Date:	Sat, 11 Apr 2009 12:46:19 +0200
From:	"Rafael J. Wysocki" <rjw@...e.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-pm@...ts.linux-foundation.org, Takashi Iwai <tiwai@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [linux-pm] [2.6.30-rc1-git2 regressions] Hibernation broken and (minor but annoying) audio problem

On Saturday 11 April 2009, Arjan van de Ven wrote:
> Linus Torvalds wrote:
> > 
> > On Sat, 11 Apr 2009, Rafael J. Wysocki wrote:
> >> On Friday 10 April 2009, Linus Torvalds wrote:
> >>> On Fri, 10 Apr 2009, Rafael J. Wysocki wrote:
> >>>> I've just verified that the resume-after-hibernation issue goes away after
> >>>> reverting commit 9710794383ee5008d67f1a6613a4717bf6de47bc
> >>>> (async: remove the temporary (2.6.29) "async is off by default" code) , so it
> >>>> is async-related.
> >>> Arjan? Clearly all the necessary fixes weren't found..
> >>>
> >>> There _is_ a module loading problem wrt initmem - I think you found that 
> >>> and we added a hack for it for the ACPI battery driver. I wonder if we're 
> >>> hitting a similar issue now with module discovery: modules that use 
> >>> "async_schedule()" to do their discovery asynchronously are now not 
> >>> necessarily fully "done" when the module is loaded. 
> >>>
> >>> And so, anything that expected the devices to be available after module 
> >>> load (like they used to) would be screwed.
> >>>
> >>> IOW, maybe something like the totally untested patch appended here (that 
> >>> should also allow us to make the ACPI battery code to go back to using 
> >>> __init).
> >> I tested it and it worked.
> > 
> > Hmm.
> > 
> > I'm not 100% sure that patch is good.
> > 
> > The reason? I think it's going to deadlock if an async caller ends up 
> > wanting to load a module, because then the nested 
> > "async_synchronize_full()" will basically want to wait for itself.
> > 
> > So it's a good test-patch, and maybe no async caller ever loads a module, 
> > but it makes me a bit nervous.
> 
> It would make a rule that async context can only use request_module_nowait().
> Not too nice, I think we can do better.
> 
> > But the fact that it fixes things for you at least means that the _reason_ 
> > for the problem is know, and maybe there are alternative solutions. Arjan?
> 
> We have async domains; for the acpi type of case we could make a "__init" domain
> that we wait on selectively... but I'm not too fond of this since it'll be fragile over time.
> 
> I suppose this got exposed now that we (just) removed the stop_machine stuff from the module loader...
> (which is a generally good improvement, don't get me wrong).
> 
> For the __init freeing case there is a even more interesting option:
> We can schedule an async task that will free the init mem of the module, and have that
> async task just wait for all its predecessors to complete before doing the actual work.
> That way the module is available and ready to its caller, while the freeing of the init mem
> will be done at a safe time.
> 
> Like this (not yet tested):
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 1196f5d..4ec90e8 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2312,6 +2312,22 @@ static noinline struct module *load_module(void __user *umod,
>   	goto free_hdr;
>   }
> 
> +static void async_module_free_initmem(void *data, async_cookie_t cookie)
> +{
> +	struct module *mod = data;
> +	async_synchronize_cookie(cookie);
> +
> +	mutex_lock(&module_mutex);
> +	/* Drop initial reference. */
> +	module_put(mod);
> +	module_free(mod, mod->module_init);
> +	mod->module_init = NULL;
> +	mod->init_size = 0;
> +	mod->init_text_size = 0;
> +	mutex_unlock(&module_mutex);
> +
> +}
> +
>   /* This is where the real work happens */
>   SYSCALL_DEFINE3(init_module, void __user *, umod,
>   		unsigned long, len, const char __user *, uargs)
> @@ -2372,15 +2388,7 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
>   	blocking_notifier_call_chain(&module_notify_list,
>   				     MODULE_STATE_LIVE, mod);
> 
> -	mutex_lock(&module_mutex);
> -	/* Drop initial reference. */
> -	module_put(mod);
> -	module_free(mod, mod->module_init);
> -	mod->module_init = NULL;
> -	mod->init_size = 0;
> -	mod->init_text_size = 0;
> -	mutex_unlock(&module_mutex);
> -
> +	async_schedule(async_module_free_initmem, mod);
>   	return 0;
>   }
> 
> 
> 
> Now the second case of "the devices are available when the module load is done".
> 
> The hard case here is that we're talking about a storage device. In 2.6.28 and before (eg well before
> any async stuff landed), SCSI probing already was done asynchronously. Same for USB. Libata I think is the same,
> by virtue of using scsi as infrastructure.
> 
> Realistically, unless you call scsi_complete_async_scans(), you could not depend on scsi devices being available
> after loading one of their modules. (from userland you could do this via the scsi_wait_scan module)
> 
> (speculation: on ATA likely the scan was done relatively quickly ... and with async it's just done with different timing. So it
> was kind of luck before)
> 
> Rafael: would it be reasonable to call "scsi_compete_async_scans()" from the resume-from-disk code ?

I think it would.

I'll try to put it in there and see if that helps.

Thanks,
Rafael
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