Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage (v2)

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

> > 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.
> > 
> > v2: protect handle_pending_slot_free() with zram rw_lock.
> > 
> 
> zram->slot_free_lock protects zram->slot_free_rq but shouldn't the zram
> rw_lock be wrapped around the whole operation like the original code
> does?  I don't know the zram code, but the original looks like it makes
> sense but in this one it looks like the locks are duplicative.
> 
> Is the down_read() in the original code be changed to down_write()?
>

I'm not touching locking around existing READ/WRITE commands.

the original code:

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

        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);
}

static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
                        int offset, struct bio *bio, int rw)
{
        int ret;

        if (rw == READ) {
                down_read(&zram->lock);
                handle_pending_slot_free(zram);
                ret = zram_bvec_read(zram, bvec, index, offset, bio);
                up_read(&zram->lock);
        } else {
                down_write(&zram->lock);
                handle_pending_slot_free(zram);
                ret = zram_bvec_write(zram, bvec, index, offset);
                up_write(&zram->lock);
        }

        return ret;
}



the new one:

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

        down_write(&zram->lock);
        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);
        up_write(&zram->lock);
}

static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
                        int offset, struct bio *bio, int rw)
{
        int ret;

        if (zram->slot_free_rq)
                handle_pending_slot_free(zram);

        if (rw == READ) {
                down_read(&zram->lock);
                ret = zram_bvec_read(zram, bvec, index, offset, bio);
                up_read(&zram->lock);
        } else {
                down_write(&zram->lock);
                ret = zram_bvec_write(zram, bvec, index, offset);
                up_write(&zram->lock);
        }

        return ret;
}


both READ and WRITE operations are still protected by down_read() for READ path
and down_write() for WRITE path. however, there is no handle_pending_slot_free()
and zram->slot_free_lock locking on every READ/WRITE, instead handle_pending_slot_free()
is called only when zram->slot_free_rq is not NULL. handle_pending_slot_free() in
turn protects zram_free_page() call by down_write(), so no READ/WRITE operations
are affected.

	-ss