Commit 293f5bc2 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

Documentation/litmus-tests: Demonstrate unordered failing cmpxchg 



This commit adds four litmus tests showing that a failing cmpxchg()
operation is unordered unless followed by an smp_mb__after_atomic()
operation.

Suggested-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Acked-by: default avatarAndrea Parri <parri.andrea@gmail.com>
parent d2c470c4

Documentation/litmus-tests/README

+16 −0
Original line number Diff line number Diff line
@@ -21,6 +21,22 @@ Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus 
    Test that atomic_set() cannot break the atomicity of atomic RMWs.

    NOTE: Require herd7 7.56 or later which supports "(void)expr".



cmpxchg-fail-ordered-1.litmus

    Demonstrate that a failing cmpxchg() operation acts as a full barrier

    when followed by smp_mb__after_atomic().



cmpxchg-fail-ordered-2.litmus

    Demonstrate that a failing cmpxchg() operation acts as an acquire

    operation when followed by smp_mb__after_atomic().



cmpxchg-fail-unordered-1.litmus

    Demonstrate that a failing cmpxchg() operation does not act as a

    full barrier.



cmpxchg-fail-unordered-2.litmus

    Demonstrate that a failing cmpxchg() operation does not act as an

    acquire operation.





locking (/locking directory)

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

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-1.litmus

0 → 100644
+34 −0
Original line number Diff line number Diff line 
C cmpxchg-fail-ordered-1



(*

 * Result: Never

 *

 * Demonstrate that a failing cmpxchg() operation will act as a full

 * barrier when followed by smp_mb__after_atomic().

 *)



{}



P0(int *x, int *y, int *z)

{

	int r0;

	int r1;



	WRITE_ONCE(*x, 1);

	r1 = cmpxchg(z, 1, 0);

	smp_mb__after_atomic();

	r0 = READ_ONCE(*y);

}



P1(int *x, int *y, int *z)

{

	int r0;



	WRITE_ONCE(*y, 1);

	r1 = cmpxchg(z, 1, 0);

	smp_mb__after_atomic();

	r0 = READ_ONCE(*x);

}



locations[0:r1;1:r1]

exists (0:r0=0 /\ 1:r0=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-2.litmus

0 → 100644
+30 −0
Original line number Diff line number Diff line 
C cmpxchg-fail-ordered-2



(*

 * Result: Never

 *

 * Demonstrate use of smp_mb__after_atomic() to make a failing cmpxchg

 * operation have acquire ordering.

 *)



{}



P0(int *x, int *y)

{

	int r0;

	int r1;



	WRITE_ONCE(*x, 1);

	r1 = cmpxchg(y, 0, 1);

}



P1(int *x, int *y)

{

	int r0;



	r1 = cmpxchg(y, 0, 1);

	smp_mb__after_atomic();

	r2 = READ_ONCE(*x);

}



exists (0:r1=0 /\ 1:r1=1 /\ 1:r2=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-1.litmus

0 → 100644
+33 −0
Original line number Diff line number Diff line 
C cmpxchg-fail-unordered-1



(*

 * Result: Sometimes

 *

 * Demonstrate that a failing cmpxchg() operation does not act as a

 * full barrier.  (In contrast, a successful cmpxchg() does act as a

 * full barrier.)

 *)



{}



P0(int *x, int *y, int *z)

{

	int r0;

	int r1;



	WRITE_ONCE(*x, 1);

	r1 = cmpxchg(z, 1, 0);

	r0 = READ_ONCE(*y);

}



P1(int *x, int *y, int *z)

{

	int r0;



	WRITE_ONCE(*y, 1);

	r1 = cmpxchg(z, 1, 0);

	r0 = READ_ONCE(*x);

}



locations[0:r1;1:r1]

exists (0:r0=0 /\ 1:r0=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-2.litmus

0 → 100644
+30 −0
Original line number Diff line number Diff line 
C cmpxchg-fail-unordered-2



(*

 * Result: Sometimes

 *

 * Demonstrate that a failing cmpxchg() operation does not act as either

 * an acquire release operation.  (In contrast, a successful cmpxchg()

 * does act as both an acquire and a release operation.)

 *)



{}



P0(int *x, int *y)

{

	int r0;

	int r1;



	WRITE_ONCE(*x, 1);

	r1 = cmpxchg(y, 0, 1);

}



P1(int *x, int *y)

{

	int r0;



	r1 = cmpxchg(y, 0, 1);

	r2 = READ_ONCE(*x);

}



exists (0:r1=0 /\ 1:r1=1 /\ 1:r2=0)