Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (09/09/13 11:33), Dan Carpenter wrote:
> On Fri, Sep 06, 2013 at 05:55:45PM +0300, Sergey Senozhatsky wrote:
> > On (09/06/13 16:42), Jerome Marchand wrote:
> > > On 09/06/2013 03:47 PM, Sergey Senozhatsky wrote:
> > > > Calling handle_pending_slot_free() for every RW operation may
> > > > cause unneccessary slot_free_lock locking, because most likely
> > > > process will see NULL slot_free_rq. handle_pending_slot_free()
> > > > only when current detects that slot_free_rq is not NULL.
> > > > 
> > > > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > > > 
> > > > ---
> > > > 
> > > >  drivers/staging/zram/zram_drv.c | 5 +++--
> > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> > > > index 91d94b5..17386e2 100644
> > > > --- a/drivers/staging/zram/zram_drv.c
> > > > +++ b/drivers/staging/zram/zram_drv.c
> > > > @@ -532,14 +532,15 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
> > > >  {
> > > >  	int ret;
> > > >  
> > > > +	if (zram->slot_free_rq)
> > > > +		handle_pending_slot_free(zram);
> > > > +
> > > 
> > > Calling handle_pending_slot_free() without holding zram->lock?
> > > That's racy.
> > 
> > sorry, my bad. it should take down_write() lock.
> > 
> 
> Or down_read() on the read path.  We leave the original as-is?
> 

Hello,

down_write() for both READ and WRITE looks ok to me (+down_write()
for zram_slot_free()). is there something I miss?

down_read() for READ in case of N active readers will force N-1
processes to spin on zram->slot_free_lock in handle_pending_slot_free().

it probably makes sense to add extra zram->slot_free_rq check for
the case when process slept on rw lock while someone was freeing
pages:

static void handle_pending_slot_free(struct zram *zram)
{
        struct zram_slot_free *free_rq;

        down_write(&zram->lock);
+	if (!zram->slot_free_rq)
+		goto out;
        spin_lock(&zram->slot_free_lock);
        while (zram->slot_free_rq) {
                free_rq = zram->slot_free_rq;
                zram->slot_free_rq = free_rq->next;
                zram_free_page(zram, free_rq->index);
                kfree(free_rq);
        }
        spin_unlock(&zram->slot_free_lock);
+out:
        up_write(&zram->lock);
}

	-ss

> regards,
> dan carpenter
>