linux-kernel - Re: [PATCH v4 24/40] RDMA/rtrs: optimize __rtrs_get_permit() by using find_and_set_bit

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Date: Thu, 27 Jun 2024 14:59:27 +0200
From: Jinpu Wang <jinpu.wang@...os.com>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org, "Md. Haris Iqbal" <haris.iqbal@...os.com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>, linux-rdma@...r.kernel.org, 
	Alexey Klimov <alexey.klimov@...aro.org>, Bart Van Assche <bvanassche@....org>, Jan Kara <jack@...e.cz>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Mirsad Todorovac <mirsad.todorovac@....unizg.hr>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Sergey Shtylyov <s.shtylyov@....ru>
Subject: Re: [PATCH v4 24/40] RDMA/rtrs: optimize __rtrs_get_permit() by using find_and_set_bit_lock()

On Thu, Jun 20, 2024 at 7:58 PM Yury Norov <yury.norov@...il.com> wrote:
>
> The function opencodes find_and_set_bit_lock() with a while-loop polling
> on test_and_set_bit_lock(). Use the dedicated function instead.
>
> Signed-off-by: Yury Norov <yury.norov@...il.com>
lgtm, thx!
Reviewed-by: Jack Wang <jinpu.wang@...os.com>
> ---
>  drivers/infiniband/ulp/rtrs/rtrs-clt.c | 16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/infiniband/ulp/rtrs/rtrs-clt.c b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
> index 88106cf5ce55..52b7728f6c63 100644
> --- a/drivers/infiniband/ulp/rtrs/rtrs-clt.c
> +++ b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
> @@ -10,6 +10,7 @@
>  #undef pr_fmt
>  #define pr_fmt(fmt) KBUILD_MODNAME " L" __stringify(__LINE__) ": " fmt
>
> +#include <linux/find_atomic.h>
>  #include <linux/module.h>
>  #include <linux/rculist.h>
>  #include <linux/random.h>
> @@ -72,18 +73,9 @@ __rtrs_get_permit(struct rtrs_clt_sess *clt, enum rtrs_clt_con_type con_type)
>         struct rtrs_permit *permit;
>         int bit;
>
> -       /*
> -        * Adapted from null_blk get_tag(). Callers from different cpus may
> -        * grab the same bit, since find_first_zero_bit is not atomic.
> -        * But then the test_and_set_bit_lock will fail for all the
> -        * callers but one, so that they will loop again.
> -        * This way an explicit spinlock is not required.
> -        */
> -       do {
> -               bit = find_first_zero_bit(clt->permits_map, max_depth);
> -               if (bit >= max_depth)
> -                       return NULL;
> -       } while (test_and_set_bit_lock(bit, clt->permits_map));
> +       bit = find_and_set_bit_lock(clt->permits_map, max_depth);
> +       if (bit >= max_depth)
> +               return NULL;
>
>         permit = get_permit(clt, bit);
>         WARN_ON(permit->mem_id != bit);
> --
> 2.43.0
>