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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
2	
3	Most of the time, you can use values from rcu_dereference() or one of
4	the similar primitives without worries.  Dereferencing (prefix "*"),
5	field selection ("->"), assignment ("="), address-of ("&"), addition and
6	subtraction of constants, and casts all work quite naturally and safely.
7	
8	It is nevertheless possible to get into trouble with other operations.
9	Follow these rules to keep your RCU code working properly:
10	
11	o	You must use one of the rcu_dereference() family of primitives
12		to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
13		will complain.  Worse yet, your code can see random memory-corruption
14		bugs due to games that compilers and DEC Alpha can play.
15		Without one of the rcu_dereference() primitives, compilers
16		can reload the value, and won't your code have fun with two
17		different values for a single pointer!  Without rcu_dereference(),
18		DEC Alpha can load a pointer, dereference that pointer, and
19		return data preceding initialization that preceded the store of
20		the pointer.
21	
22		In addition, the volatile cast in rcu_dereference() prevents the
23		compiler from deducing the resulting pointer value.  Please see
24		the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
25		for an example where the compiler can in fact deduce the exact
26		value of the pointer, and thus cause misordering.
27	
28	o	Avoid cancellation when using the "+" and "-" infix arithmetic
29		operators.  For example, for a given variable "x", avoid
30		"(x-x)".  There are similar arithmetic pitfalls from other
31		arithmetic operators, such as "(x*0)", "(x/(x+1))" or "(x%1)".
32		The compiler is within its rights to substitute zero for all of
33		these expressions, so that subsequent accesses no longer depend
34		on the rcu_dereference(), again possibly resulting in bugs due
35		to misordering.
36	
37		Of course, if "p" is a pointer from rcu_dereference(), and "a"
38		and "b" are integers that happen to be equal, the expression
39		"p+a-b" is safe because its value still necessarily depends on
40		the rcu_dereference(), thus maintaining proper ordering.
41	
42	o	Avoid all-zero operands to the bitwise "&" operator, and
43		similarly avoid all-ones operands to the bitwise "|" operator.
44		If the compiler is able to deduce the value of such operands,
45		it is within its rights to substitute the corresponding constant
46		for the bitwise operation.  Once again, this causes subsequent
47		accesses to no longer depend on the rcu_dereference(), causing
48		bugs due to misordering.
49	
50		Please note that single-bit operands to bitwise "&" can also
51		be dangerous.  At this point, the compiler knows that the
52		resulting value can only take on one of two possible values.
53		Therefore, a very small amount of additional information will
54		allow the compiler to deduce the exact value, which again can
55		result in misordering.
56	
57	o	If you are using RCU to protect JITed functions, so that the
58		"()" function-invocation operator is applied to a value obtained
59		(directly or indirectly) from rcu_dereference(), you may need to
60		interact directly with the hardware to flush instruction caches.
61		This issue arises on some systems when a newly JITed function is
62		using the same memory that was used by an earlier JITed function.
63	
64	o	Do not use the results from the boolean "&&" and "||" when
65		dereferencing.	For example, the following (rather improbable)
66		code is buggy:
67	
68			int *p;
69			int *q;
70	
71			...
72	
73			p = rcu_dereference(gp)
74			q = &global_q;
75			q += p != &oom_p1 && p != &oom_p2;
76			r1 = *q;  /* BUGGY!!! */
77	
78		The reason this is buggy is that "&&" and "||" are often compiled
79		using branches.  While weak-memory machines such as ARM or PowerPC
80		do order stores after such branches, they can speculate loads,
81		which can result in misordering bugs.
82	
83	o	Do not use the results from relational operators ("==", "!=",
84		">", ">=", "<", or "<=") when dereferencing.  For example,
85		the following (quite strange) code is buggy:
86	
87			int *p;
88			int *q;
89	
90			...
91	
92			p = rcu_dereference(gp)
93			q = &global_q;
94			q += p > &oom_p;
95			r1 = *q;  /* BUGGY!!! */
96	
97		As before, the reason this is buggy is that relational operators
98		are often compiled using branches.  And as before, although
99		weak-memory machines such as ARM or PowerPC do order stores
100		after such branches, but can speculate loads, which can again
101		result in misordering bugs.
102	
103	o	Be very careful about comparing pointers obtained from
104		rcu_dereference() against non-NULL values.  As Linus Torvalds
105		explained, if the two pointers are equal, the compiler could
106		substitute the pointer you are comparing against for the pointer
107		obtained from rcu_dereference().  For example:
108	
109			p = rcu_dereference(gp);
110			if (p == &default_struct)
111				do_default(p->a);
112	
113		Because the compiler now knows that the value of "p" is exactly
114		the address of the variable "default_struct", it is free to
115		transform this code into the following:
116	
117			p = rcu_dereference(gp);
118			if (p == &default_struct)
119				do_default(default_struct.a);
120	
121		On ARM and Power hardware, the load from "default_struct.a"
122		can now be speculated, such that it might happen before the
123		rcu_dereference().  This could result in bugs due to misordering.
124	
125		However, comparisons are OK in the following cases:
126	
127		o	The comparison was against the NULL pointer.  If the
128			compiler knows that the pointer is NULL, you had better
129			not be dereferencing it anyway.  If the comparison is
130			non-equal, the compiler is none the wiser.  Therefore,
131			it is safe to compare pointers from rcu_dereference()
132			against NULL pointers.
133	
134		o	The pointer is never dereferenced after being compared.
135			Since there are no subsequent dereferences, the compiler
136			cannot use anything it learned from the comparison
137			to reorder the non-existent subsequent dereferences.
138			This sort of comparison occurs frequently when scanning
139			RCU-protected circular linked lists.
140	
141			Note that if checks for being within an RCU read-side
142			critical section are not required and the pointer is never
143			dereferenced, rcu_access_pointer() should be used in place
144			of rcu_dereference(). The rcu_access_pointer() primitive
145			does not require an enclosing read-side critical section,
146			and also omits the smp_read_barrier_depends() included in
147			rcu_dereference(), which in turn should provide a small
148			performance gain in some CPUs (e.g., the DEC Alpha).
149	
150		o	The comparison is against a pointer that references memory
151			that was initialized "a long time ago."  The reason
152			this is safe is that even if misordering occurs, the
153			misordering will not affect the accesses that follow
154			the comparison.  So exactly how long ago is "a long
155			time ago"?  Here are some possibilities:
156	
157			o	Compile time.
158	
159			o	Boot time.
160	
161			o	Module-init time for module code.
162	
163			o	Prior to kthread creation for kthread code.
164	
165			o	During some prior acquisition of the lock that
166				we now hold.
167	
168			o	Before mod_timer() time for a timer handler.
169	
170			There are many other possibilities involving the Linux
171			kernel's wide array of primitives that cause code to
172			be invoked at a later time.
173	
174		o	The pointer being compared against also came from
175			rcu_dereference().  In this case, both pointers depend
176			on one rcu_dereference() or another, so you get proper
177			ordering either way.
178	
179			That said, this situation can make certain RCU usage
180			bugs more likely to happen.  Which can be a good thing,
181			at least if they happen during testing.  An example
182			of such an RCU usage bug is shown in the section titled
183			"EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
184	
185		o	All of the accesses following the comparison are stores,
186			so that a control dependency preserves the needed ordering.
187			That said, it is easy to get control dependencies wrong.
188			Please see the "CONTROL DEPENDENCIES" section of
189			Documentation/memory-barriers.txt for more details.
190	
191		o	The pointers are not equal -and- the compiler does
192			not have enough information to deduce the value of the
193			pointer.  Note that the volatile cast in rcu_dereference()
194			will normally prevent the compiler from knowing too much.
195	
196			However, please note that if the compiler knows that the
197			pointer takes on only one of two values, a not-equal
198			comparison will provide exactly the information that the
199			compiler needs to deduce the value of the pointer.
200	
201	o	Disable any value-speculation optimizations that your compiler
202		might provide, especially if you are making use of feedback-based
203		optimizations that take data collected from prior runs.  Such
204		value-speculation optimizations reorder operations by design.
205	
206		There is one exception to this rule:  Value-speculation
207		optimizations that leverage the branch-prediction hardware are
208		safe on strongly ordered systems (such as x86), but not on weakly
209		ordered systems (such as ARM or Power).  Choose your compiler
210		command-line options wisely!
211	
212	
213	EXAMPLE OF AMPLIFIED RCU-USAGE BUG
214	
215	Because updaters can run concurrently with RCU readers, RCU readers can
216	see stale and/or inconsistent values.  If RCU readers need fresh or
217	consistent values, which they sometimes do, they need to take proper
218	precautions.  To see this, consider the following code fragment:
219	
220		struct foo {
221			int a;
222			int b;
223			int c;
224		};
225		struct foo *gp1;
226		struct foo *gp2;
227	
228		void updater(void)
229		{
230			struct foo *p;
231	
232			p = kmalloc(...);
233			if (p == NULL)
234				deal_with_it();
235			p->a = 42;  /* Each field in its own cache line. */
236			p->b = 43;
237			p->c = 44;
238			rcu_assign_pointer(gp1, p);
239			p->b = 143;
240			p->c = 144;
241			rcu_assign_pointer(gp2, p);
242		}
243	
244		void reader(void)
245		{
246			struct foo *p;
247			struct foo *q;
248			int r1, r2;
249	
250			p = rcu_dereference(gp2);
251			if (p == NULL)
252				return;
253			r1 = p->b;  /* Guaranteed to get 143. */
254			q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
255			if (p == q) {
256				/* The compiler decides that q->c is same as p->c. */
257				r2 = p->c; /* Could get 44 on weakly order system. */
258			}
259			do_something_with(r1, r2);
260		}
261	
262	You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
263	but you should not be.  After all, the updater might have been invoked
264	a second time between the time reader() loaded into "r1" and the time
265	that it loaded into "r2".  The fact that this same result can occur due
266	to some reordering from the compiler and CPUs is beside the point.
267	
268	But suppose that the reader needs a consistent view?
269	
270	Then one approach is to use locking, for example, as follows:
271	
272		struct foo {
273			int a;
274			int b;
275			int c;
276			spinlock_t lock;
277		};
278		struct foo *gp1;
279		struct foo *gp2;
280	
281		void updater(void)
282		{
283			struct foo *p;
284	
285			p = kmalloc(...);
286			if (p == NULL)
287				deal_with_it();
288			spin_lock(&p->lock);
289			p->a = 42;  /* Each field in its own cache line. */
290			p->b = 43;
291			p->c = 44;
292			spin_unlock(&p->lock);
293			rcu_assign_pointer(gp1, p);
294			spin_lock(&p->lock);
295			p->b = 143;
296			p->c = 144;
297			spin_unlock(&p->lock);
298			rcu_assign_pointer(gp2, p);
299		}
300	
301		void reader(void)
302		{
303			struct foo *p;
304			struct foo *q;
305			int r1, r2;
306	
307			p = rcu_dereference(gp2);
308			if (p == NULL)
309				return;
310			spin_lock(&p->lock);
311			r1 = p->b;  /* Guaranteed to get 143. */
312			q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
313			if (p == q) {
314				/* The compiler decides that q->c is same as p->c. */
315				r2 = p->c; /* Locking guarantees r2 == 144. */
316			}
317			spin_unlock(&p->lock);
318			do_something_with(r1, r2);
319		}
320	
321	As always, use the right tool for the job!
322	
323	
324	EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
325	
326	If a pointer obtained from rcu_dereference() compares not-equal to some
327	other pointer, the compiler normally has no clue what the value of the
328	first pointer might be.  This lack of knowledge prevents the compiler
329	from carrying out optimizations that otherwise might destroy the ordering
330	guarantees that RCU depends on.  And the volatile cast in rcu_dereference()
331	should prevent the compiler from guessing the value.
332	
333	But without rcu_dereference(), the compiler knows more than you might
334	expect.  Consider the following code fragment:
335	
336		struct foo {
337			int a;
338			int b;
339		};
340		static struct foo variable1;
341		static struct foo variable2;
342		static struct foo *gp = &variable1;
343	
344		void updater(void)
345		{
346			initialize_foo(&variable2);
347			rcu_assign_pointer(gp, &variable2);
348			/*
349			 * The above is the only store to gp in this translation unit,
350			 * and the address of gp is not exported in any way.
351			 */
352		}
353	
354		int reader(void)
355		{
356			struct foo *p;
357	
358			p = gp;
359			barrier();
360			if (p == &variable1)
361				return p->a; /* Must be variable1.a. */
362			else
363				return p->b; /* Must be variable2.b. */
364		}
365	
366	Because the compiler can see all stores to "gp", it knows that the only
367	possible values of "gp" are "variable1" on the one hand and "variable2"
368	on the other.  The comparison in reader() therefore tells the compiler
369	the exact value of "p" even in the not-equals case.  This allows the
370	compiler to make the return values independent of the load from "gp",
371	in turn destroying the ordering between this load and the loads of the
372	return values.  This can result in "p->b" returning pre-initialization
373	garbage values.
374	
375	In short, rcu_dereference() is -not- optional when you are going to
376	dereference the resulting pointer.
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