Re: memory barrier question

[Miklos Szeredi]

On Thu, 16 Sep 2010, James Bottomley wrote:
> On Thu, 2010-09-16 at 18:56 +0200, Miklos Szeredi wrote:
> > On Thu, 16 Sep 2010, Paul E. McKenney wrote:
> > > On Thu, Sep 16, 2010 at 06:06:53PM +0200, Miklos Szeredi wrote:
> > > > On Thu, 16 Sep 2010, Paul E. McKenney wrote:
> > > > > On Thu, Sep 16, 2010 at 03:30:56PM +0100, David Howells wrote:
> > > > > > Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > > > > > 
> > > > > > > Is the rmb() really needed?
> > > > > > > 
> > > > > > > Take this code from fs/namei.c for example:
> > > > > > > 
> > > > > > > 		inode = next.dentry->d_inode;
> > > > > > > 		if (!inode)
> > > > > > > 			goto out_dput;
> > > > > > > 
> > > > > > > 		if (inode->i_op->follow_link) {
> > > > > > > 
> > > > > > > It happily dereferences dentry->d_inode without a barrier after
> > > > > > > checking it for non-null, while that d_inode might have just been
> > > > > > > initialized on another CPU with a freshly created inode.  There's
> > > > > > > absolutely no synchornization with that on this side.
> > > > > > 
> > > > > > Perhaps it's not necessary; once set, how likely is i_op to be changed once
> > > > > > I_NEW is cleared?
> > > > > 
> > > > > Are the path_get()s protecting this?
> > > > 
> > > > No, when creating a file the dentry will go from negative to positive
> > > > independently from lookup.  The dentry can get instantiated with an
> > > > inode between the path_get() and dereferencing ->d_inode.
> > > > 
> > > > > If there is no protection, then something like rcu_dereference() is
> > > > > needed for the assignment from next.dentry->d_inode.
> > > > 
> > > > Do I understand correctly that the problem is that a CPU may have a
> > > > stale cache associated with *inode, one that was loaded before the
> > > > write barrier took effect?
> > > 
> > > Yes, especially if the compiler is aggressively optimizing.
> > 
> > How do compiler optimizations make a difference?
> 
> There are two types of reorderings that cause problems if you expect the
> bus visible ordering to matter.  One is CPU issue reordering, where the
> cpu decides to output loads and stores in a different order than the
> input instruction stream actually said.  The other is compiler
> re-ordering where the compiler actually reorders the instructions to
> execute in a different order from what you'd expect by simply reading
> the C code.
> 
> We have compiler barrier instructions for the latter and barriers which
> issue CPU primitives for the former.

Right but in the concrete namei example I can't see how a compiler
optimization can make a difference.  The order of the loads is quite
clear:

   LOAD inode = next.dentry->inode
   if (inode != NULL)
   	LOAD inode->f_op

What is there the compiler can optimize?

Thanks,
Miklos
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