Messages in this thread | | | Date | Sun, 22 Oct 2023 23:23:40 +0200 | Subject | Re: [PATCH RFC v1 1/3] r8169: fix the KCSAN reported data-race in rtl_tx() while reading tp->cur_tx | From | Heiner Kallweit <> |
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On 22.10.2023 19:24, Mirsad Todorovac wrote: > On 9/27/23 21:52, Heiner Kallweit 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. > > Hi, Mr. Kallweit, > > Your words have been taken with great weight and serious concern on my behalf, so I took > some time to investigate (even after the patches were Ack-ed). > No need to be so formal. I'm just a hobbyist developer who once suddenly found himself in a maintainer position ;)
> The question is whether we deal with a data-race or a false positive reported by KCSAN. > > I can indeed independently confirm that netif_subqueue_completed_wake() eventually expands to > call to: > > include/net/netdev_queues.h: > ============================================== > /* Variant of netdev_tx_completed_queue() which guarantees smp_mb() if > * @bytes != 0, regardless of kernel config. > */ > static inline void > netdev_txq_completed_mb(struct netdev_queue *dev_queue, > unsigned int pkts, unsigned int bytes) > { > if (IS_ENABLED(CONFIG_BQL)) > netdev_tx_completed_queue(dev_queue, pkts, bytes); > else if (bytes) > smp_mb(); > } > > Obviously, smp_mb() is here. > > On my __x86_64__ it expands to an ugly hack: > > #define smp_mb() asm volatile("lock; addl $0,-132(%%rsp)" ::: "memory", "cc") > > Thus far I concur with your findings. > > But I see also Mr. Elver's point: > > 1. With the reference to: [1] https://www.kernel.org/doc/html/v6.2/core-api/wrappers/memory-barriers.html > compiler can do some "smart" optimisations that only work on single core, single threaded use. > > What comes to mind is that "struct rtl8169_private" and its members might not be on the same cache line > with the pointer struct rtl8169_private *tp. > > By the exposed matter in [1], READ_ONCE() provides a way to explain to the compiler that tp->cur_tx can > be concurrently modified by the other thread/core. > > To be honest, I haven't caught a network card error or a driver bug either, but only the KCSAN complaint. > > It would seem prudent to protect all the variables with the concurrent accesses from several cores/threads > or devices through DMA with (READ|WRITE)_ONCE. > r8169 driver supports one queue only. NAPI softirq guarantees that calls to rtl8169_poll are serialized. Threaded NAPI may be a different story, however I'm not aware of any bug report related to threaded NAPI.
> Especially since I don't see where the callback added in drivers/net/ethernet/realtek/r8169_main.c:5277 > is called from: > > netif_napi_add(dev, &tp->napi, rtl8169_poll); > > I am not trying to be an arbiter nor an authority on the rtl8169 driver, which would obviously be you. > > However, I feel compelled by something to review all the accesses to tp->cur_tx, tp->cur_tx and other > structure members against concurrent access, and systematically. With your permission, of course. > > I would appreciate your mentoring on the task if you are interested. > > Mainly, I figured out that in: > > 4575 static int rtl8169_poll(struct napi_struct *napi, int budget) > 4576 { > 4577 struct rtl8169_private *tp = container_of(napi, struct rtl8169_private, napi); > 4578 struct net_device *dev = tp->dev; > 4579 int work_done; > 4580 > 4581 rtl_tx(dev, tp, budget); > 4582 > 4583 work_done = rtl_rx(dev, tp, budget); > 4584 > 4585 if (work_done < budget && napi_complete_done(napi, work_done)) > 4586 rtl_irq_enable(tp); > 4587 > 4588 return work_done; > 4589 } > > rtl_tx() and rtl_rx() are called in sequence. Is it possible to imagine doing it in parallel? > Adding a separated TX NAPI is possible, question is whether it's worth the effort. Look for users of netif_napi_add_tx() to get an idea of separated TX NAPI. I once experimented with it in context of r8169 and didn't really find a benefit.
> Can the Realtek NIC hardware support that at all and which generations can? > Well, major issue with Realtek HW is that there's no public datasheets. Only source of information is their out-of-tree drivers (r8168, r8101, r8125).
With regard to RSS: r8125 supports it, r8168 not.
> (Of course, Ethernet might be full-duplex, but PCI Express line might not be.) > > [2] https://www.rambus.com/blogs/pci-express-4/ Full-duplex PCI 4.0 specs. > > Thank you. > > Best regards, > Mirsad Todorovac > >>> 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> >>> --- >>> 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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