linux-kernel - Re: [patch] drivers: wait for threaded probes between initcall levels

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Message-Id: <20061027221925.1041cc5e.akpm@osdl.org>
Date:	Fri, 27 Oct 2006 22:19:25 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Grant Grundler <grundler@...isc-linux.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Pavel Machek <pavel@....cz>,
	Greg KH <greg@...ah.com>,
	Stephen Hemminger <shemminger@...l.org>,
	Matthew Wilcox <matthew@....cx>, Adrian Bunk <bunk@...sta.de>,
	Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...ey.karlin.mff.cuni.cz
Subject: Re: [patch] drivers: wait for threaded probes between initcall
 levels

On Fri, 27 Oct 2006 23:09:05 -0600
Grant Grundler <grundler@...isc-linux.org> wrote:

> On Fri, Oct 27, 2006 at 04:06:26PM -0700, Andrew Morton wrote:
> > On Fri, 27 Oct 2006 23:59:30 +0100
> > Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > 
> > > Ar Gwe, 2006-10-27 am 11:42 -0700, ysgrifennodd Andrew Morton:
> > > > IOW, we want to be multithreaded _within_ an initcall level, but not between
> > > > different levels.
> > > 
> > > Thats actually insufficient. We have link ordered init sequences in
> > > large numbers of driver subtrees (ATA, watchdog, etc). We'll need
> > > several more initcall layers to fix that.
> > > 
> > 
> > It would be nice to express those dependencies in some clearer and less
> > fragile manner than link order.  I guess finer-grained initcall levels
> > would do that, but it doesn't scale very well.
> 
> Would making use of depmod data be a step in the right direction?

Nope.  The linkage-order problem is by definition applicable to
linked-into-vmlinux code, not to modules.

> ie nic driver calls extern function (e.g. pci_enable_device())
> and therefore must depend on module which provides that function.
> 
> My guess is this probably isn't 100% sufficient to replace all initcall
> levels.  But likely sufficient within a given initcall level.
> My main concern are circular dependencies (which are rare).

The simplest implementation of "A needs B to have run" is for A to simply
call B, and B arranges to not allow itself to be run more than once.

But that doesn't work in the case "A needs B to be run, but only if B is
present".  Resolving this one would require something like a fancy
"synchronisation object" against which dependers and dependees can register
interest, and a core engine which takes care of the case where a depender
registers against something which no dependees have registered.

The mind boggles.

> > But whatever.  I think multithreaded probing just doesn't pass the
> > benefit-versus-hassle test, sorry.   Make it dependent on CONFIG_GREGKH ;)
> 
> Isn't already? :)
> 
> I thought parallel PCI and SCSI probing on system with multiple NICs and
> "SCSI" storage requires udev to create devices with consistent naming.

For some reason people get upset when we rename all their devices.  They're
a humourless lot.

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