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[34.150.230.223]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5ecc64cb602sm1627480e0c.7.2026.11.06.07.58.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 07:58:56 -0800 (PST) Date: Fri, 06 Nov 2026 10:58:56 -0500 From: Willem de Bruijn To: Jingxuan Xie , netdev@vger.kernel.org Cc: Willem de Bruijn , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , John Ogness , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Message-ID: In-Reply-To: <20261103092541.273847-2-jingxuanxie@deeproute.ai> References: <20261103092541.273847-1-jingxuanxie@deeproute.ai> <20261103092541.273847-2-jingxuanxie@deeproute.ai> Subject: Re: [RFC PATCH net v3 1/1] af_packet: avoid waiting for TPACKET_V3 copies at retirement X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Jingxuan Xie wrote: > From: jingxuanxie > > TPACKET_V3 retires a full or expired block while holding > sk_receive_queue.lock. prb_retire_current_block() takes > blk_fill_in_prog_lock for write to wait for in-flight copy_bits() > before publishing the block. > > On PREEMPT_RT a copier can be preempted while holding the read lock. > The packet path or the retirement timer then blocks on the write lock > while holding sk_receive_queue.lock, and the receive path resumes only > after the copier is scheduled again. > > Try to take the write lock without blocking. If a copy is still in > progress, leave the current block and its metadata untouched: > > - the retirement timer re-arms itself and retries after the configured > interval; > - packet allocation returns NULL, so tpacket_rcv() drops this capture > record and accounts it in tp_drops, while the original packet remains > in the network stack; > - a later packet can retire the block once the copier has finished. > > Do not dispatch the next block while the current block is still open. > The existing lock order and completion ordering are preserved, and no > new state or worker is introduced. The trade-off is a higher capture > drop rate while a copier is preempted; original traffic is unaffected. > > Fixes: 632ca50f2cbd ("af_packet: TPACKET_V3: replace busy-wait loop") > Assisted-by: LLM > Signed-off-by: Jingxuan Xie No need for a cover letter for a single patch. And a v4 will be needed, because this is sent as RFC. > --- > net/packet/af_packet.c | 24 ++++++++++++++---------- > 1 file changed, 14 insertions(+), 10 deletions(-) > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c > index cf6429f09477..8fd6372ab59b 100644 > --- a/net/packet/af_packet.c > +++ b/net/packet/af_packet.c > @@ -200,8 +200,8 @@ static void packet_increment_head(struct packet_ring_buffer *buff); > static int prb_curr_blk_in_use(struct tpacket_block_desc *); > static void *prb_dispatch_next_block(struct tpacket_kbdq_core *, > struct packet_sock *); > -static void prb_retire_current_block(struct tpacket_kbdq_core *, > - struct packet_sock *, unsigned int status); > +static bool prb_retire_current_block(struct tpacket_kbdq_core *, > + struct packet_sock *, unsigned int status); no unrelated style changes > static int prb_queue_frozen(struct tpacket_kbdq_core *); > static void prb_open_block(struct tpacket_kbdq_core *, > struct tpacket_block_desc *); > @@ -711,8 +711,9 @@ static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *t) > * > */ > if (BLOCK_NUM_PKTS(pbd)) { > - /* Waiting for skb_copy_bits to finish... */ > - write_lock(&pkc->blk_fill_in_prog_lock); > + /* Retry later if a packet copy is still in progress. */ > + if (!write_trylock(&pkc->blk_fill_in_prog_lock)) > + goto refresh_timer; Since this is only a problem on RT, and it introduces drops, consider limiting the switch from write_lock to write_trylock to only RT. > write_unlock(&pkc->blk_fill_in_prog_lock); > } > > @@ -738,6 +739,7 @@ static enum hrtimer_restart prb_retire_rx_blk_timer_expired(struct hrtimer *t) > } > } > > +refresh_timer: > hrtimer_forward_now(&pkc->retire_blk_timer, pkc->interval_ktime); > spin_unlock(&po->sk.sk_receive_queue.lock); > return HRTIMER_RESTART; > @@ -946,8 +948,8 @@ static void *prb_dispatch_next_block(struct tpacket_kbdq_core *pkc, > return (void *)pkc->nxt_offset; > } > > -static void prb_retire_current_block(struct tpacket_kbdq_core *pkc, > - struct packet_sock *po, unsigned int status) > +static bool prb_retire_current_block(struct tpacket_kbdq_core *pkc, > + struct packet_sock *po, unsigned int status) again, unnecessary style changes > { > struct tpacket_block_desc *pbd = GET_CURR_PBLOCK_DESC_FROM_CORE(pkc); > > @@ -963,13 +965,14 @@ static void prb_retire_current_block(struct tpacket_kbdq_core *pkc, > * the timer-handler already handled this case. > */ > if (!(status & TP_STATUS_BLK_TMO)) { > - /* Waiting for skb_copy_bits to finish... */ > - write_lock(&pkc->blk_fill_in_prog_lock); > + /* Do not wait for copies while holding the queue lock. */ > + if (!write_trylock(&pkc->blk_fill_in_prog_lock)) > + return false; > write_unlock(&pkc->blk_fill_in_prog_lock); > } > prb_close_block(pkc, pbd, po, status); > - return; > } > + return true; > } > > static int prb_curr_blk_in_use(struct tpacket_block_desc *pbd) > @@ -1097,7 +1100,8 @@ static void *__packet_lookup_frame_in_block(struct packet_sock *po, > } > > /* Ok, close the current block */ > - prb_retire_current_block(pkc, po, 0); > + if (!prb_retire_current_block(pkc, po, 0)) > + return NULL; > > /* Now, try to dispatch the next block */ > curr = (char *)prb_dispatch_next_block(pkc, po);