[dm-devel] [PATCH v2] dm-io: deal with wandering queue limits when handling REQ_DISCARD and REQ_WRITE_SAME

[Mikulas Patocka]

On Fri, 27 Feb 2015, Mike Snitzer wrote:

> On Fri, Feb 27 2015 at  2:09pm -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > 
> > 
> > On Fri, 27 Feb 2015, Darrick J. Wong wrote:
> > 
> > > Since it's apparently possible that the queue limits for discard and
> > > write same can change while the upper level command is being sliced
> > > and diced, fix up both of them (a) to reject IO if the special command
> > > is unsupported at the start of the function and (b) read the limits
> > > once and let the commands error out on their own if the status happens
> > > to change.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
> > 
> > > +	unsigned int special_cmd_max_sectors;
> > > +
> > > +	/* Reject unsupported discard and write same requests */
> > > +	if (rw & REQ_DISCARD)
> > > +		special_cmd_max_sectors = q->limits.max_discard_sectors;
> > > +	else if (rw & REQ_WRITE_SAME)
> > > +		special_cmd_max_sectors = q->limits.max_write_same_sectors;
> > > +	if ((rw & (REQ_DISCARD | REQ_WRITE_SAME)) &&
> > > +	    special_cmd_max_sectors == 0) {
> > 
> > That results in uninitialized variable warning (although the warning is 
> > false positive). We need the macro uninitialized_var to suppress the 
> > warning.
> > 
> > It's better to use ACCESS_ONCE on variables that may be changing so that 
> > the compiler doesn't load them multiple times.
> 
> I dropped the use of ACCESS_ONCE.  We access queue_limits all over block
> related code.  If the performance is quantifiable then all accesses
> should be updated.  Until then, I'm maintaining status-quo.

ACCESS_ONCE is not there because of performance. Without ACCESS_ONCE, the 
compiler may reload the variable multple times and reintroduce the bug 
that we are trying to fix.


See this piece of code.

special_cmd_max_sectors = q->limits.max_discard_sectors;
if (special_cmd_max_sectors == 0) {
	dec_count(io, region, -EOPNOTSUPP);                                                                                                                                        
        return;                  
}
....
num_sectors = special_cmd_max_sectors;
remaining -= num_sectors;


At first sight, it seems that the variable num_sectors can't be zero. But 
in fact, it can. The compiler may eliminate the variable 
special_cmd_max_sectors and translate the code into this:

if (q->limits.max_discard_sectors == 0) {
	dec_count(io, region, -EOPNOTSUPP);                                                                                                                                        
        return;                  
}
....
num_sectors = q->limits.max_discard_sectors;
remaining -= num_sectors;

- and now, if we have the same bug that we were trying to fix.

That's why we need ACCESS_ONCE - to prevent the compiler from doing this 
transformation.

It's true that the kernel misses the ACCESS_ONCE at many places where it 
should be. But the fact that there is a lot of broken code doesn't mean 
that we should write broken code too.

Mikulas