Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Andrew Morton <akpm@osdl.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Jens Axboe <jens.axboe@oracle.com>,
	Linus Torvalds <torvalds@osdl.org>,
	Thomas Gleixner <tglx@timesys.com>, Ingo Molnar <mingo@elte.hu>,
	LKML <linux-kernel@vger.kernel.org>,
	john stultz <johnstul@us.ibm.com>,
	David Miller <davem@davemloft.net>,
	Arjan van de Ven <arjan@infradead.org>, Andi Kleen <ak@suse.de>,
	manfred@colorfullife.com, oleg@tv-sign.ru
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
Date: Fri, 17 Nov 2006 21:57:46 -0800	[thread overview]
Message-ID: <20061118055746.GB6059@us.ibm.com> (raw)
In-Reply-To: <20061117205103.847081a4.akpm@osdl.org>

On Fri, Nov 17, 2006 at 08:51:03PM -0800, Andrew Morton wrote:
> On Fri, 17 Nov 2006 23:33:45 -0500 (EST)
> Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> > On Fri, 17 Nov 2006, Paul E. McKenney wrote:
> >
> > > > Perhaps a better approach to the initialization problem would be to assume
> > > > that either:
> > > >
> > > >     1.  The srcu_struct will be initialized before it is used, or
> > > >
> > > >     2.  When it is used before initialization, the system is running
> > > > 	only one thread.
> > >
> > > Are these assumptions valid?  If so, they would indeed simplify things
> > > a bit.
> >
> > I don't know.  Maybe Andrew can tell us -- is it true that the kernel runs
> > only one thread up through the time the core_initcalls are finished?
> 
> I don't see why - a core_initcall could go off and do the
> multithreaded-pci-probing thing, or it could call kernel_thread() or
> anything.  I doubt if any core_initcall functions _do_ do that, but there
> are a lot of them.
> 
> > If not, can we create another initcall level that is guaranteed to run
> > before any threads are spawned?
> 
> It's a simple and cheap matter to create a precore_initcall() - one would
> need to document it carefully to be able to preserve whatever guarantees it
> needs.
> 
> However by the time the initcalls get run, various thing are already
> happening: SMP is up, the keventd threads are running, the CPU scheduler
> migration threads are running, ksoftirqd, softlockup-detector, etc.
> keventd is the problematic one.
> 
> So I guess you'd need a new linker section and a call from
> do_pre_smp_initcalls() or thereabouts.

Hmmm...  OK then, for the moment, I will stick with the current checks
in the primitives.  Not that I particularly like the "bulking up" of
srcu_read_lock() and srcu_read_unlock() -- but if the super-fast
version is needed, it can easily be provided either within the
confines of the subsystem that needs it, or as yet another set of
RCU-like primitives.  Hopefully this latter option can be avoided!

BTW, the reason for the hardluckref is that I don't want to inflict a
failure return from srcu_read_lock() on you guys.  The non-blocking
synchronization community has repeatedly made that sort of mistake,
and I have no intention of letting it propagate any further.  ;-)

						Thanx, Paul