summaryrefslogtreecommitdiff
path: root/lib/rcuref.c
blob: 5ec00a4a64d11cadcef29e953d1b388cc41dfcc7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
// 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
 *
 * 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 = atomic_read(&ref->refcnt);

	/* 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_cmpxchg_release(&ref->refcnt, RCUREF_NOREF, RCUREF_DEAD) != RCUREF_NOREF)
			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);