kernel-ark/Documentation/networking/netdevices.txt
Herbert Xu 932ff279a4 [NET]: Add netif_tx_lock
Various drivers use xmit_lock internally to synchronise with their
transmission routines.  They do so without setting xmit_lock_owner.
This is fine as long as netpoll is not in use.

With netpoll it is possible for deadlocks to occur if xmit_lock_owner
isn't set.  This is because if a printk occurs while xmit_lock is held
and xmit_lock_owner is not set can cause netpoll to attempt to take
xmit_lock recursively.

While it is possible to resolve this by getting netpoll to use
trylock, it is suboptimal because netpoll's sole objective is to
maximise the chance of getting the printk out on the wire.  So
delaying or dropping the message is to be avoided as much as possible.

So the only alternative is to always set xmit_lock_owner.  The
following patch does this by introducing the netif_tx_lock family of
functions that take care of setting/unsetting xmit_lock_owner.

I renamed xmit_lock to _xmit_lock to indicate that it should not be
used directly.  I didn't provide irq versions of the netif_tx_lock
functions since xmit_lock is meant to be a BH-disabling lock.

This is pretty much a straight text substitution except for a small
bug fix in winbond.  It currently uses
netif_stop_queue/spin_unlock_wait to stop transmission.  This is
unsafe as an IRQ can potentially wake up the queue.  So it is safer to
use netif_tx_disable.

The hamradio bits used spin_lock_irq but it is unnecessary as
xmit_lock must never be taken in an IRQ handler.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
2006-06-17 21:30:14 -07:00

78 lines
2.6 KiB
Plaintext

Network Devices, the Kernel, and You!
Introduction
============
The following is a random collection of documentation regarding
network devices.
struct net_device allocation rules
==================================
Network device structures need to persist even after module is unloaded and
must be allocated with kmalloc. If device has registered successfully,
it will be freed on last use by free_netdev. This is required to handle the
pathologic case cleanly (example: rmmod mydriver </sys/class/net/myeth/mtu )
There are routines in net_init.c to handle the common cases of
alloc_etherdev, alloc_netdev. These reserve extra space for driver
private data which gets freed when the network device is freed. If
separately allocated data is attached to the network device
(dev->priv) then it is up to the module exit handler to free that.
struct net_device synchronization rules
=======================================
dev->open:
Synchronization: rtnl_lock() semaphore.
Context: process
dev->stop:
Synchronization: rtnl_lock() semaphore.
Context: process
Note1: netif_running() is guaranteed false
Note2: dev->poll() is guaranteed to be stopped
dev->do_ioctl:
Synchronization: rtnl_lock() semaphore.
Context: process
dev->get_stats:
Synchronization: dev_base_lock rwlock.
Context: nominally process, but don't sleep inside an rwlock
dev->hard_start_xmit:
Synchronization: netif_tx_lock spinlock.
When the driver sets NETIF_F_LLTX in dev->features this will be
called without holding netif_tx_lock. In this case the driver
has to lock by itself when needed. It is recommended to use a try lock
for this and return -1 when the spin lock fails.
The locking there should also properly protect against
set_multicast_list
Context: BHs disabled
Notes: netif_queue_stopped() is guaranteed false
Interrupts must be enabled when calling hard_start_xmit.
(Interrupts must also be enabled when enabling the BH handler.)
Return codes:
o NETDEV_TX_OK everything ok.
o NETDEV_TX_BUSY Cannot transmit packet, try later
Usually a bug, means queue start/stop flow control is broken in
the driver. Note: the driver must NOT put the skb in its DMA ring.
o NETDEV_TX_LOCKED Locking failed, please retry quickly.
Only valid when NETIF_F_LLTX is set.
dev->tx_timeout:
Synchronization: netif_tx_lock spinlock.
Context: BHs disabled
Notes: netif_queue_stopped() is guaranteed true
dev->set_multicast_list:
Synchronization: netif_tx_lock spinlock.
Context: BHs disabled
dev->poll:
Synchronization: __LINK_STATE_RX_SCHED bit in dev->state. See
dev_close code and comments in net/core/dev.c for more info.
Context: softirq