On 20/06/2022 07:07, Damien Le Moal wrote:
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().
Right, so I could change to use __set_bit() in pm8001_find_tag(), but
rather use spinlock always.
After the patch, spinlock guarantees atomicity for both alloc and free.
Not sure there is any gain from this.
A few more points to note:
- On architectures which do not support atomic operations natively, they
have to use global spinlocks to create atomic context before doing
non-atomic bit clearing - see atomic64.c . As such, it's better to use
the already available pm8001_ha->bitmap_lock.
- spinlock does more than create atomic context, but also has barrier
semantics, so proper to use consistently for protecting the same region.
Thanks,
John
