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Message-ID: <Y2AbtAAVBjmya9qJ@example.org>
Date: Mon, 31 Oct 2022 20:02:12 +0100
From: Alexey Gladkov <legion@...nel.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: linux-kernel@...r.kernel.org, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH RESEND 2/2] ucount: Use atomic_long_try_cmpxchg in
atomic_long_inc_below
On Mon, Oct 17, 2022 at 04:40:49PM +0200, Uros Bizjak wrote:
> 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@...il.com>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Reviewed-by: Alexey Gladkov <legion@...nel.org>
Eric, I think the patch looks good. You already reviewed the previous
version [1]. What do you think about this version?
[1] https://lore.kernel.org/lkml/87pmi6z143.fsf@email.froward.int.ebiederm.org/
> ---
> kernel/ucount.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 74f5b4959feb..2c04589a61ff 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -214,16 +214,14 @@ 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,
> --
> 2.37.3
>
--
Rgrds, legion
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