2007-05-10 12:58:41

by Mika Kukkonen

Subject: [PATCH] Fix bug in mm/thrash.c function grab_swap_token()

Following bug was uncovered by compiling with '-W' flag:

CC mm/thrash.o
mm/thrash.c: In function ‘grab_swap_token’:
mm/thrash.c:52: warning: comparison of unsigned expression < 0 is always false

Field token_priority is unsigned, so decrementing first and the checking
is not a good plan. Attached patch (compile tested) reverses the order.

Btw, I'm not sure if likely() makes any sense in this new situation.

Signed-off-by: Mika Kukkonen <[email protected]>

diff --git a/mm/thrash.c b/mm/thrash.c
index 9ef9071..c4c5205 100644
--- a/mm/thrash.c
+++ b/mm/thrash.c
@@ -48,9 +48,8 @@ void grab_swap_token(void)
if (current_interval < current->mm->last_interval)
current->mm->token_priority++;
else {
- current->mm->token_priority--;
- if (unlikely(current->mm->token_priority < 0))
- current->mm->token_priority = 0;
+ if (likely(current->mm->token_priority > 0))
+ current->mm->token_priority--;
}
/* Check if we deserve the token */
if (current->mm->token_priority >

2007-05-10 15:26:36

by Satyam Sharma

Subject: Re: [PATCH] Fix bug in mm/thrash.c function grab_swap_token()

On 5/10/07, Mika Kukkonen <[email protected]> wrote:
> Following bug was uncovered by compiling with '-W' flag:
>
> CC mm/thrash.o
> mm/thrash.c: In function 'grab_swap_token':
> mm/thrash.c:52: warning: comparison of unsigned expression < 0 is always false
>
> Field token_priority is unsigned, so decrementing first and the checking
> is not a good plan. Attached patch (compile tested) reverses the order.

Heh, yeah. Actually, decrementing first and then checking < 0 to clamp
to zero is _never_ a good plan. If the original author thought he was
optimizing speed that way, he was clearly mistaken.

> Btw, I'm not sure if likely() makes any sense in this new situation.

Right, if(likely(...)) without an else wouldn't make much sense ...

> Signed-off-by: Mika Kukkonen <[email protected]>
>
> diff --git a/mm/thrash.c b/mm/thrash.c
> index 9ef9071..c4c5205 100644
> --- a/mm/thrash.c
> +++ b/mm/thrash.c
> @@ -48,9 +48,8 @@ void grab_swap_token(void)
> if (current_interval < current->mm->last_interval)
> current->mm->token_priority++;
> else {
> - current->mm->token_priority--;
> - if (unlikely(current->mm->token_priority < 0))
> - current->mm->token_priority = 0;
> + if (likely(current->mm->token_priority > 0))
> + current->mm->token_priority--;
> }
> /* Check if we deserve the token */
> if (current->mm->token_priority >
>