On Tue, 7 Jul 2020 17:28:04 +0206 John Ogness wrote:
> A busy-wait loop is used to implement waiting for bits to be copied
> from the skb to the kernel buffer before retiring a block. This is
> a problem on PREEMPT_RT because the copying task could be preempted
> by the busy-waiting task and thus live lock in the busy-wait loop.
>
> Replace the busy-wait logic with an rwlock_t. This provides lockdep
> coverage and makes the code RT ready.
>
> Signed-off-by: John Ogness <john.ogn...@linutronix.de>
Is taking a lock and immediately releasing it better than a completion?
Seems like the lock is guaranteed to dirty a cache line, which would
otherwise be avoided here.
Willem, would you be able to take a look as well? Is this path
performance sensitive in real life?
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 29bd405adbbd..dd1eec2dd6ef 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -593,6 +593,7 @@ static void init_prb_bdqc(struct packet_sock *po,
> req_u->req3.tp_block_size);
> p1->tov_in_jiffies = msecs_to_jiffies(p1->retire_blk_tov);
> p1->blk_sizeof_priv = req_u->req3.tp_sizeof_priv;
> + rwlock_init(&p1->blk_fill_in_prog_lock);
>
> p1->max_frame_len = p1->kblk_size - BLK_PLUS_PRIV(p1->blk_sizeof_priv);
> prb_init_ft_ops(p1, req_u);
> @@ -659,10 +660,9 @@ static void prb_retire_rx_blk_timer_expired(struct
> timer_list *t)
> *
> */
> if (BLOCK_NUM_PKTS(pbd)) {
> - while (atomic_read(&pkc->blk_fill_in_prog)) {
> - /* Waiting for skb_copy_bits to finish... */
> - cpu_relax();
> - }
> + /* Waiting for skb_copy_bits to finish... */
> + write_lock(&pkc->blk_fill_in_prog_lock);
> + write_unlock(&pkc->blk_fill_in_prog_lock);
> }
>
> if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
> @@ -921,10 +921,9 @@ static void prb_retire_current_block(struct
> tpacket_kbdq_core *pkc,
> * the timer-handler already handled this case.
> */
> if (!(status & TP_STATUS_BLK_TMO)) {
> - while (atomic_read(&pkc->blk_fill_in_prog)) {
> - /* Waiting for skb_copy_bits to finish... */
> - cpu_relax();
> - }
> + /* Waiting for skb_copy_bits to finish... */
> + write_lock(&pkc->blk_fill_in_prog_lock);
> + write_unlock(&pkc->blk_fill_in_prog_lock);
> }
> prb_close_block(pkc, pbd, po, status);
> return;
> @@ -944,7 +943,8 @@ static int prb_queue_frozen(struct tpacket_kbdq_core *pkc)
> static void prb_clear_blk_fill_status(struct packet_ring_buffer *rb)
> {
> struct tpacket_kbdq_core *pkc = GET_PBDQC_FROM_RB(rb);
> - atomic_dec(&pkc->blk_fill_in_prog);
> +
> + read_unlock(&pkc->blk_fill_in_prog_lock);
> }
>
> static void prb_fill_rxhash(struct tpacket_kbdq_core *pkc,
> @@ -998,7 +998,7 @@ static void prb_fill_curr_block(char *curr,
> pkc->nxt_offset += TOTAL_PKT_LEN_INCL_ALIGN(len);
> BLOCK_LEN(pbd) += TOTAL_PKT_LEN_INCL_ALIGN(len);
> BLOCK_NUM_PKTS(pbd) += 1;
> - atomic_inc(&pkc->blk_fill_in_prog);
> + read_lock(&pkc->blk_fill_in_prog_lock);
> prb_run_all_ft_ops(pkc, ppd);
> }
>
> diff --git a/net/packet/internal.h b/net/packet/internal.h
> index 907f4cd2a718..fd41ecb7f605 100644
> --- a/net/packet/internal.h
> +++ b/net/packet/internal.h
> @@ -39,7 +39,7 @@ struct tpacket_kbdq_core {
> char *nxt_offset;
> struct sk_buff *skb;
>
> - atomic_t blk_fill_in_prog;
> + rwlock_t blk_fill_in_prog_lock;
>
> /* Default is set to 8ms */
> #define DEFAULT_PRB_RETIRE_TOV (8)
