From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Date: Thu, 18 Oct 2018 10:43:13 +0200
> on 32bit, lockdep notices that virtnet_open() and refill_work() invoke
> try_fill_recv() from process context while virtnet_receive() invokes the
> same function from BH context. The problem that the seqcounter within
> u64_stats_update_begin() may deadlock if it is interrupted by BH and
> then acquired again.
>
> Introduce u64_stats_update_begin_bh() which disables BH on 32bit
> architectures. Since the BH might interrupt the worker, this new
> function should not limited to SMP like the others which are expected
> to be used in softirq.
>
> With this change we might lose increments but this is okay. The
> important part that the two 32bit parts of the 64bit counter are not
> corrupted.
>
> Fixes: 461f03dc99cf6 ("virtio_net: Add kick stats").
> Suggested-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Trying to get down to the bottom of this:
1) virtnet_receive() runs from softirq but only if NAPI is active and
enabled. It is in this context that it invokes try_fill_recv().
2) refill_work() runs from process context, but disables NAPI (and
thus invocation of virtnet_receive()) before calling
try_fill_recv().
3) virtnet_open() invokes from process context as well, but before the
NAPI instances are enabled, it is same as case #2.
4) virtnet_restore_up() is the same situations as #3.
Therefore I agree that this is a false positive, and simply lockdep
cannot see the NAPI synchronization done by case #2.
I think we shouldn't add unnecessary BH disabling here, and instead
find some way to annotate this for lockdep's sake.
Thank you.
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