Re: [PATCH RFC v1 1/3] r8169: fix the KCSAN reported data-race in rtl_tx() while reading tp->cur_tx
From: Mirsad Goran Todorovac
Date: Fri Oct 13 2023 - 01:02:10 EST
On 9/28/2023 8:02 AM, Marco Elver wrote:
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>
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
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
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