linux-kernel - Re: memory barrier question

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Message-Id: <E1OxQIg-0005Nw-WE@pomaz-ex.szeredi.hu>
Date:	Sun, 19 Sep 2010 22:15:51 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	paulmck@...ux.vnet.ibm.com
CC:	benh@...nel.crashing.org, miklos@...redi.hu,
	James.Bottomley@...senPartnership.com, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: memory barrier question

On Sun, 19 Sep 2010, Paul E. McKenney wrote:
> Give it a few years.  There are reportedly already other compilers that do
> this, which is not too surprising given that the perception of insanity
> is limited to lockless parallel code.  If you have single-threaded code,
> such as code and data under a lock (where the data is never accessed
> without holding that lock), then this sort of optimization is pretty safe.
> I still don't like it, but the compiler guys would argue that this is
> because I am one of those insane parallel-programming guys.
> 
> Furthermore, there are other ways to get into trouble.  If the code
> continued as follows:
> 
>    LOAD inode = next.dentry->inode
>    if (inode != NULL)
>    	LOAD inode->f_op
> 	do_something_using_lots_of_registers();
> 	LOAD inode->some_other_field
> 
> and if the code expected ->f_op and ->some_other_field to be from the
> same inode structure, severe disappointment could ensue.  This is because
> the compiler is within its rights to reload from next.dentry->inode,
> especially given register pressure.  In fact, the compiler would be within
> its rights to reload from next.dentry->inode in the "LOAD inode->f_op"
> statement.  And it might well get NULL from such a reload.

Except the VFS doesn't allow that.  dentry->inode can go from NULL to
non-NULL anytime but will only go from non-NULL to NULL when there are
no possible external references to the dentry.

The compiler and the CPU cannot move the "LOAD inode->some_field"
before the "LOAD dentry->inode", because of the conditional, right?

Thanks,
Miklos
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