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Date:   Fri, 13 Oct 2023 07:01:15 +0200
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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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To:     Marco Elver <elver@google.com>,
        Heiner Kallweit <hkallweit1@gmail.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        nic_swsd@realtek.com, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>
References: <20230927184158.243575-1-mirsad.todorovac@alu.unizg.hr>
 <0a201a6f-90dd-403c-97d0-94372be1e3e6@gmail.com>
 <CANpmjNP=jY2NzrXX4mgY5cL5tqZeLb18gQkzVhg1OofNtJ1VSw@mail.gmail.com>
From:   Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
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Organization: Academy of Fine Arts, University of Zagreb
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On 9/28/2023 8:02 AM, Marco Elver wrote:
> 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>

Hi, Marco,

Does this Acked-by: cover all of the [123]/3 in the patch series?

I guess I should resubmit the patches as the formal ones as patchwork will
not pick up a PATH RFC?

Thanks,
Mirsad Todorovac

>>> ---
>>> 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);
>>>        }
>>>   }
>>

-- 
Mirsad Todorovac
Sistem inženjer
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