linux-kernel - Re: [PATCH 3/4] scsi: pm8001: Use non-atomic bitmap ops for tag alloc + free

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Date:   Mon, 20 Jun 2022 15:07:06 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Hannes Reinecke <hare@...e.de>, John Garry <john.garry@...wei.com>,
        jinpu.wang@...ud.ionos.com, jejb@...ux.ibm.com,
        martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ajish.Koshy@...rochip.com
Subject: Re: [PATCH 3/4] scsi: pm8001: Use non-atomic bitmap ops for tag alloc
 + free

On 6/20/22 15:00, Hannes Reinecke wrote:
> On 6/10/22 18:46, John Garry wrote:
>> In pm8001_tag_alloc() we don't require atomic set_bit() as we are already
>> in atomic context. In pm8001_tag_free() we should use the same host
>> spinlock to protect clearing the tag (and then don't require the atomic
>> clear_bit()).
>>
>> Signed-off-by: John Garry <john.garry@...wei.com>
>> ---
>>   drivers/scsi/pm8001/pm8001_sas.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/scsi/pm8001/pm8001_sas.c b/drivers/scsi/pm8001/pm8001_sas.c
>> index 3a863d776724..8e3f2f9ddaac 100644
>> --- a/drivers/scsi/pm8001/pm8001_sas.c
>> +++ b/drivers/scsi/pm8001/pm8001_sas.c
>> @@ -66,7 +66,11 @@ static int pm8001_find_tag(struct sas_task *task, u32 *tag)
>>   void pm8001_tag_free(struct pm8001_hba_info *pm8001_ha, u32 tag)
>>   {
>>   	void *bitmap = pm8001_ha->tags;
>> -	clear_bit(tag, bitmap);
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&pm8001_ha->bitmap_lock, flags);
>> +	__clear_bit(tag, bitmap);
>> +	spin_unlock_irqrestore(&pm8001_ha->bitmap_lock, flags);
>>   }
>>   
> This spin lock is pretty much pointless; clear_bit() is always atomic.

But __clear_bit() is not atomic. I think it was the point of this patch,
to not use atomics and use the spinlock instead to protect bitmap.

Before the patch, pm8001_tag_alloc() takes the spinlock *and* use the
atomic set_bit(), which is an overkill. pm8001_tag_free() only clears the
bit using the the atomic clear_bit().

After the patch, spinlock guarantees atomicity for both alloc and free.

Not sure there is any gain from this.

> 
> Cheers,
> 
> Hannes


-- 
Damien Le Moal
Western Digital Research