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- [Medium] mana_change_mtu() adds a new use of the boolean
channel_changing as an ad-hoc lock.
--
net: mana: swap queue sets in mana_change_mtu
With this patch, mana_change_mtu() builds a full replacement queue set
sized for the new MTU and swaps it in. It no longer does a detach and
attach. The MTU is carried in the queue set as configured_mtu, and
ndev->mtu is published only after RSS configuration succeeds.
> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c
> b/drivers/net/ethernet/microsoft/mana/mana_en.c
> index c80a06ffbe508..c1ff4d9b8d45f 100644
> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
> @@ -956,32 +956,52 @@ int mana_pre_alloc_rxbufs(struct mana_port_context
> *mpc, int new_mtu, int num_qu
> static int mana_change_mtu(struct net_device *ndev, int new_mtu)
> {
[ ... ]
> + /* Fail-close may release the Ethernet vport reference. */
> + mutex_lock(&mpc->vport_mutex);
> + if (mpc->channel_changing) {
> + mutex_unlock(&mpc->vport_mutex);
> + return -EBUSY;
> + }
> + mpc->channel_changing = true;
> + mutex_unlock(&mpc->vport_mutex);
> +
> + scratch = mana_qset_scratch_alloc(mpc);
> + if (!scratch) {
> + err = -ENOMEM;
> + goto clear_flag;
> }
>
> - WRITE_ONCE(ndev->mtu, new_mtu);
> + err = mana_alloc_qset(mpc, scratch, mpc->num_queues,
> + mpc->rx_queue_size, mpc->tx_queue_size,
> + mpc->priv_flags, new_mtu, &newq);
> + if (err)
> + goto free_scratch;
>
> - err = mana_attach(ndev);
> + err = mana_publish_qset(mpc, &newq, &oldq);
> if (err) {
> - netdev_err(ndev, "mana_attach failed: %d\n", err);
> - WRITE_ONCE(ndev->mtu, old_mtu);
> + mana_free_qset(scratch, &newq);
> + goto free_scratch;
> }
>
> -out:
> - mana_pre_dealloc_rxbufs(mpc);
> + mana_free_qset(scratch, &oldq);
> +
> +free_scratch:
> + mana_publish_close_if_needed(mpc);
> + mana_qset_scratch_free(scratch);
> +clear_flag:
> + mutex_lock(&mpc->vport_mutex);
> + mpc->channel_changing = false;
> + mutex_unlock(&mpc->vport_mutex);
> return err;
> }
[Severity: Medium]
Here, channel_changing seems to act as a lock around a code section rather
than as a record of object state. Is that the intent?
The flag is set under vport_mutex, and then the mutex is dropped. The whole
sleeping replacement runs after that: mana_qset_scratch_alloc(),
mana_alloc_qset(), mana_publish_qset(), mana_free_qset(),
mana_publish_close_if_needed() and mana_qset_scratch_free(). The flag is
cleared again at clear_flag.
The only reader is on the RDMA side, and it fails instead of waiting:
mana_cfg_vport() {
...
mutex_lock(&apc->vport_mutex);
if (apc->vport_use_count > 0 ||
(check_channel_changing && apc->channel_changing)) {
mutex_unlock(&apc->vport_mutex);
return -EBUSY;
}
...
}
As a result, RAW-QP creation gets a transient -EBUSY during any MTU change
on a running port. Lockdep also cannot see this exclusion.
vport_mutex probably can't just be held across the region, because the
fail-close path takes it itself:
mana_publish_close_if_needed()
-> mana_dealloc_queues()
-> mana_destroy_vport()
-> mana_uncfg_vport()
-> mutex_lock(&apc->vport_mutex)
Can the -EBUSY check at the top of mana_change_mtu() ever trigger? Every
writer of channel_changing holds RTNL for its whole region, and RDMA only
reads the flag. The writers are mana_change_mtu(), the queue reset work,
the set_channels, set_ringparam and set_priv_flags handlers, and the XDP
setup path.
The same pattern already exists in the queue reset work,
mana_set_channels() and mana_set_priv_flags(). This patch extends it to
the MTU path, and it is still there at the end of the series. Could a real
lock, or some way for RDMA admission to wait, be used here?
[ ... ]
--
Sashiko AI review ·
https://netdev-ai.bots.linux.dev/sashiko/#/patchset/cover.1791710735.git.weh%40linux.microsoft.com
