Re: [PATCH] lightnvm: pblk: fix race condition on GC

["Javier González" <javier@javigon.com>]

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> On 27 Jan 2019, at 07.54, Heiner Litz <hlitz@ucsc.edu> wrote:
> 
> This patch fixes a race condition where a write is mapped to the last
> sectors of a line. The write is synced to the device but the L2P is not
> updated yet. When the line is garbage collected before the L2P update is
> performed, the sectors are ignored by the GC logic and the line is freed
> before all sectors are moved. When the L2P is finally updated, it contains
> a mapping to a freed line, subsequent reads of the corresponding LBAs fail.

Hi Heiner,

This has been an interesting issue to debug - good catch!

> 
> Note that looking up the L2P and checking the ppa in the write buffer needs
> to be performed atomically, hence the refactor of pblk_lookup_l2p_rand.
> 
> Signed-off-by: Heiner Litz <hlitz@ucsc.edu>
> ---
> drivers/lightnvm/pblk-read.c | 27 +++++++++++++++++++++++++--
> 1 file changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/lightnvm/pblk-read.c b/drivers/lightnvm/pblk-read.c
> index 3789185144da..7c556b2218e4 100644
> --- a/drivers/lightnvm/pblk-read.c
> +++ b/drivers/lightnvm/pblk-read.c
> @@ -529,13 +529,35 @@ static int read_ppalist_rq_gc(struct pblk *pblk, struct nvm_rq *rqd,
> 	int valid_secs = 0;
> 	int i;
> 
> -	pblk_lookup_l2p_rand(pblk, ppa_list_l2p, lba_list, nr_secs);
> -
> +	spin_lock(&pblk->trans_lock);
> 	for (i = 0; i < nr_secs; i++) {
> 		if (lba_list[i] == ADDR_EMPTY)
> 			continue;
> 
> +		ppa_list_l2p[i] = pblk_trans_map_get(pblk, lba_list[i]);
> 		ppa_gc = addr_to_gen_ppa(pblk, paddr_list_gc[i], line->id);
> +
> +		/* Obtain ppa from cache if the sector has been synced to the
> +		   device but the L2P has not been updated yet */
> +		if(pblk_addr_in_cache(ppa_list_l2p[i])) {
> +			struct pblk_rb *rb = &pblk->rwb;
> +			struct pblk_rb_entry *entry;
> +			struct pblk_w_ctx *w_ctx;
> +			u64 pos = pblk_addr_to_cacheline(ppa_list_l2p[i]);
> +
> +#ifdef CONFIG_NVM_PBLK_DEBUG
> +			/* Ensure that the access will not cause an overflow */
> +			BUG_ON(pos >= rb->nr_entries);
> +#endif
> +
> +			entry = &rb->entries[pos];
> +			w_ctx = &entry->w_ctx;
> +			if (pblk_ppa_comp(w_ctx->ppa, ppa_gc)) {
> +				rqd->ppa_list[valid_secs++] = ppa_gc;
> +				continue;
> +			}
> +		}
> +
> 		if (!pblk_ppa_comp(ppa_list_l2p[i], ppa_gc)) {
> 			paddr_list_gc[i] = lba_list[i] = ADDR_EMPTY;
> 			continue;
> @@ -543,6 +565,7 @@ static int read_ppalist_rq_gc(struct pblk *pblk, struct nvm_rq *rqd,
> 
> 		rqd->ppa_list[valid_secs++] = ppa_list_l2p[i];
> 	}
> +	spin_unlock(&pblk->trans_lock);
> 
> #ifdef CONFIG_NVM_PBLK_DEBUG
> 	atomic_long_add(valid_secs, &pblk->inflight_reads);
> --
> 2.17.1


Here is a suggestion: Why not add an atomic counter to the
line stating the sectors that are synced in the L2P table and then
loosely wait (i.e., check and sleep / schedule) until the counter
reaches 0 on pblk_line_close_ws()? This way you guarantee that the line
does not close - and therefore never reaches the GC lists - before all
the L2P entries for that line point to the media. Any other form of
synchronization that puts the burden at pblk_line_close_ws() would also
work for me.

In essence, I would rather pay the price on a per-line basis than
blocking the trans_lock longer for each I/O.

Thoughts?

Javier


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