linux-kernel - Re: [PATCH net v2] rds: introduce acquire/release ordering in acquire/release_in

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Date: Fri, 15 Mar 2024 14:32:12 +0100
From: Michal Kubiak <michal.kubiak@...el.com>
To: Yewon Choi <woni9911@...il.com>
CC: Allison Henderson <allison.henderson@...cle.com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, <netdev@...r.kernel.org>,
	<linux-rdma@...r.kernel.org>, <rds-devel@....oracle.com>,
	<linux-kernel@...r.kernel.org>, "Dae R. Jeong" <threeearcat@...il.com>
Subject: Re: [PATCH net v2] rds: introduce acquire/release ordering in
 acquire/release_in_xmit()

On Fri, Mar 15, 2024 at 06:28:38PM +0900, Yewon Choi wrote:
> acquire/release_in_xmit() work as bit lock in rds_send_xmit(), so they
> are expected to ensure acquire/release memory ordering semantics.
> However, test_and_set_bit/clear_bit() don't imply such semantics, on
> top of this, following smp_mb__after_atomic() does not guarantee release
> ordering (memory barrier actually should be placed before clear_bit()).
> 
> Instead, we use clear_bit_unlock/test_and_set_bit_lock() here.
> 
> Fixes: 0f4b1c7e89e6 ("rds: fix rds_send_xmit() serialization")
> Fixes: 1f9ecd7eacfd ("RDS: Pass rds_conn_path to rds_send_xmit()")
> Signed-off-by: Yewon Choi <woni9911@...il.com>
> ---
> Changes in v1 -> v2:
> - Added missing Fixes tags
> 
>  net/rds/send.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/net/rds/send.c b/net/rds/send.c
> index 5e57a1581dc6..8f38009721b7 100644
> --- a/net/rds/send.c
> +++ b/net/rds/send.c
> @@ -103,13 +103,12 @@ EXPORT_SYMBOL_GPL(rds_send_path_reset);
>  
>  static int acquire_in_xmit(struct rds_conn_path *cp)
>  {
> -	return test_and_set_bit(RDS_IN_XMIT, &cp->cp_flags) == 0;
> +	return test_and_set_bit_lock(RDS_IN_XMIT, &cp->cp_flags) == 0;
>  }
>  
>  static void release_in_xmit(struct rds_conn_path *cp)
>  {
> -	clear_bit(RDS_IN_XMIT, &cp->cp_flags);
> -	smp_mb__after_atomic();
> +	clear_bit_unlock(RDS_IN_XMIT, &cp->cp_flags);
>  	/*
>  	 * We don't use wait_on_bit()/wake_up_bit() because our waking is in a
>  	 * hot path and finding waiters is very rare.  We don't want to walk
> -- 
> 2.43.0
> 

LGTM

Thanks,
Reviewed-by: Michal Kubiak <michal.kubiak@...el.com>