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Date: Wed, 24 Jan 2024 10:21:48 +0100
From: Christoph Hellwig <hch@lst.de>
To: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Damien Le Moal <dlemoal@kernel.org>,
	Keith Busch <kbusch@kernel.org>, Sagi Grimberg <sagi@grimberg.me>,
	linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
	virtualization@lists.linux.dev
Subject: Re: [PATCH 03/15] block: add an API to atomically update queue
 limits
Message-ID: <20240124092148.GA28770@lst.de>
References: <20240122173645.1686078-1-hch@lst.de> <20240122173645.1686078-4-hch@lst.de> <de3cd926-c78c-4e68-bee0-2f571ccdaf8d@suse.de>
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On Wed, Jan 24, 2024 at 07:08:58AM +0100, Hannes Reinecke wrote:
>> + * that there is outstanding I/O by other means.
>> + */
>> +static inline struct queue_limits
>> +queue_limits_start_update(struct request_queue *q)
>> +	__acquires(q->limits_lock)
>> +{
>> +	mutex_lock(&q->limits_lock);
>> +	return q->limits;
>> +}
>
> I'm slightly confused about the lifetime of the returned structure.
> By my understanding, the returned 'struct queue_limit' is allocated
> on the stack of the caller, right?
> Shouldn't we note this somewhere such that people don't start passing
> the structure around, or, worse, calling 'kfree()' on it?

queue_limits_start_update returns the structure by value.  So the
lifetime is that of whatever variable you assign it to, which better
be on the stack.  Tryign to kfree a non-pointer type will get the
compiler complain, as does passing a reference to it outside the
function these days.