mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2026-07-19 21:40:29 +02:00
Jakub KicinskiandGreg Kroah-Hartman
628e82e270
[ Upstream commitc91c46de6b] A lot of drivers follow the same scheme to stop / start queues without introducing locks between xmit and NAPI tx completions. I'm guessing they all copy'n'paste each other's code. The original code dates back all the way to e1000 and Linux 2.6.19. Smaller drivers shy away from the scheme and introduce a lock which may cause deadlocks in netpoll. Provide macros which encapsulate the necessary logic. The macros do not prevent false wake ups, the extra barrier required to close that race is not worth it. See discussion in: https://lore.kernel.org/all/c39312a2-4537-14b4-270c-9fe1fbb91e89@gmail.com/ Acked-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: Jakub Kicinski <kuba@kernel.org> Stable-dep-of:95ecba62e2("net: fix races in netdev_tx_sent_queue()/dev_watchdog()") Signed-off-by: Sasha Levin <sashal@kernel.org>
125 lines
3.1 KiB
ReStructuredText
125 lines
3.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=====================
|
|
Softnet Driver Issues
|
|
=====================
|
|
|
|
Transmit path guidelines
|
|
========================
|
|
|
|
Stop queues in advance
|
|
----------------------
|
|
|
|
The ndo_start_xmit method must not return NETDEV_TX_BUSY under
|
|
any normal circumstances. It is considered a hard error unless
|
|
there is no way your device can tell ahead of time when its
|
|
transmit function will become busy.
|
|
|
|
Instead it must maintain the queue properly. For example,
|
|
for a driver implementing scatter-gather this means::
|
|
|
|
static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
|
|
struct net_device *dev)
|
|
{
|
|
struct drv *dp = netdev_priv(dev);
|
|
|
|
lock_tx(dp);
|
|
...
|
|
/* This is a hard error log it. */
|
|
if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
|
|
netif_stop_queue(dev);
|
|
unlock_tx(dp);
|
|
printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
|
|
dev->name);
|
|
return NETDEV_TX_BUSY;
|
|
}
|
|
|
|
... queue packet to card ...
|
|
... update tx consumer index ...
|
|
|
|
if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
|
|
netif_stop_queue(dev);
|
|
|
|
...
|
|
unlock_tx(dp);
|
|
...
|
|
return NETDEV_TX_OK;
|
|
}
|
|
|
|
And then at the end of your TX reclamation event handling::
|
|
|
|
if (netif_queue_stopped(dp->dev) &&
|
|
TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
|
|
netif_wake_queue(dp->dev);
|
|
|
|
For a non-scatter-gather supporting card, the three tests simply become::
|
|
|
|
/* This is a hard error log it. */
|
|
if (TX_BUFFS_AVAIL(dp) <= 0)
|
|
|
|
and::
|
|
|
|
if (TX_BUFFS_AVAIL(dp) == 0)
|
|
|
|
and::
|
|
|
|
if (netif_queue_stopped(dp->dev) &&
|
|
TX_BUFFS_AVAIL(dp) > 0)
|
|
netif_wake_queue(dp->dev);
|
|
|
|
Lockless queue stop / wake helper macros
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. kernel-doc:: include/net/netdev_queues.h
|
|
:doc: Lockless queue stopping / waking helpers.
|
|
|
|
No exclusive ownership
|
|
----------------------
|
|
|
|
An ndo_start_xmit method must not modify the shared parts of a
|
|
cloned SKB.
|
|
|
|
Timely completions
|
|
------------------
|
|
|
|
Do not forget that once you return NETDEV_TX_OK from your
|
|
ndo_start_xmit method, it is your driver's responsibility to free
|
|
up the SKB and in some finite amount of time.
|
|
|
|
For example, this means that it is not allowed for your TX
|
|
mitigation scheme to let TX packets "hang out" in the TX
|
|
ring unreclaimed forever if no new TX packets are sent.
|
|
This error can deadlock sockets waiting for send buffer room
|
|
to be freed up.
|
|
|
|
If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you
|
|
must not keep any reference to that SKB and you must not attempt
|
|
to free it up.
|
|
|
|
Probing guidelines
|
|
==================
|
|
|
|
Address validation
|
|
------------------
|
|
|
|
Any hardware layer address you obtain for your device should
|
|
be verified. For example, for ethernet check it with
|
|
linux/etherdevice.h:is_valid_ether_addr()
|
|
|
|
Close/stop guidelines
|
|
=====================
|
|
|
|
Quiescence
|
|
----------
|
|
|
|
After the ndo_stop routine has been called, the hardware must
|
|
not receive or transmit any data. All in flight packets must
|
|
be aborted. If necessary, poll or wait for completion of
|
|
any reset commands.
|
|
|
|
Auto-close
|
|
----------
|
|
|
|
The ndo_stop routine will be called by unregister_netdevice
|
|
if device is still UP.
|