[PATCH v2 4/9] refcount: Use atomic_*_overflow()

[Peter Zijlstra]

Use the shiny new atomic_*_overflow() functions in order to have better
code-gen.

Strictly speaking, these ops operate on [0, INT_MIN] rather than
[0, INT_MAX] but this is harmless. At worst another op finds the
refcount is negative and goes saturate, which is always a possibility
anyway.

Notably refcount_inc() case no longer distinguishes between
inc-from-zero and inc-negative in the fast path, this improves
code-gen:

    4b88:       b8 01 00 00 00          mov    $0x1,%eax
    4b8d:       f0 0f c1 43 28          lock xadd %eax,0x28(%rbx)
    4b92:       85 c0                   test   %eax,%eax
    4b94:       74 1b                   je     4bb1 <alloc_perf_context+0xf1>
    4b96:       8d 50 01                lea    0x1(%rax),%edx
    4b99:       09 c2                   or     %eax,%edx
    4b9b:       78 20                   js     4bbd <alloc_perf_context+0xfd>

to:

    4768:       b8 01 00 00 00          mov    $0x1,%eax
    476d:       f0 0f c1 43 28          lock xadd %eax,0x28(%rbx)
    4772:       85 c0                   test   %eax,%eax
    4774:       7e 14                   jle    478a <alloc_perf_context+0xea>


without sacrificing on functionality; the only thing that suffers is
the reported error condition, which might now 'spuriously' report
'saturated' instead of 'inc-from-zero'.

refcount_dec_and_test() is also improved:

    aa40:       b8 ff ff ff ff          mov    $0xffffffff,%eax
    aa45:       f0 0f c1 07             lock xadd %eax,(%rdi)
    aa49:       83 f8 01                cmp    $0x1,%eax
    aa4c:       74 05                   je     aa53 <ring_buffer_put+0x13>
    aa4e:       85 c0                   test   %eax,%eax
    aa50:       7e 1e                   jle    aa70 <ring_buffer_put+0x30>
    aa52:       c3                      ret

to:

    a980:       b8 ff ff ff ff          mov    $0xffffffff,%eax
    a985:       f0 0f c1 07             lock xadd %eax,(%rdi)
    a989:       83 e8 01                sub    $0x1,%eax
    a98c:       78 20                   js     a9ae <ring_buffer_put+0x2e>
    a98e:       74 01                   je     a991 <ring_buffer_put+0x11>
    a990:       c3                      ret

XXX atomic_dec_and_test_overflow() is strictly stronger ordered than
refcount_dec_and_test() is defined -- Power and Arrghh64 ?

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/refcount.h |   17 +++++++++++++----
 lib/refcount.c           |    5 +++++
 2 files changed, 18 insertions(+), 4 deletions(-)

--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -264,7 +264,10 @@ static inline void __refcount_inc(refcou
  */
 static inline void refcount_inc(refcount_t *r)
 {
-	__refcount_inc(r, NULL);
+	atomic_inc_overflow(&r->refs, Eoverflow);
+	return;
+Eoverflow:
+	refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
 }
 
 static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
@@ -330,7 +333,10 @@ static inline __must_check bool __refcou
  */
 static inline __must_check bool refcount_dec_and_test(refcount_t *r)
 {
-	return __refcount_dec_and_test(r, NULL);
+	return atomic_dec_and_test_overflow(&r->refs, Eoverflow);
+Eoverflow:
+	refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
+	return false;
 }
 
 static inline void __refcount_dec(refcount_t *r, int *oldp)
@@ -340,7 +346,7 @@ static inline void __refcount_dec(refcou
 	if (oldp)
 		*oldp = old;
 
-	if (unlikely(old <= 1))
+	if (unlikely(old - 1 <= 0))
 		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 
@@ -356,7 +362,10 @@ static inline void __refcount_dec(refcou
  */
 static inline void refcount_dec(refcount_t *r)
 {
-	__refcount_dec(r, NULL);
+	atomic_dec_overflow(&r->refs, Eoverflow);
+	return;
+Eoverflow:
+	refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -12,8 +12,13 @@
 
 void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
 {
+	int old = refcount_read(r);
 	refcount_set(r, REFCOUNT_SATURATED);
 
+	/* racy; who cares */
+	if (old == 1 && t == REFCOUNT_ADD_OVF)
+		t = REFCOUNT_ADD_UAF;
+
 	switch (t) {
 	case REFCOUNT_ADD_NOT_ZERO_OVF:
 		REFCOUNT_WARN("saturated; leaking memory");