Re: [RFC 0/2] refcount: attempt to avoid imbalance warnings

[Linus Torvalds]

On Thu, Jun 30, 2022 at 6:59 AM Alexander Aring <aahringo@xxxxxxxxxx> wrote:
>
> I send this patch series as RFC because it was necessary to do a kref
> change after adding __cond_lock() to refcount_dec_and_lock()
> functionality.

Can you try something like this instead?

This is two separate patches - one for sparse, and one for the kernel.

This is only *very* lightly tested (ie I tested it on a single kernel
file that used refcount_dec_and_lock())

                Linus
 linearize.c          | 24 ++++++++++++++++++++++--
 validation/context.c | 15 +++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/linearize.c b/linearize.c
index d9aed61b..8dd005af 100644
--- a/linearize.c
+++ b/linearize.c
@@ -1537,6 +1537,8 @@ static pseudo_t linearize_call_expression(struct entrypoint *ep, struct expressi
 	add_one_insn(ep, insn);
 
 	if (ctype) {
+		struct basic_block *post_call = NULL;
+
 		FOR_EACH_PTR(ctype->contexts, context) {
 			int in = context->in;
 			int out = context->out;
@@ -1547,8 +1549,21 @@ static pseudo_t linearize_call_expression(struct entrypoint *ep, struct expressi
 				in = 0;
 			}
 			if (out < 0) {
-				check = 0;
-				out = 0;
+				struct basic_block *set_context;
+				if (retval == VOID) {
+					warning(expr->pos, "nonsensical conditional output context with no condition");
+					break;
+				}
+				if (check || in) {
+					warning(expr->pos, "nonsensical conditional input context");
+					break;
+				}
+				if (!post_call)
+					post_call = alloc_basic_block(ep, expr->pos);
+				set_context = alloc_basic_block(ep, expr->pos);
+				add_branch(ep, retval, set_context, post_call);
+				set_activeblock(ep, set_context);
+				out = -out;
 			}
 			context_diff = out - in;
 			if (check || context_diff) {
@@ -1560,6 +1575,11 @@ static pseudo_t linearize_call_expression(struct entrypoint *ep, struct expressi
 			}
 		} END_FOR_EACH_PTR(context);
 
+		if (post_call) {
+			add_goto(ep, post_call);
+			set_activeblock(ep, post_call);
+		}
+
 		if (ctype->modifiers & MOD_NORETURN)
 			add_unreachable(ep);
 	}
diff --git a/validation/context.c b/validation/context.c
index b9500dc7..f8962f19 100644
--- a/validation/context.c
+++ b/validation/context.c
@@ -10,6 +10,10 @@ static void r(void) __attribute__((context(1,0)))
 	__context__(-1);
 }
 
+// Negative output means "conditional positive output"
+extern int cond_get(void) __attribute((context(0,-1)));
+extern void nonsensical_cond_get(void) __attribute((context(0,-1)));
+
 extern int _ca(int fail);
 #define ca(fail) __cond_lock(_ca(fail))
 
@@ -19,6 +23,17 @@ static void good_paired1(void)
 	r();
 }
 
+static void good_conditional(void)
+{
+	if (cond_get())
+		r();
+}
+
+static void nonsensical_conditional(void)
+{
+	nonsensical_cond_get();
+}
+
 static void good_paired2(void)
 {
 	a();
 include/linux/compiler_types.h | 2 ++
 include/linux/refcount.h       | 6 +++---
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index d08dfcb0ac68..4f2a819fd60a 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -24,6 +24,7 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 /* context/locking */
 # define __must_hold(x)	__attribute__((context(x,1,1)))
 # define __acquires(x)	__attribute__((context(x,0,1)))
+# define __cond_acquires(x) __attribute__((context(x,0,-1)))
 # define __releases(x)	__attribute__((context(x,1,0)))
 # define __acquire(x)	__context__(x,1)
 # define __release(x)	__context__(x,-1)
@@ -50,6 +51,7 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 /* context/locking */
 # define __must_hold(x)
 # define __acquires(x)
+# define __cond_acquires(x)
 # define __releases(x)
 # define __acquire(x)	(void)0
 # define __release(x)	(void)0
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index b8a6e387f8f9..a62fcca97486 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -361,9 +361,9 @@ static inline void refcount_dec(refcount_t *r)
 
 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);
-extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
+extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
+extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
 						       spinlock_t *lock,
-						       unsigned long *flags);
+						       unsigned long *flags) __cond_acquires(lock);
 #endif /* _LINUX_REFCOUNT_H */