netdev - Re: [PATCH 1/6] net/rds: Avoid queuing superfluous send and recv work

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Message-ID: <20250326094245.094cef0d@kernel.org>
Date: Wed, 26 Mar 2025 09:42:45 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Allison Henderson <allison.henderson@...cle.com>
Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/6] net/rds: Avoid queuing superfluous send and recv
 work

On Wed, 12 Mar 2025 07:50:11 +0000 Allison Henderson wrote:
> Thread A:					Thread B:
> -----------------------------------		-----------------------------------
> 						Calls rds_sendmsg()
> 						   Calls rds_send_xmit()
> 						      Calls rds_cond_queue_send_work()   
> Calls rds_send_worker()					
>   calls rds_clear_queued_send_work_bit()		   
>     clears RDS_SEND_WORK_QUEUED in cp->cp_flags		
>       						         checks RDS_SEND_WORK_QUEUED in cp->cp_flags

But if the two threads run in parallel what prevents this check 
to happen fully before the previous line on the "Thread A" side?

Please take a look at netif_txq_try_stop() for an example of 
a memory-barrier based algo.

>       						         Queues work on on cp->cp_send_w
>     Calls rds_send_xmit()
>        Calls rds_cond_queue_send_work()
>           skips queueing work on cp->cp_send_w