[Feature]add MT2731_MP2_MR2_SVN388 baseline version
Change-Id: Ief04314834b31e27effab435d3ca8ba33b499059
diff --git a/src/kernel/linux/v4.14/Documentation/RCU/rcu_dereference.txt b/src/kernel/linux/v4.14/Documentation/RCU/rcu_dereference.txt
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+PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
+
+Most of the time, you can use values from rcu_dereference() or one of
+the similar primitives without worries. Dereferencing (prefix "*"),
+field selection ("->"), assignment ("="), address-of ("&"), addition and
+subtraction of constants, and casts all work quite naturally and safely.
+
+It is nevertheless possible to get into trouble with other operations.
+Follow these rules to keep your RCU code working properly:
+
+o You must use one of the rcu_dereference() family of primitives
+ to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
+ will complain. Worse yet, your code can see random memory-corruption
+ bugs due to games that compilers and DEC Alpha can play.
+ Without one of the rcu_dereference() primitives, compilers
+ can reload the value, and won't your code have fun with two
+ different values for a single pointer! Without rcu_dereference(),
+ DEC Alpha can load a pointer, dereference that pointer, and
+ return data preceding initialization that preceded the store of
+ the pointer.
+
+ In addition, the volatile cast in rcu_dereference() prevents the
+ compiler from deducing the resulting pointer value. Please see
+ the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
+ for an example where the compiler can in fact deduce the exact
+ value of the pointer, and thus cause misordering.
+
+o You are only permitted to use rcu_dereference on pointer values.
+ The compiler simply knows too much about integral values to
+ trust it to carry dependencies through integer operations.
+ There are a very few exceptions, namely that you can temporarily
+ cast the pointer to uintptr_t in order to:
+
+ o Set bits and clear bits down in the must-be-zero low-order
+ bits of that pointer. This clearly means that the pointer
+ must have alignment constraints, for example, this does
+ -not- work in general for char* pointers.
+
+ o XOR bits to translate pointers, as is done in some
+ classic buddy-allocator algorithms.
+
+ It is important to cast the value back to pointer before
+ doing much of anything else with it.
+
+o Avoid cancellation when using the "+" and "-" infix arithmetic
+ operators. For example, for a given variable "x", avoid
+ "(x-(uintptr_t)x)" for char* pointers. The compiler is within its
+ rights to substitute zero for this sort of expression, so that
+ subsequent accesses no longer depend on the rcu_dereference(),
+ again possibly resulting in bugs due to misordering.
+
+ Of course, if "p" is a pointer from rcu_dereference(), and "a"
+ and "b" are integers that happen to be equal, the expression
+ "p+a-b" is safe because its value still necessarily depends on
+ the rcu_dereference(), thus maintaining proper ordering.
+
+o If you are using RCU to protect JITed functions, so that the
+ "()" function-invocation operator is applied to a value obtained
+ (directly or indirectly) from rcu_dereference(), you may need to
+ interact directly with the hardware to flush instruction caches.
+ This issue arises on some systems when a newly JITed function is
+ using the same memory that was used by an earlier JITed function.
+
+o Do not use the results from relational operators ("==", "!=",
+ ">", ">=", "<", or "<=") when dereferencing. For example,
+ the following (quite strange) code is buggy:
+
+ int *p;
+ int *q;
+
+ ...
+
+ p = rcu_dereference(gp)
+ q = &global_q;
+ q += p > &oom_p;
+ r1 = *q; /* BUGGY!!! */
+
+ As before, the reason this is buggy is that relational operators
+ are often compiled using branches. And as before, although
+ weak-memory machines such as ARM or PowerPC do order stores
+ after such branches, but can speculate loads, which can again
+ result in misordering bugs.
+
+o Be very careful about comparing pointers obtained from
+ rcu_dereference() against non-NULL values. As Linus Torvalds
+ explained, if the two pointers are equal, the compiler could
+ substitute the pointer you are comparing against for the pointer
+ obtained from rcu_dereference(). For example:
+
+ p = rcu_dereference(gp);
+ if (p == &default_struct)
+ do_default(p->a);
+
+ Because the compiler now knows that the value of "p" is exactly
+ the address of the variable "default_struct", it is free to
+ transform this code into the following:
+
+ p = rcu_dereference(gp);
+ if (p == &default_struct)
+ do_default(default_struct.a);
+
+ On ARM and Power hardware, the load from "default_struct.a"
+ can now be speculated, such that it might happen before the
+ rcu_dereference(). This could result in bugs due to misordering.
+
+ However, comparisons are OK in the following cases:
+
+ o The comparison was against the NULL pointer. If the
+ compiler knows that the pointer is NULL, you had better
+ not be dereferencing it anyway. If the comparison is
+ non-equal, the compiler is none the wiser. Therefore,
+ it is safe to compare pointers from rcu_dereference()
+ against NULL pointers.
+
+ o The pointer is never dereferenced after being compared.
+ Since there are no subsequent dereferences, the compiler
+ cannot use anything it learned from the comparison
+ to reorder the non-existent subsequent dereferences.
+ This sort of comparison occurs frequently when scanning
+ RCU-protected circular linked lists.
+
+ Note that if checks for being within an RCU read-side
+ critical section are not required and the pointer is never
+ dereferenced, rcu_access_pointer() should be used in place
+ of rcu_dereference(). The rcu_access_pointer() primitive
+ does not require an enclosing read-side critical section,
+ and also omits the smp_read_barrier_depends() included in
+ rcu_dereference(), which in turn should provide a small
+ performance gain in some CPUs (e.g., the DEC Alpha).
+
+ o The comparison is against a pointer that references memory
+ that was initialized "a long time ago." The reason
+ this is safe is that even if misordering occurs, the
+ misordering will not affect the accesses that follow
+ the comparison. So exactly how long ago is "a long
+ time ago"? Here are some possibilities:
+
+ o Compile time.
+
+ o Boot time.
+
+ o Module-init time for module code.
+
+ o Prior to kthread creation for kthread code.
+
+ o During some prior acquisition of the lock that
+ we now hold.
+
+ o Before mod_timer() time for a timer handler.
+
+ There are many other possibilities involving the Linux
+ kernel's wide array of primitives that cause code to
+ be invoked at a later time.
+
+ o The pointer being compared against also came from
+ rcu_dereference(). In this case, both pointers depend
+ on one rcu_dereference() or another, so you get proper
+ ordering either way.
+
+ That said, this situation can make certain RCU usage
+ bugs more likely to happen. Which can be a good thing,
+ at least if they happen during testing. An example
+ of such an RCU usage bug is shown in the section titled
+ "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
+
+ o All of the accesses following the comparison are stores,
+ so that a control dependency preserves the needed ordering.
+ That said, it is easy to get control dependencies wrong.
+ Please see the "CONTROL DEPENDENCIES" section of
+ Documentation/memory-barriers.txt for more details.
+
+ o The pointers are not equal -and- the compiler does
+ not have enough information to deduce the value of the
+ pointer. Note that the volatile cast in rcu_dereference()
+ will normally prevent the compiler from knowing too much.
+
+ However, please note that if the compiler knows that the
+ pointer takes on only one of two values, a not-equal
+ comparison will provide exactly the information that the
+ compiler needs to deduce the value of the pointer.
+
+o Disable any value-speculation optimizations that your compiler
+ might provide, especially if you are making use of feedback-based
+ optimizations that take data collected from prior runs. Such
+ value-speculation optimizations reorder operations by design.
+
+ There is one exception to this rule: Value-speculation
+ optimizations that leverage the branch-prediction hardware are
+ safe on strongly ordered systems (such as x86), but not on weakly
+ ordered systems (such as ARM or Power). Choose your compiler
+ command-line options wisely!
+
+
+EXAMPLE OF AMPLIFIED RCU-USAGE BUG
+
+Because updaters can run concurrently with RCU readers, RCU readers can
+see stale and/or inconsistent values. If RCU readers need fresh or
+consistent values, which they sometimes do, they need to take proper
+precautions. To see this, consider the following code fragment:
+
+ struct foo {
+ int a;
+ int b;
+ int c;
+ };
+ struct foo *gp1;
+ struct foo *gp2;
+
+ void updater(void)
+ {
+ struct foo *p;
+
+ p = kmalloc(...);
+ if (p == NULL)
+ deal_with_it();
+ p->a = 42; /* Each field in its own cache line. */
+ p->b = 43;
+ p->c = 44;
+ rcu_assign_pointer(gp1, p);
+ p->b = 143;
+ p->c = 144;
+ rcu_assign_pointer(gp2, p);
+ }
+
+ void reader(void)
+ {
+ struct foo *p;
+ struct foo *q;
+ int r1, r2;
+
+ p = rcu_dereference(gp2);
+ if (p == NULL)
+ return;
+ r1 = p->b; /* Guaranteed to get 143. */
+ q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
+ if (p == q) {
+ /* The compiler decides that q->c is same as p->c. */
+ r2 = p->c; /* Could get 44 on weakly order system. */
+ }
+ do_something_with(r1, r2);
+ }
+
+You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
+but you should not be. After all, the updater might have been invoked
+a second time between the time reader() loaded into "r1" and the time
+that it loaded into "r2". The fact that this same result can occur due
+to some reordering from the compiler and CPUs is beside the point.
+
+But suppose that the reader needs a consistent view?
+
+Then one approach is to use locking, for example, as follows:
+
+ struct foo {
+ int a;
+ int b;
+ int c;
+ spinlock_t lock;
+ };
+ struct foo *gp1;
+ struct foo *gp2;
+
+ void updater(void)
+ {
+ struct foo *p;
+
+ p = kmalloc(...);
+ if (p == NULL)
+ deal_with_it();
+ spin_lock(&p->lock);
+ p->a = 42; /* Each field in its own cache line. */
+ p->b = 43;
+ p->c = 44;
+ spin_unlock(&p->lock);
+ rcu_assign_pointer(gp1, p);
+ spin_lock(&p->lock);
+ p->b = 143;
+ p->c = 144;
+ spin_unlock(&p->lock);
+ rcu_assign_pointer(gp2, p);
+ }
+
+ void reader(void)
+ {
+ struct foo *p;
+ struct foo *q;
+ int r1, r2;
+
+ p = rcu_dereference(gp2);
+ if (p == NULL)
+ return;
+ spin_lock(&p->lock);
+ r1 = p->b; /* Guaranteed to get 143. */
+ q = rcu_dereference(gp1); /* Guaranteed non-NULL. */
+ if (p == q) {
+ /* The compiler decides that q->c is same as p->c. */
+ r2 = p->c; /* Locking guarantees r2 == 144. */
+ }
+ spin_unlock(&p->lock);
+ do_something_with(r1, r2);
+ }
+
+As always, use the right tool for the job!
+
+
+EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
+
+If a pointer obtained from rcu_dereference() compares not-equal to some
+other pointer, the compiler normally has no clue what the value of the
+first pointer might be. This lack of knowledge prevents the compiler
+from carrying out optimizations that otherwise might destroy the ordering
+guarantees that RCU depends on. And the volatile cast in rcu_dereference()
+should prevent the compiler from guessing the value.
+
+But without rcu_dereference(), the compiler knows more than you might
+expect. Consider the following code fragment:
+
+ struct foo {
+ int a;
+ int b;
+ };
+ static struct foo variable1;
+ static struct foo variable2;
+ static struct foo *gp = &variable1;
+
+ void updater(void)
+ {
+ initialize_foo(&variable2);
+ rcu_assign_pointer(gp, &variable2);
+ /*
+ * The above is the only store to gp in this translation unit,
+ * and the address of gp is not exported in any way.
+ */
+ }
+
+ int reader(void)
+ {
+ struct foo *p;
+
+ p = gp;
+ barrier();
+ if (p == &variable1)
+ return p->a; /* Must be variable1.a. */
+ else
+ return p->b; /* Must be variable2.b. */
+ }
+
+Because the compiler can see all stores to "gp", it knows that the only
+possible values of "gp" are "variable1" on the one hand and "variable2"
+on the other. The comparison in reader() therefore tells the compiler
+the exact value of "p" even in the not-equals case. This allows the
+compiler to make the return values independent of the load from "gp",
+in turn destroying the ordering between this load and the loads of the
+return values. This can result in "p->b" returning pre-initialization
+garbage values.
+
+In short, rcu_dereference() is -not- optional when you are going to
+dereference the resulting pointer.