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Subject: Re: [PATCH v1] nvme: avoid race in shutdown namespace removal
To:     Christoph Hellwig <hch@infradead.org>,
        Hannes Reinecke <hare@suse.de>
Cc:     Daniel Wagner <dwagner@suse.de>, linux-nvme@lists.infradead.org,
        linux-kernel@vger.kernel.org, Keith Busch <kbusch@kernel.org>
References: <20210830093618.97657-1-dwagner@suse.de>
 <20210830100443.b2zkdp4l3vi2gz42@carbon.lan>
 <742108c3-7c76-0bc3-fc13-06acf975c25c@suse.de>
 <YS+AYA+5/o8Qj08Q@infradead.org>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <7677d695-aa57-ee91-3b32-65570cefdff4@grimberg.me>
Date:   Wed, 1 Sep 2021 17:41:57 +0300
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>> Most likely. Do drop me a mail on how to create a reproducer for that; it's
>> not exactly trivial as you need to patch qemu for that
>> (and, of course, those patches will not go upstream as they again hit a
>> section which the maintainer deemed to be reworked any time now. So of
>> course he can't possibly apply them.)
>> (I seem to have a particular spell of bad luck, seeing that it's the _third_
>> time this happened to me :-( )
> 
> Soo.  What is the problem in simply checking in nvme_find_ns_head that
> h->list is non-empty?  E.g. this variant of the patch from Daniel:

Don't see why this won't work...

> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index d535b00d65816..ce91655fa29bb 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -3523,7 +3523,9 @@ static struct nvme_ns_head *nvme_find_ns_head(struct nvme_subsystem *subsys,
>   	lockdep_assert_held(&subsys->lock);
>   
>   	list_for_each_entry(h, &subsys->nsheads, entry) {
> -		if (h->ns_id == nsid && nvme_tryget_ns_head(h))
> +		if (h->ns_id != nsid)
> +			continue;
> +		if (!list_empty(&h->list) && nvme_tryget_ns_head(h))
>   			return h;
>   	}
>   
> @@ -3835,7 +3837,11 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>   
>   	mutex_lock(&ns->ctrl->subsys->lock);
>   	list_del_rcu(&ns->siblings);
> -	mutex_unlock(&ns->ctrl->subsys->lock);
> +	if (list_empty(&ns->head->list)) {
> +		list_del_init(&ns->head->entry);
> +		last_path = true;
> +	}
> +	mutex_unlock(&ns->head->subsys->lock);
>   
>   	/* guarantee not available in head->list */
>   	synchronize_rcu();
> @@ -3855,13 +3861,6 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>   	list_del_init(&ns->list);
>   	up_write(&ns->ctrl->namespaces_rwsem);
>   
> -	/* Synchronize with nvme_init_ns_head() */
> -	mutex_lock(&ns->head->subsys->lock);
> -	if (list_empty(&ns->head->list)) {
> -		list_del_init(&ns->head->entry);
> -		last_path = true;
> -	}
> -	mutex_unlock(&ns->head->subsys->lock);
>   	if (last_path)
>   		nvme_mpath_shutdown_disk(ns->head);
>   	nvme_put_ns(ns);
> 
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>