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Date: Fri, 10 Jan 2025 10:33:11 +0100
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Damien Le Moal <dlemoal@kernel.org>, Ming Lei <ming.lei@redhat.com>,
	Nilay Shroff <nilay@linux.ibm.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org, nbd@other.debian.org,
	linux-scsi@vger.kernel.org, usb-storage@lists.one-eyed-alien.net,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>
Subject: Re: [PATCH 06/11] block: fix queue freeze vs limits lock order in
 sysfs store methods
Message-ID: <20250110093311.GA9083@lst.de>
References: <20250109055810.1402918-1-hch@lst.de> <20250109055810.1402918-7-hch@lst.de> <8cfe7690-0101-42e7-ba97-6c6b717c4706@oracle.com>
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On Thu, Jan 09, 2025 at 01:07:47PM +0000, John Garry wrote:
> Do you mean that the sysfs_lock could be removed in future? I would have 
> thought that queue limits lock could be used for the same thing, but I am 
> probably failing to see some lock nesting/ordering issues...

More or less.  Think about it: what does it even try to protect?

Readіng/writing sysfs files vs itself and file removal it serialized by
sysfs/kernfs internally.

Any information tweaked in sysfs usually also has other places that can
modify it.  So we'll need a lock independent of sysfs for that anyway.
A big part, buy by far all of that is covered by limits_lock.

Serializing creating/removing sysfs attribues is supposed to be
serialized using sysfs_dir_lock, although that needs a careful audit.

It's also used to serialize a few debugfs things, but we'll need to look
carefully for what exactly and switch that over to debugfs_mutex or
something new.

And then there's a bunch of misc cruft that also needs a careful look.