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@xxxxxxxxxx>
>> ---
>> 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
