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(). The rcu_access_pointer() primitive
 126                 does not require an enclosing read-side critical section,
 127                 and also omits the smp_read_barrier_depends() included in
 128                 rcu_dereference(), which in turn should provide a small
 129                 performance gain in some CPUs (e.g., the DEC Alpha).
 130
 131         o       The comparison is against a pointer that references memory
 132                 that was initialized "a long time ago."  The reason
 133                 this is safe is that even if misordering occurs, the
 134                 misordering will not affect the accesses that follow
 135                 the comparison.  So exactly how long ago is "a long
 136                 time ago"?  Here are some possibilities:
 137
 138                 o       Compile time.
 139
 140                 o       Boot time.
 141
 142                 o       Module-init time for module code.
 143
 144                 o       Prior to kthread creation for kthread code.
 145
 146                 o       During some prior acquisition of the lock that
 147                         we now hold.
 148
 149                 o       Before mod_timer() time for a timer handler.
 150
 151                 There are many other possibilities involving the Linux
 152                 kernel's wide array of primitives that cause code to
 153                 be invoked at a later time.
 154
 155         o       The pointer being compared against also came from
 156                 rcu_dereference().  In this case, both pointers depend
 157                 on one rcu_dereference() or another, so you get proper
 158                 ordering either way.
 159
 160                 That said, this situation can make certain RCU usage
 161                 bugs more likely to happen.  Which can be a good thing,
 162                 at least if they happen during testing.  An example
 163                 of such an RCU usage bug is shown in the section titled
 164                 "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
 165
 166         o       All of the accesses following the comparison are stores,
 167                 so that a control dependency preserves the needed ordering.
 168                 That said, it is easy to get control dependencies wrong.
 169                 Please see the "CONTROL DEPENDENCIES" section of
 170                 Documentation/memory-barriers.txt for more details.
 171
 172         o       The pointers are not equal -and- the compiler does
 173                 not have enough information to deduce the value of the
 174                 pointer.  Note that the volatile cast in rcu_dereference()
 175                 will normally prevent the compiler from knowing too much.
 176
 177                 However, please note that if the compiler knows that the
 178                 pointer takes on only one of two values, a not-equal
 179                 comparison will provide exactly the information that the
 180                 compiler needs to deduce the value of the pointer.
 181
 182 o       Disable any value-speculation optimizations that your compiler
 183         might provide, especially if you are making use of feedback-based
 184         optimizations that take data collected from prior runs.  Such
 185         value-speculation optimizations reorder operations by design.
 186
 187         There is one exception to this rule:  Value-speculation
 188         optimizations that leverage the branch-prediction hardware are
 189         safe on strongly ordered systems (such as x86), but not on weakly
 190         ordered systems (such as ARM or Power).  Choose your compiler
 191         command-line options wisely!
 192
 193
 194 EXAMPLE OF AMPLIFIED RCU-USAGE BUG
 195
 196 Because updaters can run concurrently with RCU readers, RCU readers can
 197 see stale and/or inconsistent values.  If RCU readers need fresh or
 198 consistent values, which they sometimes do, they need to take proper
 199 precautions.  To see this, consider the following code fragment:
 200
 201         struct foo {
 202                 int a;
 203                 int b;
 204                 int c;
 205         };
 206         struct foo *gp1;
 207         struct foo *gp2;
 208
 209         void updater(void)
 210         {
 211                 struct foo *p;
 212
 213                 p = kmalloc(...);
 214                 if (p == NULL)
 215                         deal_with_it();
 216                 p->a = 42;  /* Each field in its own cache line. */
 217                 p->b = 43;
 218                 p->c = 44;
 219                 rcu_assign_pointer(gp1, p);
 220                 p->b = 143;
 221                 p->c = 144;
 222                 rcu_assign_pointer(gp2, p);
 223         }
 224
 225         void reader(void)
 226         {
 227                 struct foo *p;
 228                 struct foo *q;
 229                 int r1, r2;
 230
 231                 p = rcu_dereference(gp2);
 232                 if (p == NULL)
 233                         return;
 234                 r1 = p->b;  /* Guaranteed to get 143. */
 235                 q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
 236                 if (p == q) {
 237                         /* The compiler decides that q->c is same as p->c. */
 238                         r2 = p->c; /* Could get 44 on weakly order system. */
 239                 }
 240                 do_something_with(r1, r2);
 241         }
 242
 243 You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
 244 but you should not be.  After all, the updater might have been invoked
 245 a second time between the time reader() loaded into "r1" and the time
 246 that it loaded into "r2".  The fact that this same result can occur due
 247 to some reordering from the compiler and CPUs is beside the point.
 248
 249 But suppose that the reader needs a consistent view?
 250
 251 Then one approach is to use locking, for example, as follows:
 252
 253         struct foo {
 254                 int a;
 255                 int b;
 256                 int c;
 257                 spinlock_t lock;
 258         };
 259         struct foo *gp1;
 260         struct foo *gp2;
 261
 262         void updater(void)
 263         {
 264                 struct foo *p;
 265
 266                 p = kmalloc(...);
 267                 if (p == NULL)
 268                         deal_with_it();
 269                 spin_lock(&p->lock);
 270                 p->a = 42;  /* Each field in its own cache line. */
 271                 p->b = 43;
 272                 p->c = 44;
 273                 spin_unlock(&p->lock);
 274                 rcu_assign_pointer(gp1, p);
 275                 spin_lock(&p->lock);
 276                 p->b = 143;
 277                 p->c = 144;
 278                 spin_unlock(&p->lock);
 279                 rcu_assign_pointer(gp2, p);
 280         }
 281
 282         void reader(void)
 283         {
 284                 struct foo *p;
 285                 struct foo *q;
 286                 int r1, r2;
 287
 288                 p = rcu_dereference(gp2);
 289                 if (p == NULL)
 290                         return;
 291                 spin_lock(&p->lock);
 292                 r1 = p->b;  /* Guaranteed to get 143. */
 293                 q = rcu_dereference(gp1);  /* Guaranteed non-NULL. */
 294                 if (p == q) {
 295                         /* The compiler decides that q->c is same as p->c. */
 296                         r2 = p->c; /* Locking guarantees r2 == 144. */
 297                 }
 298                 spin_unlock(&p->lock);
 299                 do_something_with(r1, r2);
 300         }
 301
 302 As always, use the right tool for the job!
 303
 304
 305 EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
 306
 307 If a pointer obtained from rcu_dereference() compares not-equal to some
 308 other pointer, the compiler normally has no clue what the value of the
 309 first pointer might be.  This lack of knowledge prevents the compiler
 310 from carrying out optimizations that otherwise might destroy the ordering
 311 guarantees that RCU depends on.  And the volatile cast in rcu_dereference()
 312 should prevent the compiler from guessing the value.
 313
 314 But without rcu_dereference(), the compiler knows more than you might
 315 expect.  Consider the following code fragment:
 316
 317         struct foo {
 318                 int a;
 319                 int b;
 320         };
 321         static struct foo variable1;
 322         static struct foo variable2;
 323         static struct foo *gp = &variable1;
 324
 325         void updater(void)
 326         {
 327                 initialize_foo(&variable2);
 328                 rcu_assign_pointer(gp, &variable2);
 329                 /*
 330                  * The above is the only store to gp in this translation unit,
 331                  * and the address of gp is not exported in any way.
 332                  */
 333         }
 334
 335         int reader(void)
 336         {
 337                 struct foo *p;
 338
 339                 p = gp;
 340                 barrier();
 341                 if (p == &variable1)
 342                         return p->a; /* Must be variable1.a. */
 343                 else
 344                         return p->b; /* Must be variable2.b. */
 345         }
 346
 347 Because the compiler can see all stores to "gp", it knows that the only
 348 possible values of "gp" are "variable1" on the one hand and "variable2"
 349 on the other.  The comparison in reader() therefore tells the compiler
 350 the exact value of "p" even in the not-equals case.  This allows the
 351 compiler to make the return values independent of the load from "gp",
 352 in turn destroying the ordering between this load and the loads of the
 353 return values.  This can result in "p->b" returning pre-initialization
 354 garbage values.
 355
 356 In short, rcu_dereference() is -not- optional when you are going to
 357 dereference the resulting pointer.