On 2018/10/19 上午7:23, David Miller wrote:
From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Date: Thu, 18 Oct 2018 10:43:13 +0200on 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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