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Date: Mon, 26 Nov 2007 21:29:36 -0500 (EST) From: Alan Stern <stern@...land.harvard.edu> To: Andrew Morton <akpm@...ux-foundation.org> cc: greg@...ah.com, <kay.sievers@...y.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] Kobjects: drop child->parent ref at unregistration On Mon, 26 Nov 2007, Andrew Morton wrote: > On Mon, 19 Nov 2007 10:53:40 -0500 (EST) > Alan Stern <stern@...land.harvard.edu> wrote: > > > This patch (as1015) reverts changes that were made to the driver core > > about four years ago. The intent back then was to avoid certain kinds > > of invalid memory accesses by leaving kernel objects allocated as long > > as any of their children were still allocated. The original and > > correct approach was to wait only as long as any children were still > > _registered_; that's what this patch reinstates. > > What happened with this? As far as I know, it's on Greg's queue. > > This fixes a problem in the SCSI core made visible by the class_device > > to regular device conversion: A reference loop (scsi_device holds > > reference to request_queue, which is the child of a gendisk, which is > > the child of the scsi_device) prevents the data structures from being > > released, even though they are deregistered okay. > > > > It's possible that this change will cause a few bugs to surface, > > things that have been hidden for several years. They can be fixed > > easily enough by having the child device take an explicit reference to > > the parent whenever needed. > > > > How will such bugs manifest? Ideally via a nice printk and a stack trace > followed by damage avoidance. They will manifest in the same way as any other use-after-free bug: an oops message and either death of the current process or a system hang. Obviously I'm not aware of any such bugs -- if I were, I'd fix them. Greg has expressed concern that some USB serial drivers might have this problem. I'll do what testing I can (not much because I don't have any USB serial devices). > If it's via a mysterious crash or something similarly obscure then can we > improve that? I can't think of anything offhand. Maybe someone else can. Alan Stern - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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