Re: [PATCH] kernel: fix data race in put_pid

Peter Zijlstra
2015-09-18T12:02:03Z

On Fri, Sep 18, 2015 at 01:50:01PM +0200, Dmitry Vyukov wrote:
> > +#ifndef atomic_read_ctrl
> > +static inline int atomic_read_ctrl(atomic_t *v)
> > +{
> > + int val = atomic_read(v);
> > + smp_read_barrier_depends(); /* Enforce control dependency. */
> > + return val;
> > +}
> > +#endif
> > +
> > /*
> > * Relaxed variants of xchg, cmpxchg and some atomic operations.
> > *
>
> Looks good to me.
> Should we add atomic64_read_ctrl for completeness? I have not seen
> cases where it was needed, though.

Sure, and while doing another spin, let me go update the documentation
too.

---
Subject: atomic: Implement atomic_read_ctrl()
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 18 Sep 2015 13:22:52 +0200

Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
more conveniently use atomics in control dependencies.

Since we can assume atomic_read() implies a READ_ONCE(), we must only
emit an extra smp_read_barrier_depends() in order to upgrade to
READ_ONCE_CTRL() semantics.

Cc: oleg@redhat.com
Cc: torvalds@linux-foundation.org
Cc: will.deacon@arm.com
Cc: paulmck@linux.vnet.ibm.com
Requested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
Documentation/memory-barriers.txt | 17 +++++++++--------
include/linux/atomic.h | 18 ++++++++++++++++++
2 files changed, 27 insertions(+), 8 deletions(-)

--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -637,7 +637,8 @@ to optimize the original example by elim
b = p; /* BUG: Compiler and CPU can both reorder!!! */

Finally, the READ_ONCE_CTRL() includes an smp_read_barrier_depends()
-that DEC Alpha needs in order to respect control depedencies.
+that DEC Alpha needs in order to respect control depedencies. Alternatively
+use one of atomic{,64}_read_ctrl().

So don't leave out the READ_ONCE_CTRL().

@@ -796,9 +797,9 @@ site: https://www.cl.cam.ac.uk/~pes20/pp

In summary:

- (*) Control dependencies must be headed by READ_ONCE_CTRL().
- Or, as a much less preferable alternative, interpose
- smp_read_barrier_depends() between a READ_ONCE() and the
+ (*) Control dependencies must be headed by READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl(). Or, as a much less preferable alternative,
+ interpose smp_read_barrier_depends() between a READ_ONCE() and the
control-dependent write.

(*) Control dependencies can order prior loads against later stores.
@@ -820,10 +821,10 @@ site: https://www.cl.cam.ac.uk/~pes20/pp
and WRITE_ONCE() can help to preserve the needed conditional.

(*) Control dependencies require that the compiler avoid reordering the
- dependency into nonexistence. Careful use of READ_ONCE_CTRL()
- or smp_read_barrier_depends() can help to preserve your control
- dependency. Please see the Compiler Barrier section for more
- information.
+ dependency into nonexistence. Careful use of READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl() or smp_read_barrier_depends() can help to
+ preserve your control dependency. Please see the Compiler Barrier
+ section for more information.

(*) Control dependencies pair normally with other types of barriers.

--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -4,6 +4,24 @@
#include <asm/atomic.h>
#include <asm/barrier.h>

+#ifndef atomic_read_ctrl
+static inline int atomic_read_ctrl(atomic_t *v)
+{
+ int val = atomic_read(v);
+ smp_read_barrier_depends(); /* Enforce control dependency. */
+ return val;
+}
+#endif
+
+#ifndef atomic64_read_ctrl
+static inline int atomic64_read_ctrl(atomic64_t *v)
+{
+ int val = atomic64_read(v);
+ smp_read_barrier_depends(); /* Enforce control dependency. */
+ return val;
+}
+#endif
+
/*
* Relaxed variants of xchg, cmpxchg and some atomic operations.
*