linux-kernel - Re: [PATCH 1/1] (v3) SYSVIPC - Fix the ipc structures initialization

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Date:	Mon, 27 Oct 2008 16:42:41 +0100
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	cboulte@...il.com
Cc:	akpm@...ux-foundation.org, manfred@...orfullife.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] (v3) SYSVIPC - Fix the ipc structures
	initialization

On Mon, 2008-10-27 at 12:04 +0100, Nadia Derbey wrote:
> On Mon, 2008-10-27 at 11:32 +0100, cboulte@...il.com wrote:
> > On Mon, Oct 27, 2008 at 8:28 AM,  <Nadia.Derbey@...l.net> wrote:
> > >
> > > This patch is a fix for Bugzilla bug
> > > http://bugzilla.kernel.org/show_bug.cgi?id=11796.
> > >
> > > To summarize, a simple testcase is concurrently running an infinite loop on
> > > msgctl(IPC_STAT) and a call to msgget():
> > >
> > > while (1)
> > >   msgctl(id, IPC_STAT)    1
> > >                           |
> > >                           |
> > >                           |
> > >                           2 id = msgget(key, IPC_CREAT)
> > >                           |
> > >                           |
> > >                           |
> > >
> > > In the interval [1-2], the id doesn't exist yet.
> > >
> > > In that test, the problem is the following:
> > > When we are calling ipc_addid() from msgget() the msq structure is not
> > > completely initialized. So idr_get_new() is inserting a pointer into the
> > > idr tree, and the structure which is pointed to has, among other fields,
> > > its lock uninitialized.
> > >
> > > Since msgctl(IPC_STAT) is looping while (1), idr_find() returns the
> > > pointer as soon as it is inserted into the IDR tree. And ipc_lock()
> > > calls spin_lock(&mqs->lock), while we have not initialized that lock
> > > yet.
> > >
> > > This patch moves the spin_lock_init() before the call to ipc_addid().
> > > It also sets the "deleted" flag to 1 in the window between msg structure
> > > allocation and msg structure locking in ipc_addid().
> > >
> > >
> > > Regards,
> > > Nadia
> > >
> > >
> > > Signed-off-by: Nadia Derbey <Nadia.Derbey@...l.net>
> > >
> > > ---
> > >  ipc/util.c |   16 ++++++++++++++--
> > >  1 file changed, 14 insertions(+), 2 deletions(-)
> > >
> > > Index: linux-2.6.27/ipc/util.c
> > > ===================================================================
> > > --- linux-2.6.27.orig/ipc/util.c        2008-10-23 15:20:46.000000000 +0200
> > > +++ linux-2.6.27/ipc/util.c     2008-10-24 17:48:33.000000000 +0200
> > > @@ -266,6 +266,17 @@ int ipc_addid(struct ipc_ids* ids, struc
> > >        if (ids->in_use >= size)
> > >                return -ENOSPC;
> > >
> > > +       spin_lock_init(&new->lock);
> > > +
> > > +       /*
> > > +        * We have a window between the time new is inserted into the idr
> > > +        * tree and the time it is actually locked.
> > > +        * In order to be safe during that window set the new ipc structure
> > > +        * as deleted: a concurrent ipc_lock() will see it as not present
> > > +        * until the initialization phase is complete.
> > > +        */
> > > +       new->deleted = 1;
> > > +
> > >        err = idr_get_new(&ids->ipcs_idr, new, &id);
> > >        if (err)
> > >                return err;
> > > @@ -280,10 +291,11 @@ int ipc_addid(struct ipc_ids* ids, struc
> > >                ids->seq = 0;
> > >
> > >        new->id = ipc_buildid(id, new->seq);
> > > -       spin_lock_init(&new->lock);
> > > -       new->deleted = 0;
> > >        rcu_read_lock();
> > >        spin_lock(&new->lock);
> > > +
> > > +       new->deleted = 0;
> > > +
> > >        return id;
> > >  }
> > >
> > >
> > > --
> > >
> > 
> > Still got the lock... I'm using a 4 cpus node: Intel Xeon @ 2.8GHz...
> > don't know if it has an impact.
> ???
> The bad new, is that it becomes unreprodicible on my side.
> For my part, I've got 2 2.8 GHz Xeon CPUs.
> 
> Will review the code once more.
> 
> Thanks!
> Nadia
> 
> > The only way I found to have no lock, it's to spin_lock the ipc
> > _before_ inserting it into the idr.
> > 
> > Best regards, c.
> > 

I agree with you that it's more logical and correct to take the lock
before inserting the ipc structure (i.e. making it visible to readers).

But I wanted to understand what's wrong with
1. new->lock init
2. new->deleted = 1
3. insert(new)

I've been looking at the code again and again and the only thing I see
could have happened, is that instructions have been reordered and the
insertion done before the lock actually being initialized.
This means that a memory barrier is missing (this would explain why your
fix works: the spin_lock acts as a barrier).
But this memory barrier is supposed to be invoked by
rcu_assign_pointer() in idr_get_new(). So may be there's a problem with
the idr code.
Before going into a review of this code, I'd like to confirm what I'm
saying, doing the following (I'm sorry to ask you do it, but I can't
reproduce the problem in my side anymore): would you mind adding a
smp_wmb() just before the idr_get_new in ipc_addid() and tell me if this
solves the problem.
(BTW, I didn't ask you before, but I guess you're getting the same call
trace?)

Regards,
Nadia

-- 
Nadia Derbey <Nadia.Derbey@...l.net>

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