Commit 020e6c22 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

kcsan: Add example to data_race() kerneldoc header 



Although the data_race() kerneldoc header accurately states what it does,
some of the implications and usage patterns are non-obvious.  Therefore,
add a brief locking example and also state how to have KCSAN ignore
accesses while also preventing the compiler from folding, spindling,
or otherwise mutilating the access.

[ paulmck: Apply Bart Van Assche feedback. ]
[ paulmck: Apply feedback from Marco Elver. ]

Reported-by: default avatarBart Van Assche <bvanassche@acm.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Jens Axboe <axboe@kernel.dk>
parent 1613e604

include/linux/compiler.h

+9 −1
Original line number Diff line number Diff line
@@ -194,9 +194,17 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, 
 * This data_race() macro is useful for situations in which data races

 * should be forgiven.  One example is diagnostic code that accesses

 * shared variables but is not a part of the core synchronization design.

 * For example, if accesses to a given variable are protected by a lock,

 * except for diagnostic code, then the accesses under the lock should

 * be plain C-language accesses and those in the diagnostic code should

 * use data_race().  This way, KCSAN will complain if buggy lockless

 * accesses to that variable are introduced, even if the buggy accesses

 * are protected by READ_ONCE() or WRITE_ONCE().

 *

 * This macro *does not* affect normal code generation, but is a hint

 * to tooling that data races here are to be ignored.

 * to tooling that data races here are to be ignored.  If the access must

 * be atomic *and* KCSAN should ignore the access, use both data_race()

 * and READ_ONCE(), for example, data_race(READ_ONCE(x)).

 */

#define data_race(expr)							\

({									\

tools/memory-model/Documentation/access-marking.txt

+23 −1
Original line number Diff line number Diff line
@@ -24,6 +24,11 @@ The Linux kernel provides the following access-marking options: 
4.	WRITE_ONCE(), for example, "WRITE_ONCE(a, b);"

	The various forms of atomic_set() also fit in here.



5.	__data_racy, for example "int __data_racy a;"



6.	KCSAN's negative-marking assertions, ASSERT_EXCLUSIVE_ACCESS()

	and ASSERT_EXCLUSIVE_WRITER(), are described in the

	"ACCESS-DOCUMENTATION OPTIONS" section below.



These may be used in combination, as shown in this admittedly improbable

example:
@@ -205,6 +210,23 @@ because doing otherwise prevents KCSAN from detecting violations of your 
code's synchronization rules.





Use of __data_racy

------------------



Adding the __data_racy type qualifier to the declaration of a variable

causes KCSAN to treat all accesses to that variable as if they were

enclosed by data_race().  However, __data_racy does not affect the

compiler, though one could imagine hardened kernel builds treating the

__data_racy type qualifier as if it was the volatile keyword.



Note well that __data_racy is subject to the same pointer-declaration

rules as are other type qualifiers such as const and volatile.

For example:



	int __data_racy *p; // Pointer to data-racy data.

	int *__data_racy p; // Data-racy pointer to non-data-racy data.





ACCESS-DOCUMENTATION OPTIONS

============================
@@ -342,7 +364,7 @@ as follows: 


Because foo is read locklessly, all accesses are marked.  The purpose

of the ASSERT_EXCLUSIVE_WRITER() is to allow KCSAN to check for a buggy

concurrent lockless write.

concurrent write, whether marked or not.





Lock-Protected Writes With Heuristic Lockless Reads