kernel / Documentation / RCU / rcu_dereference.txt

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	Avoid cancellation when using the "+" and "-" infix arithmetic
	operators.  For example, for a given variable "x", avoid
	"(x-x)".  There are similar arithmetic pitfalls from other
	arithmetic operators, such as "(x*0)", "(x/(x+1))" or "(x%1)".
	The compiler is within its rights to substitute zero for all of
	these expressions, 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	Avoid all-zero operands to the bitwise "&" operator, and
	similarly avoid all-ones operands to the bitwise "|" operator.
	If the compiler is able to deduce the value of such operands,
	it is within its rights to substitute the corresponding constant
	for the bitwise operation.  Once again, this causes subsequent
	accesses to no longer depend on the rcu_dereference(), causing
	bugs due to misordering.

	Please note that single-bit operands to bitwise "&" can also
	be dangerous.  At this point, the compiler knows that the
	resulting value can only take on one of two possible values.
	Therefore, a very small amount of additional information will
	allow the compiler to deduce the exact value, which again can
	result in misordering.

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 the boolean "&&" and "||" when
	dereferencing.	For example, the following (rather improbable)
	code is buggy:

		int *p;
		int *q;

		...

		p = rcu_dereference(gp)
		q = &global_q;
		q += p != &oom_p1 && p != &oom_p2;
		r1 = *q;  /* BUGGY!!! */

	The reason this is buggy is that "&&" and "||" are often compiled
	using branches.  While weak-memory machines such as ARM or PowerPC
	do order stores after such branches, they can speculate loads,
	which can result in misordering bugs.

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.