linux-kernel - Re: [PATCH 1/3] scsi/libfc: Remove get_cpu() semantics in fc_exch_em

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Date:   Wed, 17 Nov 2021 08:25:41 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     martin.petersen@...cle.com, jejb@...ux.ibm.com, hare@...e.de,
        tglx@...utronix.de, linux-scsi@...r.kernel.org,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/3] scsi/libfc: Remove get_cpu() semantics in
 fc_exch_em_alloc()

On 2021-11-16 18:59:54 [-0800], Davidlohr Bueso wrote:
> The get_cpu() in fc_exch_em_alloc() was introduced in:
> 
>     f018b73af6db ([SCSI] libfc, libfcoe, fcoe: use smp_processor_id() only when preempt disabled)
> 
> for no other reason than to simply use smp_processor_id()
> without getting a warning, because everything is done with
> the pool->lock held anyway. However, get_cpu(), by disabling
> preemption, does not play well with PREEMPT_RT, particularly
> when acquiring a regular (and thus sleepable) spinlock.
> 
> Therefore remove the get_cpu() and just use the unstable value
> as we will have CPU locality guarantees next by taking the lock.
> The window of migration, as noted by Sebastian, is small and
> even if it happens the result is correct.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

> diff --git a/drivers/scsi/libfc/fc_exch.c b/drivers/scsi/libfc/fc_exch.c
> index 841000445b9a..be37bfb2814d 100644
> --- a/drivers/scsi/libfc/fc_exch.c
> +++ b/drivers/scsi/libfc/fc_exch.c
> @@ -825,10 +825,9 @@ static struct fc_exch *fc_exch_em_alloc(struct fc_lport *lport,
>  	}
>  	memset(ep, 0, sizeof(*ep));
>  
> -	cpu = get_cpu();
> +	cpu = raw_smp_processor_id();
>  	pool = per_cpu_ptr(mp->pool, cpu);
>  	spin_lock_bh(&pool->lock);
> -	put_cpu();

The `cpu' variable is later ORed into ep->oxid. I haven't figured out
why this is important/ required. The ep variable/ `fc_exch' is allocated
from a per-CPU memory pool and is later released to the same pool but
the pool's CPU number is not obtained from fc_exch::oxid but there is a
pointer to its pool stored in fc_exch::em. 
So it remains a mystery to why the CPU number is stored here.

>  	/* peek cache of free slot */
>  	if (pool->left != FC_XID_UNKNOWN) {

Sebastian