[PATCH 0/2] ucount: Fix and improve atomic_long_inc_below

[PATCH 0/2] ucount: Fix and improve atomic_long_inc_below

[Uros Bizjak]

The series fixes wrong argument type and improves atomic_long_inc_below
by using atomic_long_try_cmpxchg instead of atomic_long_cmpxchg.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>

Uros Bizjak (2):
  ucount: Fix atomic_long_inc_below argument type
  ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below

 kernel/ucount.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

-- 
2.35.3

[PATCH 1/2] ucount: Fix atomic_long_inc_below argument type

[Uros Bizjak]

The type of u argument of atomic_long_inc_below should be long
to avoid unwanted truncation to int.

Fixes: f9c82a4ea89c ("Increase size of ucounts to atomic_long_t")
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/ucount.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 06ea04d44685..974ac1585009 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -216,7 +216,7 @@ void put_ucounts(struct ucounts *ucounts)
 	}
 }
 
-static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
+static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
 {
 	long c, old;
 	c = atomic_long_read(v);
-- 
2.35.3

[PATCH 2/2] ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below

[Uros Bizjak]

Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_below.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves
a compare after cmpxchg (and related move instruction in front of cmpxchg).

Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/ucount.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 974ac1585009..5d980460104e 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -218,16 +218,14 @@ void put_ucounts(struct ucounts *ucounts)
 
 static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
 {
-	long c, old;
-	c = atomic_long_read(v);
-	for (;;) {
+	long c = atomic_long_read(v);
+
+	do {
 		if (unlikely(c >= u))
 			return false;
-		old = atomic_long_cmpxchg(v, c, c+1);
-		if (likely(old == c))
-			return true;
-		c = old;
-	}
+	} while (!atomic_long_try_cmpxchg(v, &c, c+1));
+
+	return true;
 }
 
 struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
-- 
2.35.3

Re: [PATCH 1/2] ucount: Fix atomic_long_inc_below argument type

[Eric W. Biederman]

Uros Bizjak <ubizjak@gmail.com> writes:

> The type of u argument of atomic_long_inc_below should be long
> to avoid unwanted truncation to int.

Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>

>
> Fixes: f9c82a4ea89c ("Increase size of ucounts to atomic_long_t")
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  kernel/ucount.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 06ea04d44685..974ac1585009 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -216,7 +216,7 @@ void put_ucounts(struct ucounts *ucounts)
>  	}
>  }
>  
> -static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
> +static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
>  {
>  	long c, old;
>  	c = atomic_long_read(v);

Re: [PATCH 0/2] ucount: Fix and improve atomic_long_inc_below

[Eric W. Biederman]

Uros Bizjak <ubizjak@gmail.com> writes:

> The series fixes wrong argument type and improves atomic_long_inc_below
> by using atomic_long_try_cmpxchg instead of atomic_long_cmpxchg.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>

The bug fix at least looks correct.  I haven't reviewed the
atomic_long_try_cmpxchg yet.

Thanks,
Eric


>
> Uros Bizjak (2):
>   ucount: Fix atomic_long_inc_below argument type
>   ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below
>
>  kernel/ucount.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)