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SubjectRe: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module libcrc32c"
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On Tue, 1 Jun 2010 02:18:34 am Andrew Morton wrote:
> On Mon, 31 May 2010 21:32:27 +0930 Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> > Problem: it's hard to avoid an init routine stumbling over a
> > request_module these days. And it's not clear it's always a bad idea:
> > for example, a module like kvm with dynamic dependencies on kvm-intel
> > or kvm-amd would be neater if it could simply request_module the right
> > one.
> >
> > In this particular case, it's libcrc32c:
> >
> > libcrc32c_mod_init
> > crypto_alloc_shash
> > crypto_alloc_tfm
> > crypto_find_alg
> > crypto_alg_mod_lookup
> > crypto_larval_lookup
> > request_module
> >
> > If another module is waiting for libcrc32c to finish initializing
> > (ie. bne2 depends on libcrc32c) then it does so holding the module
> > lock, and our request_module() can't make progress until that is
> > released.
> >
> > Waiting without the lock isn't all that hard: we just need to pass the
> > -EBUSY up the call chain so we can sleep where we don't hold the lock.
> > Error reporting is a bit trickier: we need to copy the name of the
> > unfinished module before releasing the lock.
>
> Who's returning -EBUSY? request_module()? If so, are you requiring
> that all code which might call request_module() be correctly
> propagating error codes back? Please spell this all out?

Sorry if I was unclear. It's all inside module.c. Here's the updated
commentry:

If another module is waiting inside resolve_symbol() for libcrc32c to
finish initializing (ie. bne2 depends on libcrc32c) then it does so
holding the module lock, and our request_module() can't make progress
until that is released.

Waiting inside resolve_symbol() without the lock isn't all that hard:
we just need to pass the -EBUSY up the call chain so we can sleep
where we don't hold the lock. Error reporting is a bit trickier: we
need to copy the name of the unfinished module before releasing the
lock.

Hope that helps,
Rusty.


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