Date: Wed, 17 Oct 2012 17:19:53 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Michal Simek <monstr@monstr.eu>
Cc: Jonas Bonn <jonas@southpole.se>, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Haavard Skinnemoen <hskinnemoen@gmail.com>,
        Mike Frysinger <vapier@gentoo.org>,
        Jesper Nilsson <jesper.nilsson@axis.com>,
        David Howells <dhowells@redhat.com>, Tony Luck <tony.luck@intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Hirokazu Takata <takata@linux-m32r.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "James E.J. Bottomley" <jejb@parisc-linux.org>,
        Richard Kuo <rkuo@codeaurora.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Lennox Wu <lennox.wu@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Paul Mundt <lethal@linux-sh.org>, Chris Zankel <chris@zankel.net>,
        Chris Metcalf <cmetcalf@tilera.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Guan Xuetao <gxt@mprc.pku.edu.cn>
Subject: Re: new execve/kernel_thread design
Message-ID: <20121017161953.GZ2616@ZenIV.linux.org.uk>
References: <20121016223508.GR2616@ZenIV.linux.org.uk>
 <CACM3HyEpypULRWUc5ZnLnZ=uOWf3_j=9PXZiJrT_BXyGcQe9yg@mail.gmail.com>
 <CAHTX3dKDsOiUwtXxAUj-F4Y4rO+UZZDR4ufx_nxf+vuhmz46WA@mail.gmail.com>
 <20121017160702.GY2616@ZenIV.linux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20121017160702.GY2616@ZenIV.linux.org.uk>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1775
Lines: 29

On Wed, Oct 17, 2012 at 05:07:03PM +0100, Al Viro wrote:
> What happens during boot is this:
> 	* init_task (not to be confused with init) is used as current during
> infrastructure initializations.  Once everything needed for scheduler and
> for working fork is set, we spawn two threads - future init and future
> kthreadd.  The last thing we do with init_task is telling init that kthreadd
> has been spawned.  After that init_task turns itself into an idle thread.
> 	* future init waits for kthreadd to be spawned (it would be more
> natural to fork them in opposite order, but we want init to have PID 1 -
> too much stuff in userland depends on that).  Then it does the rest of
> initialization, including setting up initramfs contents.  And does
> kernel_execve() on /init.  Note that this is a task that had been created
> by kernel_thread() and is currently in function called from
> ret_from_kernel_thread().  Its kernel stack has been set up by copy_thread().
> That's where pt_regs need to be set up; note that they'll be passed to
> start_thread() before you return to userland.  If there are any magic bits
> in pt_regs needed by return-from-syscall code, set them in kthread case of
> copy_thread().

PS: I suspect that we end up with the wrong value in childregs->msr;
start_thread() only add MSR_UMS there.  I'd suggest running the kernel
with these patches + printk childregs->msr the very first time start_thread()
is called and see what it prints, then working kernel + such printk and
compare the results...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/