Messages in this thread | | | From | Marco Elver <> | Date | Thu, 28 Sep 2023 08:02:58 +0200 | Subject | Re: [PATCH RFC v1 1/3] r8169: fix the KCSAN reported data-race in rtl_tx() while reading tp->cur_tx |
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On Wed, 27 Sept 2023 at 21:52, Heiner Kallweit <hkallweit1@gmail.com> 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@gmail.com> > > Cc: nic_swsd@realtek.com > > Cc: "David S. Miller" <davem@davemloft.net> > > Cc: Eric Dumazet <edumazet@google.com> > > Cc: Jakub Kicinski <kuba@kernel.org> > > Cc: Paolo Abeni <pabeni@redhat.com> > > Cc: Marco Elver <elver@google.com> > > Cc: netdev@vger.kernel.org > > Link: https://lore.kernel.org/lkml/dc7fc8fa-4ea4-e9a9-30a6-7c83e6b53188@alu.unizg.hr/ > > Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Acked-by: Marco Elver <elver@google.com>
> > --- > > 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); > > } > > } >
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