Re: [PATCH net-next v6 07/13] net: mana: swap queue sets in mana_change_mtu

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Deadlock on 32-bit systems due to u64_stats_update_begin with preemption enabled
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commit 9fbccd82cfbdb308859b32b732f45f8415ee4bd0
Author: Long Li <longli@microsoft.com>
net: mana: swap queue sets in mana_change_mtu

This commit carries the MTU in the queue set and replaces the RX buffers from
it. It swaps queue sets in mana_change_mtu by allocating a new scratch context,
publishing the new queue set, and cleaning up the old one upon success or
failure.

> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
[ ... ]
> @@ -4392,6 +4419,8 @@ int mana_publish_qset(struct mana_port_context *apc, struct mana_qset *newq,
>  	synchronize_net();
>  	mana_fold_qset_rx_stats(apc, out_old);
>  
> +	WRITE_ONCE(ndev->mtu, apc->configured_mtu);
> +
>  	/* Publish restored fields before reopening the gate, as on success. */
>  	smp_wmb();

[Severity: High]
Can this lead to a deadlock on 32-bit systems?

The mana_publish_qset function executes under the rtnl lock with preemption
enabled, which leads to this call chain:

mana_publish_qset()
  mana_fold_qset_rx_stats()
    mana_fold_rxq_stats() {
        u64_stats_update_begin(&dst->syncp);
        dst->packets += src->packets;
        dst->bytes += src->bytes;
        ...
        u64_stats_update_end(&dst->syncp);
    }

Because preemption is enabled, on 32-bit architectures, a reader such as
ndo_get_stats64() could preempt the writer while the sequence count is odd.
The reader would then spin forever in u64_stats_fetch_retry() waiting for
an even sequence count, starving the preempted writer and resulting in a
deadlock.

Should preemption or interrupts be disabled around this stats update to
ensure readers cannot preempt the writer?

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Sashiko AI review · https://sashiko.dev/#/patchset/cover.1790795005.git.weh@linux.microsoft.com?part=7