rcuref_put -- Release one reference for a rcuref reference count
126+ * @ref: Pointer to the reference count
127+ *
128+ * Can be invoked from any context.
129+ *
130+ * Provides release memory ordering, such that prior loads and stores are done
131+ * before, and provides an acquire ordering on success such that free()
132+ * must come after.
133+ *
134+ * Return:
135+ *
136+ * True if this was the last reference with no future references
137+ * possible. This signals the caller that it can safely schedule the
138+ * object, which is protected by the reference counter, for
139+ * deconstruction.
140+ *
141+ * False if there are still active references or the put() raced
142+ * with a concurrent get()/put() pair. Caller is not allowed to
143+ * deconstruct the protected object.
144+ */
145+static inline __must_check bool rcuref_put(rcuref_t *ref)
146+{
147+ bool released;
148+
149+ preempt_disable();
150+ released = __rcuref_put(ref);
151+ preempt_enable();
152+ return released;
153+}
154+
155+#endif
include/linux/types.h+6−0
@@ -175,6 +175,12 @@ typedef struct { · 174 unmodified lines
175175 } atomic64_t;
176176 #endif
177177
178+typedef struct {
179+ atomic_t refcnt;
180+} rcuref_t;
181+
182+#define RCUREF_INIT(i) { .refcnt = ATOMIC_INIT(i - 1) }
183+
178184 struct list_head {
179185 struct list_head *next, *prev;
180186 };
lib/Makefile+1−1
@@ -47,7 +47,7 @@ obj-y += bcd.o sort.o parser.o debug_locks.o random32.o \ · 46 unmodified lines
4747 list_sort.o uuid.o iov_iter.o clz_ctz.o \
4848 bsearch.o find_bit.o llist.o memweight.o kfifo.o \
4949 percpu-refcount.o rhashtable.o base64.o \
50− once.o refcount.o usercopy.o errseq.o bucket_locks.o \
50+ once.o refcount.o rcuref.o usercopy.o errseq.o bucket_locks.o \
5151 generic-radix-tree.o
5252 obj-$(CONFIG_STRING_SELFTEST) += test_string.o
5353 obj-y += string_helpers.o
lib/rcuref.c+281−0
@@ -0,0 +1,281 @@
1+// SPDX-License-Identifier: GPL-2.0-only
2+
3+/*
4+ * rcuref - A scalable reference count implementation for RCU managed objects
5+ *
6+ * rcuref is provided to replace open coded reference count implementations
7+ * based on atomic_t. It protects explicitely RCU managed objects which can
8+ * be visible even after the last reference has been dropped and the object
9+ * is heading towards destruction.
10+ *
11+ * A common usage pattern is:
12+ *
13+ * get()
14+ * rcu_read_lock();
15+ * p = get_ptr();
16+ * if (p && !atomic_inc_not_zero(&p->refcnt))
17+ * p = NULL;
18+ * rcu_read_unlock();
19+ * return p;
20+ *
21+ * put()
22+ * if (!atomic_dec_return(&->refcnt)) {
23+ * remove_ptr(p);
24+ * kfree_rcu((p, rcu);
25+ * }
26+ *
27+ * atomic_inc_not_zero() is implemented with a try_cmpxchg() loop which has
28+ * O(N^2) behaviour under contention with N concurrent operations.
29+ *
30+ * rcuref uses atomic_add_negative_relaxed() for the fast path, which scales
31+ * better under contention.
32+ *
33+ * Why not refcount?
34+ * =================
35+ *
36+ * In principle it should be possible to make refcount use the rcuref
37+ * scheme, but the destruction race described below cannot be prevented
38+ * unless the protected object is RCU managed.
39+ *
40+ * Theory of operation
41+ * ===================
42+ *
43+ * rcuref uses an unsigned integer reference counter. As long as the
44+ * counter value is greater than or equal to RCUREF_ONEREF and not larger
45+ * than RCUREF_MAXREF the reference is alive:
46+ *
47+ * ONEREF MAXREF SATURATED RELEASED DEAD NOREF
48+ * 0 0x7FFFFFFF 0x8000000 0xA0000000 0xBFFFFFFF 0xC0000000 0xE0000000 0xFFFFFFFF
49+ * <---valid --------> <-------saturation zone-------> <-----dead zone----->
50+ *
51+ * The get() and put() operations do unconditional increments and
52+ * decrements. The result is checked after the operation. This optimizes
53+ * for the fast path.
54+ *
55+ * If the reference count is saturated or dead, then the increments and
56+ * decrements are not harmful as the reference count still stays in the
57+ * respective zones and is always set back to STATURATED resp. DEAD. The
58+ * zones have room for 2^28 racing operations in each direction, which
59+ * makes it practically impossible to escape the zones.
60+ *
61+ * Once the last reference is dropped the reference count becomes
62+ * RCUREF_NOREF which forces rcuref_put() into the slowpath operation. The
63+ * slowpath then tries to set the reference count from RCUREF_NOREF to
64+ * RCUREF_DEAD via a cmpxchg(). This opens a small window where a
65+ * concurrent rcuref_get() can acquire the reference count and bring it
66+ * back to RCUREF_ONEREF or even drop the reference again and mark it DEAD.
67+ *
68+ * If the cmpxchg() succeeds then a concurrent rcuref_get() will result in
69+ * DEAD + 1, which is inside the dead zone. If that happens the reference
70+ * count is put back to DEAD.
71+ *
72+ * The actual race is possible due to the unconditional increment and
73+ * decrements in rcuref_get() and rcuref_put():
74+ *
75+ * T1 T2
76+ * get() put()
77+ * if (atomic_add_negative(-1, &ref->refcnt))
78+ * succeeds-> atomic_cmpxchg(&ref->refcnt, NOREF, DEAD);
79+ *
80+ * atomic_add_negative(1, &ref->refcnt); <- Elevates refcount to DEAD + 1
81+ *
82+ * As the result of T1's add is negative, the get() goes into the slow path
83+ * and observes refcnt being in the dead zone which makes the operation fail.
84+ *
85+ * Possible critical states:
86+ *
87+ * Context Counter References Operation
88+ * T1 0 1 init()
89+ * T2 1 2 get()
90+ * T1 0 1 put()
91+ * T2 -1 0 put() tries to mark dead
92+ * T1 0 1 get()
93+ * T2 0 1 put() mark dead fails
94+ * T1 -1 0 put() tries to mark dead
95+ * T1 DEAD 0 put() mark dead succeeds
96+ * T2 DEAD+1 0 get() fails and puts it back to DEAD
97+ *
98+ * Of course there are more complex scenarios, but the above illustrates
99+ * the working principle. The rest is left to the imagination of the
100+ * reader.
101+ *
102+ * Deconstruction race
103+ * ===================
104+ *
105+ * The release operation must be protected by prohibiting a grace period in
106+ * order to prevent a possible use after free:
107+ *
108+ * T1 T2
109+ * put() get()
110+ * // ref->refcnt = ONEREF
111+ * if (!atomic_add_negative(-1, &ref->refcnt))
112+ * return false; <- Not taken
113+ *
114+ * // ref->refcnt == NOREF
115+ * --> preemption
116+ * // Elevates ref->refcnt to ONEREF
117+ * if (!atomic_add_negative(1, &ref->refcnt))
118+ * return true; <- taken
119+ *
120+ * if (put(&p->ref)) { <-- Succeeds
121+ * remove_pointer(p);
122+ * kfree_rcu(p, rcu);
123+ * }
124+ *
125+ * RCU grace period ends, object is freed
126+ *
127+ * atomic_cmpxchg(&ref->refcnt, NOREF, DEAD); <- UAF
128+ *
129+ * This is prevented by disabling preemption around the put() operation as
130+ * that's in most kernel configurations cheaper than a rcu_read_lock() /
131+ * rcu_read_unlock() pair and in many cases even a NOOP. In any case it
132+ * prevents the grace period which keeps the object alive until all put()
133+ * operations complete.
134+ *
135+ * Saturation protection
136+ * =====================
137+ *
138+ * The reference count has a saturation limit RCUREF_MAXREF (INT_MAX).
139+ * Once this is exceedded the reference count becomes stale by setting it
140+ * to RCUREF_SATURATED, which will cause a memory leak, but it prevents
141+ * wrap arounds which obviously cause worse problems than a memory
142+ * leak. When saturation is reached a warning is emitted.
143+ *
144+ * Race conditions
145+ * ===============
146+ *
147+ * All reference count increment/decrement operations are unconditional and
148+ * only verified after the fact. This optimizes for the good case and takes
149+ * the occasional race vs. a dead or already saturated refcount into
150+ * account. The saturation and dead zones are large enough to accomodate
151+ * for that.
152+ *
153+ * Memory ordering
154+ * ===============
155+ *
156+ * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
157+ * and provide only what is strictly required for refcounts.
158+ *
159+ * The increments are fully relaxed; these will not provide ordering. The
160+ * rationale is that whatever is used to obtain the object to increase the
161+ * reference count on will provide the ordering. For locked data
162+ * structures, its the lock acquire, for RCU/lockless data structures its
163+ * the dependent load.
164+ *
165+ * rcuref_get() provides a control dependency ordering future stores which
166+ * ensures that the object is not modified when acquiring a reference
167+ * fails.
168+ *
169+ * rcuref_put() provides release order, i.e. all prior loads and stores
170+ * will be issued before. It also provides a control dependency ordering
171+ * against the subsequent destruction of the object.
172+ *
173+ * If rcuref_put() successfully dropped the last reference and marked the
174+ * object DEAD it also provides acquire ordering.
175+ */
176+
177+#include <linux/export.h>
178+#include <linux/rcuref.h>
179+
180+/**
181+ * rcuref_get_slowpath - Slowpath of rcuref_get()
182+ * @ref: Pointer to the reference count
183+ *
184+ * Invoked when the reference count is outside of the valid zone.
185+ *
186+ * Return:
187+ * False if the reference count was already marked dead
188+ *
189+ * True if the reference count is saturated, which prevents the
190+ * object from being deconstructed ever.
191+ */
192+bool rcuref_get_slowpath(rcuref_t *ref)
193+{
194+ unsigned int cnt = atomic_read(&ref->refcnt);
195+
196+ /*
197+ * If the reference count was already marked dead, undo the
198+ * increment so it stays in the middle of the dead zone and return
199+ * fail.
200+ */
201+ if (cnt >= RCUREF_RELEASED) {
202+ atomic_set(&ref->refcnt, RCUREF_DEAD);
203+ return false;
204+ }
205+
206+ /*
207+ * If it was saturated, warn and mark it so. In case the increment
208+ * was already on a saturated value restore the saturation
209+ * marker. This keeps it in the middle of the saturation zone and
210+ * prevents the reference count from overflowing. This leaks the
211+ * object memory, but prevents the obvious reference count overflow
212+ * damage.
213+ */
214+ if (WARN_ONCE(cnt > RCUREF_MAXREF, "rcuref saturated - leaking memory"))
215+ atomic_set(&ref->refcnt, RCUREF_SATURATED);
216+ return true;
217+}
218+EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
219+
220+/**
221+ * rcuref_put_slowpath - Slowpath of __rcuref_put()
222+ * @ref: Pointer to the reference count
223+ *
224+ * Invoked when the reference count is outside of the valid zone.
225+ *
226+ * Return:
227+ * True if this was the last reference with no future references
228+ * possible. This signals the caller that it can safely schedule the
229+ * object, which is protected by the reference counter, for
230+ * deconstruction.
231+ *
232+ * False if there are still active references or the put() raced
233+ * with a concurrent get()/put() pair. Caller is not allowed to
234+ * deconstruct the protected object.
235+ */
236+bool rcuref_put_slowpath(rcuref_t *ref)
237+{
238+ unsigned int cnt = atomic_read(&ref->refcnt);
239+
240+ /* Did this drop the last reference? */
241+ if (likely(cnt == RCUREF_NOREF)) {
242+ /*
243+ * Carefully try to set the reference count to RCUREF_DEAD.
244+ *
245+ * This can fail if a concurrent get() operation has
246+ * elevated it again or the corresponding put() even marked
247+ * it dead already. Both are valid situations and do not
248+ * require a retry. If this fails the caller is not
249+ * allowed to deconstruct the object.
250+ */
251+ if (atomic_cmpxchg_release(&ref->refcnt, RCUREF_NOREF, RCUREF_DEAD) != RCUREF_NOREF)
252+ return false;
253+
254+ /*
255+ * The caller can safely schedule the object for
256+ * deconstruction. Provide acquire ordering.
257+ */
258+ smp_acquire__after_ctrl_dep();
259+ return true;
260+ }
261+
262+ /*
263+ * If the reference count was already in the dead zone, then this
264+ * put() operation is imbalanced. Warn, put the reference count back to
265+ * DEAD and tell the caller to not deconstruct the object.
266+ */
267+ if (WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")) {
268+ atomic_set(&ref->refcnt, RCUREF_DEAD);
269+ return false;
270+ }
271+
272+ /*
273+ * This is a put() operation on a saturated refcount. Restore the
274+ * mean saturation value and tell the caller to not deconstruct the
275+ * object.
276+ */
277+ if (cnt > RCUREF_MAXREF)
278+ atomic_set(&ref->refcnt, RCUREF_SATURATED);
279+ return false;
280+}
281+EXPORT_SYMBOL_GPL(rcuref_put_slowpath);

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