Re: New bug in patch and existing Linux code - race withinstall_page() (was: Re: [PATCH] 2.6.14 patch for supportingmadvise(MADV_REMOVE))

[Badari Pulavarty]

On Wed, 2005-11-02 at 21:55 +0000, Hugh Dickins wrote:
> On Wed, 2 Nov 2005, Badari Pulavarty wrote:
> > On Wed, 2005-11-02 at 20:54 +0100, Blaisorblade wrote:
> > > > +       /* XXX - Do we need both i_sem and i_allocsem all the way ? */
> > > > +       down(&inode->i_sem);
> > > > +       down_write(&inode->i_alloc_sem);
> > > > +       unmap_mapping_range(mapping, offset, (end - offset), 1);
> > > In my opinion, as already said, unmap_mapping_range can be called without 
> > > these two locks, as it operates only on mappings for the file.
> > > 
> > > However currently it's called with these locks held in vmtruncate, but I think 
> > > the locks are held in that case only because we need to truncate the file, 
> > > and are hold in excess also across this call.
> > 
> > I agree, I can push down the locking only for ->truncate_range - if
> > no one has objections. (But again, it so special case - no one really
> > cares about the performance of this interface ?).
> 
> I can't remember why i_alloc_sem got introduced, and don't have time to
> work it out: something to do with direct I/O races, perhaps?  Someone
> else must advise, perhaps you will be able to drop that one.

Yep. i_alloc_sem is supposed to protect DIO races with truncate.

> But I think you'd be very unwise to drop i_sem too.  i_mmap_lock gets
> dropped whenever preemption demands here, i_sem is what's preventing
> someone else coming along and doing a concurrent truncate or remove.
> You don't want that.
> 
> Sorry, I've not yet had time to study your patch: I do intend to,
> but cannot promise when.  I fear it won't be as easy as making
> these occasional responses.

Thanks Hugh. For now, I will leave those locks alone. We can re-visit
later, if we really care about the performance of this interface.
Better be safe than sorry.

Thanks,
Badari

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