Documentation/RCU/rcu_dereference.txt

   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       You are only permitted to use rcu_dereference on pointer values.
  29         The compiler simply knows too much about integral values to
  30         trust it to carry dependencies through integer operations.
  31         There are a very few exceptions, namely that you can temporarily
  32         cast the pointer to uintptr_t in order to:
  33
  34         o       Set bits and clear bits down in the must-be-zero low-order
  35                 bits of that pointer.  This clearly means that the pointer
  36                 must have alignment constraints, for example, this does
  37                 -not- work in general for char* pointers.
  38
  39         o       XOR bits to translate pointers, as is done in some
  40                 classic buddy-allocator algorithms.
  41
  42         It is important to cast the value back to pointer before
  43         doing much of anything else with it.
  44
  45 o       Avoid cancellation when using the "+" and "-" infix arithmetic
  46         operators.  For example, for a given variable "x", avoid
  47         "(x-(uintptr_t)x)" for char* pointers.  The compiler is within its
  48         rights to substitute zero for this sort of expression, so that
  49         subsequent accesses no longer depend on the rcu_dereference(),
  50         again possibly resulting in bugs due to misordering.
  51
  52         Of course, if "p" is a pointer from rcu_dereference(), and "a"
  53         and "b" are integers that happen to be equal, the expression
  54         "p+a-b" is safe because its value still necessarily depends on
  55         the rcu_dereference(), thus maintaining proper ordering.
  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 relational operators ("==", "!=",
  65         ">", ">=", "<", or "<=") when dereferencing.  For example,
  66         the following (quite strange) 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_p;
  76                 r1 = *q;  /* BUGGY!!! */
  77
  78         As before, the reason this is buggy is that relational operators
  79         are often compiled using branches.  And as before, although
  80         weak-memory machines such as ARM or PowerPC do order stores
  81         after such branches, but can speculate loads, which can again
  82         result in misordering bugs.
  83
  84 o       Be very careful about comparing pointers obtained from
  85         rcu_dereference() against non-NULL values.  As Linus Torvalds
  86         explained, if the two pointers are equal, the compiler could
  87         substitute the pointer you are comparing against for the pointer
  88         obtained from rcu_dereference().  For example:
  89
  90                 p = rcu_dereference(gp);
  91                 if (p == &default_struct)
  92                         do_default(p->a);
  93
  94         Because the compiler now knows that the value of "p" is exactly
  95         the address of the variable "default_struct", it is free to
  96         transform this code into the following:
  97
  98                 p = rcu_dereference(gp);
  99                 if (p == &default_struct)
 100                         do_default(default_struct.a);
 101
 102         On ARM and Power hardware, the load from "default_struct.a"
 103         can now be speculated, such that it might happen before the
 104         rcu_dereference().  This could result in bugs due to misordering.
 105
 106         However, comparisons are OK in the following cases:
 107
 108         o       The comparison was against the NULL pointer.  If the
 109                 compiler knows that the pointer is NULL, you had better
 110                 not be dereferencing it anyway.  If the comparison is
 111                 non-equal, the compiler is none the wiser.  Therefore,
 112                 it is safe to compare pointers from rcu_dereference()
 113                 against NULL pointers.
 114
 115         o       The pointer is never dereferenced after being compared.
 116                 Since there are no subsequent dereferences, the compiler
 117                 cannot use anything it learned from the comparison
 118                 to reorder the non-existent subsequent dereferences.
 119                 This sort of comparison occurs frequently when scanning
 120                 RCU-protected circular linked lists.
 121
 122                 Note that if checks for being within an RCU read-side
 123                 critical section are not required and the pointer is never
 124                 dereferenced, rcu_access_pointer() should be used in place
 125                 of rcu_dereference().
 126
 127         o       The comparison is against a pointer that references memory
 128                 that was initialized "a long time ago."  The reason
 129                 this is safe is that even if misordering occurs, the
 130                 misordering will not affect the accesses that follow
 131                 the comparison.  So exactly how long ago is "a long
 132                 time ago"?  Here are some possibilities:
 133
 134                 o       Compile time.
 135
 136                 o       Boot time.
 137
 138                 o       Module-init time for module code.
 139
 140                 o       Prior to kthread creation for kthread code.
 141
 142                 o       During some prior acquisition of the lock that
 143                         we now hold.
 144
 145                 o       Before mod_timer() time for a timer handler.
 146
 147                 There are many other possibilities involving the Linux
 148                 kernel's wide array of primitives that cause code to
 149                 be invoked at a later time.
 150
 151         o       The pointer being compared against also came from
 152                 rcu_dereference().  In this case, both pointers depend
 153                 on one rcu_dereference() or another, so you get proper
 154                 ordering either way.
 155
 156                 That said, this situation can make certain RCU usage
 157                 bugs more likely to happen.  Which can be a good thing,
 158                 at least if they happen during testing.  An example
 159                 of such an RCU usage bug is shown in the section titled
 160                 "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
 161
 162         o       All of the accesses following the comparison are stores,
 163                 so that a control dependency preserves the needed ordering.
 164                 That said, it is easy to get control dependencies wrong.
 165                 Please see the "CONTROL DEPENDENCIES" section of
 166                 Documentation/memory-barriers.txt for more details.
 167
 168         o       The pointers are not equal -and- the compiler does
 169                 not have enough information to deduce the value of the
 170                 pointer.  Note that the volatile cast in rcu_dereference()
 171                 will normally prevent the compiler from knowing too much.
 172
 173                 However, please note that if the compiler knows that the
 174                 pointer takes on only one of two values, a not-equal
 175                 comparison will provide exactly the information that the
 176                 compiler needs to deduce the value of the pointer.
 177
 178 o       Disable any value-speculation optimizations that your compiler
 179         might provide, especially if you are making use of feedback-based
 180         optimizations that take data collected from prior runs.  Such
 181         value-speculation optimizations reorder operations by design.
 182
 183         There is one exception to this rule:  Value-speculation
 184         optimizations that leverage the branch-prediction hardware are
 185         safe on strongly ordered systems (such as x86), but not on weakly
 186         ordered systems (such as ARM or Power).  Choose your compiler
 187         command-line options wisely!
 188
 189
 190 EXAMPLE OF AMPLIFIED RCU-USAGE BUG
 191
 192 Because updaters can run concurrently with RCU readers, RCU readers can
 193 see stale and/or inconsistent values.  If RCU readers need fresh or
 194 consistent values, which they sometimes do, they need to take proper
 195 precautions.  To see this, consider the following code fragment:
 196
 197         struct foo {
 198                 int a;
 199                 int b;
 200                 int c;
 201         };
 202         struct foo *gp1;
 203         struct foo *gp2;
 204
 205         void updater(void)
 206         {
 207                 struct foo *p;
 208
 209                 p = kmalloc(...);
 210                 if (p == NULL)
 211                         deal_with_it();
 212                 p->a = 42;  /* Each field in its own cache line. */
 213                 p->b = 43;
 214                 p->c = 44;
 215                 rcu_assign_pointer(gp1, p);
 216                 p->b = 143;
 217                 p->c = 144;
 218                 rcu_assign_pointer(gp2, p);
 219         }
 220
 221         void reader(void)
 222         {
 223                 struct foo *p;
 224                 struct foo *q;
 225                 int r1, r2;
 226
 227                 p = rcu_dereference(gp2);
 228                 if (p == NULL)
 229                         return;
 230                 r1 = p->b;  /* Guaranteed to get 143. */
 231                 q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
 232                 if (p == q) {
 233                         /* The compiler decides that q->c is same as p->c. */
 234                         r2 = p->c; /* Could get 44 on weakly order system. */
 235                 }
 236                 do_something_with(r1, r2);
 237         }
 238
 239 You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
 240 but you should not be.  After all, the updater might have been invoked
 241 a second time between the time reader() loaded into "r1" and the time
 242 that it loaded into "r2".  The fact that this same result can occur due
 243 to some reordering from the compiler and CPUs is beside the point.
 244
 245 But suppose that the reader needs a consistent view?
 246
 247 Then one approach is to use locking, for example, as follows:
 248
 249         struct foo {
 250                 int a;
 251                 int b;
 252                 int c;
 253                 spinlock_t lock;
 254         };
 255         struct foo *gp1;
 256         struct foo *gp2;
 257
 258         void updater(void)
 259         {
 260                 struct foo *p;
 261
 262                 p = kmalloc(...);
 263                 if (p == NULL)
 264                         deal_with_it();
 265                 spin_lock(&p->lock);
 266                 p->a = 42;  /* Each field in its own cache line. */
 267                 p->b = 43;
 268                 p->c = 44;
 269                 spin_unlock(&p->lock);
 270                 rcu_assign_pointer(gp1, p);
 271                 spin_lock(&p->lock);
 272                 p->b = 143;
 273                 p->c = 144;
 274                 spin_unlock(&p->lock);
 275                 rcu_assign_pointer(gp2, p);
 276         }
 277
 278         void reader(void)
 279         {
 280                 struct foo *p;
 281                 struct foo *q;
 282                 int r1, r2;
 283
 284                 p = rcu_dereference(gp2);
 285                 if (p == NULL)
 286                         return;
 287                 spin_lock(&p->lock);
 288                 r1 = p->b;  /* Guaranteed to get 143. */
 289                 q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
 290                 if (p == q) {
 291                         /* The compiler decides that q->c is same as p->c. */
 292                         r2 = p->c; /* Locking guarantees r2 == 144. */
 293                 }
 294                 spin_unlock(&p->lock);
 295                 do_something_with(r1, r2);
 296         }
 297
 298 As always, use the right tool for the job!
 299
 300
 301 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
 302
 303 If a pointer obtained from rcu_dereference() compares not-equal to some
 304 other pointer, the compiler normally has no clue what the value of the
 305 first pointer might be.  This lack of knowledge prevents the compiler
 306 from carrying out optimizations that otherwise might destroy the ordering
 307 guarantees that RCU depends on.  And the volatile cast in rcu_dereference()
 308 should prevent the compiler from guessing the value.
 309
 310 But without rcu_dereference(), the compiler knows more than you might
 311 expect.  Consider the following code fragment:
 312
 313         struct foo {
 314                 int a;
 315                 int b;
 316         };
 317         static struct foo variable1;
 318         static struct foo variable2;
 319         static struct foo *gp = &variable1;
 320
 321         void updater(void)
 322         {
 323                 initialize_foo(&variable2);
 324                 rcu_assign_pointer(gp, &variable2);
 325                 /*
 326                  * The above is the only store to gp in this translation unit,
 327                  * and the address of gp is not exported in any way.
 328                  */
 329         }
 330
 331         int reader(void)
 332         {
 333                 struct foo *p;
 334
 335                 p = gp;
 336                 barrier();
 337                 if (p == &variable1)
 338                         return p->a; /* Must be variable1.a. */
 339                 else
 340                         return p->b; /* Must be variable2.b. */
 341         }
 342
 343 Because the compiler can see all stores to "gp", it knows that the only
 344 possible values of "gp" are "variable1" on the one hand and "variable2"
 345 on the other.  The comparison in reader() therefore tells the compiler
 346 the exact value of "p" even in the not-equals case.  This allows the
 347 compiler to make the return values independent of the load from "gp",
 348 in turn destroying the ordering between this load and the loads of the
 349 return values.  This can result in "p->b" returning pre-initialization
 350 garbage values.
 351
 352 In short, rcu_dereference() is -not- optional when you are going to
 353 dereference the resulting pointer.