Re: [PATCH RFC v1 1/3] r8169: fix the KCSAN reported data-race in rtl_tx() while reading tp->cur_tx

From: Marco Elver
Date: Thu Sep 28 2023 - 02:03:46 EST

On Wed, 27 Sept 2023 at 21:52, Heiner Kallweit <hkallweit1@xxxxxxxxx> wrote:
>
> On 27.09.2023 20:41, Mirsad Goran Todorovac wrote:
> > KCSAN reported the following data-race:
> >
> > ==================================================================
> > BUG: KCSAN: data-race in rtl8169_poll [r8169] / rtl8169_start_xmit [r8169]
> >
> > write (marked) to 0xffff888102474b74 of 4 bytes by task 5358 on cpu 29:
> > rtl8169_start_xmit (drivers/net/ethernet/realtek/r8169_main.c:4254) r8169
> > dev_hard_start_xmit (./include/linux/netdevice.h:4889 ./include/linux/netdevice.h:4903 net/core/dev.c:3544 net/core/dev.c:3560)
> > sch_direct_xmit (net/sched/sch_generic.c:342)
> > __dev_queue_xmit (net/core/dev.c:3817 net/core/dev.c:4306)
> > ip_finish_output2 (./include/linux/netdevice.h:3082 ./include/net/neighbour.h:526 ./include/net/neighbour.h:540 net/ipv4/ip_output.c:233)
> > __ip_finish_output (net/ipv4/ip_output.c:311 net/ipv4/ip_output.c:293)
> > ip_finish_output (net/ipv4/ip_output.c:328)
> > ip_output (net/ipv4/ip_output.c:435)
> > ip_send_skb (./include/net/dst.h:458 net/ipv4/ip_output.c:127 net/ipv4/ip_output.c:1486)
> > udp_send_skb (net/ipv4/udp.c:963)
> > udp_sendmsg (net/ipv4/udp.c:1246)
> > inet_sendmsg (net/ipv4/af_inet.c:840 (discriminator 4))
> > sock_sendmsg (net/socket.c:730 net/socket.c:753)
> > __sys_sendto (net/socket.c:2177)
> > __x64_sys_sendto (net/socket.c:2185)
> > do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
> > entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
> >
> > read to 0xffff888102474b74 of 4 bytes by interrupt on cpu 21:
> > rtl8169_poll (drivers/net/ethernet/realtek/r8169_main.c:4397 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)
> > common_interrupt (arch/x86/kernel/irq.c:247 (discriminator 14))
> > asm_common_interrupt (./arch/x86/include/asm/idtentry.h:636)
> > 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: 0x002f4815 -> 0x002f4816
> >
> > 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 write side of drivers/net/ethernet/realtek/r8169_main.c is:
> > ==================
> > 4251 /* rtl_tx needs to see descriptor changes before updated tp->cur_tx */
> > 4252 smp_wmb();
> > 4253
> > → 4254 WRITE_ONCE(tp->cur_tx, tp->cur_tx + frags + 1);
> > 4255
> > 4256 stop_queue = !netif_subqueue_maybe_stop(dev, 0, rtl_tx_slots_avail(tp),
> > 4257 R8169_TX_STOP_THRS,
> > 4258 R8169_TX_START_THRS);
> >
> > The read side is the function rtl_tx():
> >
> > 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 (tp->cur_tx != dirty_tx && skb)
> > 4398 rtl8169_doorbell(tp);
> > 4399 }
> > 4400 }
> >
> > Obviously from the code, an earlier detected data-race for tp->cur_tx was fixed in the
> > line 4363:
> >
> > 4363 while (READ_ONCE(tp->cur_tx) != dirty_tx) {
> >
> > but the same solution is required for protecting the other access to tp->cur_tx:
> >
> > → 4397 if (READ_ONCE(tp->cur_tx) != dirty_tx && skb)
> > 4398 rtl8169_doorbell(tp);
> >
> > The write in the line 4254 is protected with WRITE_ONCE(), but the read in the line 4397
> > might have suffered read tearing under some compiler optimisations.
> >
> > The fix eliminated the KCSAN data-race report for this bug.
> >
> > It is yet to be evaluated what happens if tp->cur_tx changes between the test in line 4363
> > and line 4397. This test should certainly not be cached by the compiler in some register
> > for such a long time, while asynchronous writes to tp->cur_tx might have occurred in line
> > 4254 in the meantime.
> >
>
> netif_subqueue_completed_wake() has barriers ensuring that no cached value for tp->cur_tx
> is used in line 4397. I'm not aware of any reported issues with an obvious link to the
> potentential issue you describe.
> I don't have a strong opinion on these patches. They shouldn't hurt, and if they make
> KCSAN happy, why not.

Barries don't protect unmarked accesses from being miscompiled. So the
use of barriers and marked accesses like READ_ONCE() is correct:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
That said, actually encountering a miscompilation depends on
architecture and compiler. Using the right marked accesses just
ensures things don't suddenly break because the compiler decided to be
a little more clever.

> > Fixes: 94d8a98e6235c ("r8169: reduce number of workaround doorbell rings")
> > 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>

> > ---
> > v1:
> > the initial patch proposal. fixes the KCSAN warning.
> >
> > drivers/net/ethernet/realtek/r8169_main.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
> > index 6351a2dc13bc..281aaa851847 100644
> > --- a/drivers/net/ethernet/realtek/r8169_main.c
> > +++ b/drivers/net/ethernet/realtek/r8169_main.c
> > @@ -4394,7 +4394,7 @@ static void rtl_tx(struct net_device *dev, struct rtl8169_private *tp,
> > * If skb is NULL then we come here again once a tx irq is
> > * triggered after the last fragment is marked transmitted.
> > */
> > - if (tp->cur_tx != dirty_tx && skb)
> > + if (READ_ONCE(tp->cur_tx) != dirty_tx && skb)
> > rtl8169_doorbell(tp);
> > }
> > }
>