Re: [PATCH v2 3/3] r8169: fix the KCSAN reported data-race in rtl_tx while reading TxDescArray[entry].opts1

From: Simon Horman
Date: Wed Oct 18 2023 - 09:56:35 EST

On Tue, Oct 17, 2023 at 10:43:36PM +0200, Mirsad Todorovac wrote:
> On 10/17/23 22:01, Simon Horman wrote:
> > On Mon, Oct 16, 2023 at 11:47:56PM +0200, Mirsad Goran Todorovac wrote:
> > > KCSAN reported the following data-race:
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4368 drivers/net/ethernet/realtek/r8169_main.c:4581) r8169
> > >
> > > race at unknown origin, with read to 0xffff888140d37570 of 4 bytes by interrupt on cpu 21:
> > > rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4368 drivers/net/ethernet/realtek/r8169_main.c:4581) r8169
> > > __napi_poll (net/core/dev.c:6527)
> > > net_rx_action (net/core/dev.c:6596 net/core/dev.c:6727)
> > > __do_softirq (kernel/softirq.c:553)
> > > __irq_exit_rcu (kernel/softirq.c:427 kernel/softirq.c:632)
> > > irq_exit_rcu (kernel/softirq.c:647)
> > > sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1074 (discriminator 14))
> > > asm_sysvec_apic_timer_interrupt (./arch/x86/include/asm/idtentry.h:645)
> > > cpuidle_enter_state (drivers/cpuidle/cpuidle.c:291)
> > > cpuidle_enter (drivers/cpuidle/cpuidle.c:390)
> > > call_cpuidle (kernel/sched/idle.c:135)
> > > do_idle (kernel/sched/idle.c:219 kernel/sched/idle.c:282)
> > > cpu_startup_entry (kernel/sched/idle.c:378 (discriminator 1))
> > > start_secondary (arch/x86/kernel/smpboot.c:210 arch/x86/kernel/smpboot.c:294)
> > > secondary_startup_64_no_verify (arch/x86/kernel/head_64.S:433)
> > >
> > > value changed: 0xb0000042 -> 0x00000000
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 21 PID: 0 Comm: swapper/21 Tainted: G L 6.6.0-rc2-kcsan-00143-gb5cbe7c00aa0 #41
> > > Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
> > > ==================================================================
> > >
> > > The read side is in
> > >
> > > drivers/net/ethernet/realtek/r8169_main.c
> > > =========================================
> > > 4355 static void rtl_tx(struct net_device *dev, struct rtl8169_private *tp,
> > > 4356 int budget)
> > > 4357 {
> > > 4358 unsigned int dirty_tx, bytes_compl = 0, pkts_compl = 0;
> > > 4359 struct sk_buff *skb;
> > > 4360
> > > 4361 dirty_tx = tp->dirty_tx;
> > > 4362
> > > 4363 while (READ_ONCE(tp->cur_tx) != dirty_tx) {
> > > 4364 unsigned int entry = dirty_tx % NUM_TX_DESC;
> > > 4365 u32 status;
> > > 4366
> > > → 4367 status = le32_to_cpu(tp->TxDescArray[entry].opts1);
> > > 4368 if (status & DescOwn)
> > > 4369 break;
> > > 4370
> > > 4371 skb = tp->tx_skb[entry].skb;
> > > 4372 rtl8169_unmap_tx_skb(tp, entry);
> > > 4373
> > > 4374 if (skb) {
> > > 4375 pkts_compl++;
> > > 4376 bytes_compl += skb->len;
> > > 4377 napi_consume_skb(skb, budget);
> > > 4378 }
> > > 4379 dirty_tx++;
> > > 4380 }
> > > 4381
> > > 4382 if (tp->dirty_tx != dirty_tx) {
> > > 4383 dev_sw_netstats_tx_add(dev, pkts_compl, bytes_compl);
> > > 4384 WRITE_ONCE(tp->dirty_tx, dirty_tx);
> > > 4385
> > > 4386 netif_subqueue_completed_wake(dev, 0, pkts_compl, bytes_compl,
> > > 4387 rtl_tx_slots_avail(tp),
> > > 4388 R8169_TX_START_THRS);
> > > 4389 /*
> > > 4390 * 8168 hack: TxPoll requests are lost when the Tx packets are
> > > 4391 * too close. Let's kick an extra TxPoll request when a burst
> > > 4392 * of start_xmit activity is detected (if it is not detected,
> > > 4393 * it is slow enough). -- FR
> > > 4394 * If skb is NULL then we come here again once a tx irq is
> > > 4395 * triggered after the last fragment is marked transmitted.
> > > 4396 */
> > > 4397 if (READ_ONCE(tp->cur_tx) != dirty_tx && skb)
> > > 4398 rtl8169_doorbell(tp);
> > > 4399 }
> > > 4400 }
> > >
> > > tp->TxDescArray[entry].opts1 is reported to have a data-race and READ_ONCE() fixes
> > > this KCSAN warning.
> > >
> > > 4366
> > > → 4367 status = le32_to_cpu(READ_ONCE(tp->TxDescArray[entry].opts1));
> > > 4368 if (status & DescOwn)
> > > 4369 break;
> > > 4370
> > >
> > > Fixes: ^1da177e4c3f4 ("initial git repository build")
> >
> > Hi Mirsad,
> >
> > The fixes tag above seems wrong.
>
> Hi, Simon,
>
> It is taken directly from "git blame" as you can check for yourself.
>
> It is supposed to tag the commits prior to the introduction of git.
>
> If you have a better idea how to denote those, I will be happy to learn,
> but I have no better clue than what "git blame" gives ...

Interesting, thanks for the explanation.
I do think it's more usual, in such cases, to use the following.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

> Best regards,
> Mirsad Todorovac
> > > Cc: Heiner Kallweit <hkallweit1@xxxxxxxxx>
> > > Cc: nic_swsd@xxxxxxxxxxx
> > > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> > > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > > Cc: Marco Elver <elver@xxxxxxxxxx>
> > > Cc: netdev@xxxxxxxxxxxxxxx
> > > Link: https://lore.kernel.org/lkml/dc7fc8fa-4ea4-e9a9-30a6-7c83e6b53188@xxxxxxxxxxxx/
> > > Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>
> > > Acked-by: Marco Elver <elver@xxxxxxxxxx>
> >
> > ...
>