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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 18, 2015 at 10:28:20AM +0100, Will Deacon wrote:
> On Fri, Sep 18, 2015 at 10:06:46AM +0100, Dmitry Vyukov wrote:

> > Can we have something along the lines of:
> > 
> > #define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)
> 
> Funnily enough, I had this exact same discussion off-list yesterday
> afternoon, since I wrote some code relying on a ctrl dependency from
> an atomic_read to an atomic_xchg_relaxed.

Something a little like so should work fine I suppose.

---
Subject: atomic: Implement atomic_read_ctrl()

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.

Requested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/atomic.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 00a5763e850e..c4072421ea7f 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -4,6 +4,15 @@
 #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
+
 /*
  * Relaxed variants of xchg, cmpxchg and some atomic operations.
  *