af_packet: TPACKET_V3: replace busy-wait loop

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.ogness@linutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 7b436eb..781fee9 100644 (file)
--- 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 907f4cd..fd41ecb 100644 (file)
--- 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)