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Date:   Thu, 27 Aug 2020 07:54:23 -0000
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Subject: [tip: locking/core] locking/refcount: Provide __refcount API to
 obtain the old value
Cc:     "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>, x86 <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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The following commit has been merged into the locking/core branch of tip:

Commit-ID:     a435b9a14356587cf512ea6473368a579373c74c
Gitweb:        https://git.kernel.org/tip/a435b9a14356587cf512ea6473368a579373c74c
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Wed, 29 Jul 2020 13:00:57 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 26 Aug 2020 12:42:02 +02:00

locking/refcount: Provide __refcount API to obtain the old value

David requested means to obtain the old/previous value from the
refcount API for tracing purposes.

Duplicate (most of) the API as __refcount*() with an additional
'int *' argument into which, if !NULL, the old value will be stored.

Requested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200729111120.GA2638@hirez.programming.kicks-ass.net
---
 include/linux/refcount.h | 65 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 57 insertions(+), 8 deletions(-)

diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0e3ee25..7fabb1a 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -165,7 +165,7 @@ static inline unsigned int refcount_read(const refcount_t *r)
  *
  * Return: false if the passed refcount is 0, true otherwise
  */
-static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
+static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
 {
 	int old = refcount_read(r);
 
@@ -174,12 +174,20 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 			break;
 	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 
+	if (oldp)
+		*oldp = old;
+
 	if (unlikely(old < 0 || old + i < 0))
 		refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
 
 	return old;
 }
 
+static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
+{
+	return __refcount_add_not_zero(i, r, NULL);
+}
+
 /**
  * refcount_add - add a value to a refcount
  * @i: the value to add to the refcount
@@ -196,16 +204,24 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
  * cases, refcount_inc(), or one of its variants, should instead be used to
  * increment a reference count.
  */
-static inline void refcount_add(int i, refcount_t *r)
+static inline void __refcount_add(int i, refcount_t *r, int *oldp)
 {
 	int old = atomic_fetch_add_relaxed(i, &r->refs);
 
+	if (oldp)
+		*oldp = old;
+
 	if (unlikely(!old))
 		refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
 	else if (unlikely(old < 0 || old + i < 0))
 		refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
 }
 
+static inline void refcount_add(int i, refcount_t *r)
+{
+	__refcount_add(i, r, NULL);
+}
+
 /**
  * refcount_inc_not_zero - increment a refcount unless it is 0
  * @r: the refcount to increment
@@ -219,9 +235,14 @@ static inline void refcount_add(int i, refcount_t *r)
  *
  * Return: true if the increment was successful, false otherwise
  */
+static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
+{
+	return __refcount_add_not_zero(1, r, oldp);
+}
+
 static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
 {
-	return refcount_add_not_zero(1, r);
+	return __refcount_inc_not_zero(r, NULL);
 }
 
 /**
@@ -236,9 +257,14 @@ static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
  * Will WARN if the refcount is 0, as this represents a possible use-after-free
  * condition.
  */
+static inline void __refcount_inc(refcount_t *r, int *oldp)
+{
+	__refcount_add(1, r, oldp);
+}
+
 static inline void refcount_inc(refcount_t *r)
 {
-	refcount_add(1, r);
+	__refcount_inc(r, NULL);
 }
 
 /**
@@ -261,10 +287,13 @@ static inline void refcount_inc(refcount_t *r)
  *
  * Return: true if the resulting refcount is 0, false otherwise
  */
-static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
+static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
 {
 	int old = atomic_fetch_sub_release(i, &r->refs);
 
+	if (oldp)
+		*oldp = old;
+
 	if (old == i) {
 		smp_acquire__after_ctrl_dep();
 		return true;
@@ -276,6 +305,11 @@ static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
 	return false;
 }
 
+static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
+{
+	return __refcount_sub_and_test(i, r, NULL);
+}
+
 /**
  * refcount_dec_and_test - decrement a refcount and test if it is 0
  * @r: the refcount
@@ -289,9 +323,14 @@ static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
  *
  * Return: true if the resulting refcount is 0, false otherwise
  */
+static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
+{
+	return __refcount_sub_and_test(1, r, oldp);
+}
+
 static inline __must_check bool refcount_dec_and_test(refcount_t *r)
 {
-	return refcount_sub_and_test(1, r);
+	return __refcount_dec_and_test(r, NULL);
 }
 
 /**
@@ -304,12 +343,22 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
  * Provides release memory ordering, such that prior loads and stores are done
  * before.
  */
-static inline void refcount_dec(refcount_t *r)
+static inline void __refcount_dec(refcount_t *r, int *oldp)
 {
-	if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
+	int old = atomic_fetch_sub_release(1, &r->refs);
+
+	if (oldp)
+		*oldp = old;
+
+	if (unlikely(old <= 1))
 		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 
+static inline void refcount_dec(refcount_t *r)
+{
+	__refcount_dec(r, NULL);
+}
+
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 extern __must_check bool refcount_dec_not_one(refcount_t *r);
 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);