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SubjectRe: USB device cannot be reconnected and khubd "blocked for more than 120 seconds"
On Tue, 15 Jan 2013, Linus Torvalds wrote:

> Tejun, comments? You can see the whole thread on lkml, but the basic
> problem is that the module loading doing the unconditional
> async_synchronize_full() has caused problems, because we have
>
> - load module A
> - module A does per-controller async discovery of its devices (eg
> scsi or ata probing)
> - in the async thread, it initializes somethign that needs another
> module B (in this case the default IO scheduler module)
> - modprobe for B loads the IO scheduler module successfully
> at the end of the module load, it does
> async_synchronize_full() to make sure load_module won't return before
> the module is ready
> *DEADLOCK*, because the async_synchronize_full() thing
> actually waits for not the module B async code (it didn't have any),
> but for the module *A* async code, which is waiting for module B to
> finish.
>
> Now, I'll happily argue that we shouldn't have this kind of "load
> modules from random context" behavior in the kernel, and I think the
> block layer is to blame for doing the IO scheduler load at an insane
> time. So "don't do that then" would be the best solution.

It may not be so easy. When the SCSI async thread probes the new disk,
it has to do I/O. So it needs to use a scheduler.

But maybe it could use a built-in trivial scheduler until the proper
one is loaded. Then the loading could be asynchronous.

Alan Stern



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