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Date:   Mon, 28 Aug 2023 14:09:40 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
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Subject: Re: [PATCH 01/30] block: also call ->open for incremental
 partition opens
Message-ID: <20230828120940.GB10552@lst.de>
References: <20230608110258.189493-1-hch@lst.de> <20230608110258.189493-2-hch@lst.de> <20230825024457.GD95084@ZenIV>
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On Fri, Aug 25, 2023 at 03:44:57AM +0100, Al Viro wrote:
> That got me curious about the ->bd_openers - do we need it atomic?
> Most of the users (and all places that do modifications) are
> under ->open_mutex; the only exceptions are
> 	* early sync logics in blkdev_put(); it's explicitly racy -
> see the comment there.
> 	* callers of disk_openers() in loop and nbd (the ones in
> zram are under ->open_mutex).  There's driver-private exclusion
> around those, but in any case - READ_ONCE() is no worse than
> atomic_read() in those cases.
> 
> Is there something subtle I'm missing here?

No.  When I had to add unlocked readers I did the READ_ONCE initially,
but reviewers though the atomic_t would be better.  I didn't really feel
like arguing so went with this version.