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net: usb: cdc_ncm: don't enable BHs in tx_fixup under netpoll

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netpoll holds the txq lock and calls ndo_start_xmit() with IRQs off but
BHs on. The spin_unlock_bh() in cdc_ncm_tx_fixup() then runs pending
softirqs. If one of them transmits, it spins on the txq lock that
netpoll holds. With netconsole on a cdc_ncm device this ends in a soft
or hard lockup.

Every caller of cdc_ncm_tx_fixup() holds the txq lock with BHs or IRQs
off, so the softirq user of ctx->mtx cannot interrupt it. Use plain
spin_lock().

Fixes: c84ff1d6df ("cdc_ncm: reduce driver latency in the data path")
Reported-by: Mikkel Eskildsen <mke@visotia.com>
Closes: https://lore.kernel.org/netdev/PRAP189MB185019A5A8D411482BB6E0CDA9982@PRAP189MB1850.EURP189.PROD.OUTLOOK.COM/
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261024-cdc-ncm-netpoll-v3-1-6284ca03f967@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Nicolai Buchwitz 2026-10-24 15:23:06 +02:00 committed by Jakub Kicinski
parent 9e2ba4a177
commit 108ca34cf4
1 changed files with 5 additions and 2 deletions
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7
drivers/net/usb/cdc_ncm.c
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@ -1535,14 +1535,17 @@ cdc_ncm_tx_fixup(struct usbnet *dev, struct sk_buff *skb, gfp_t flags)
if (ctx == NULL)
goto error;
spin_lock_bh(&ctx->mtx);
/* BHs or IRQs are always off here. netpoll only disables IRQs, and
* spin_unlock_bh() would then run softirqs with the txq lock held.
*/
spin_lock(&ctx->mtx);
if (ctx->is_ndp16)
skb_out = cdc_ncm_fill_tx_frame(dev, skb, cpu_to_le32(USB_CDC_NCM_NDP16_NOCRC_SIGN));
else
skb_out = cdc_ncm_fill_tx_frame(dev, skb, cpu_to_le32(USB_CDC_NCM_NDP32_NOCRC_SIGN));
spin_unlock_bh(&ctx->mtx);
spin_unlock(&ctx->mtx);
return skb_out;
error: