Re: [PATCH 0/4] Fix a crash when block device is read and block size is changed at the same time

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 26 Sep 2012, Jeff Moyer wrote:

> Mikulas Patocka <mpatocka@redhat.com> writes:
> 
> > On Tue, 25 Sep 2012, Jeff Moyer wrote:
> >
> >> Jeff Moyer <jmoyer@redhat.com> writes:
> >> 
> >> > Mikulas Patocka <mpatocka@redhat.com> writes:
> >> >
> >> >> Hi Jeff
> >> >>
> >> >> Thanks for testing.
> >> >>
> >> >> It would be interesting ... what happens if you take the patch 3, leave 
> >> >> "struct percpu_rw_semaphore bd_block_size_semaphore" in "struct 
> >> >> block_device", but remove any use of the semaphore from fs/block_dev.c? - 
> >> >> will the performance be like unpatched kernel or like patch 3? It could be 
> >> >> that the change in the alignment affects performance on your CPU too, just 
> >> >> differently than on my CPU.
> >> >
> >> > It turns out to be exactly the same performance as with the 3rd patch
> >> > applied, so I guess it does have something to do with cache alignment.
> >> > Here is the patch (against vanilla) I ended up testing.  Let me know if
> >> > I've botched it somehow.
> >> >
> >> > So, I next up I'll play similar tricks to what you did (padding struct
> >> > block_device in all kernels) to eliminate the differences due to
> >> > structure alignment and provide a clear picture of what the locking
> >> > effects are.
> >> 
> >> After trying again with the same padding you used in the struct
> >> bdev_inode, I see no performance differences between any of the
> >> patches.  I tried bumping up the number of threads to saturate the
> >> number of cpus on a single NUMA node on my hardware, but that resulted
> >> in lower IOPS to the device, and hence consumption of less CPU time.
> >> So, I believe my results to be inconclusive.
> >
> > For me, the fourth patch with RCU-based locks performed better, so I am 
> > submitting that.
> >
> >> After talking with Vivek about the problem, he had mentioned that it
> >> might be worth investigating whether bd_block_size could be protected
> >> using SRCU.  I looked into it, and the one thing I couldn't reconcile is
> >> updating both the bd_block_size and the inode->i_blkbits at the same
> >> time.  It would involve (afaiui) adding fields to both the inode and the
> >> block_device data structures and using rcu_assign_pointer  and
> >> rcu_dereference to modify and access the fields, and both fields would
> >> need to protected by the same struct srcu_struct.  I'm not sure whether
> >> that's a desirable approach.  When I started to implement it, it got
> >> ugly pretty quickly.  What do others think?
> >
> > Using RCU doesn't seem sensible to me (except for lock implementation, as 
> > it is in patch 4). The major problem is that the block layer reads 
> > blocksize multiple times and when different values are read, a crash may 
> > happen - RCU doesn't protect you against that - if you read a variable 
> > multiple times in a RCU-protected section, you can still get different 
> > results.
> 
> SRCU is sleepable, so could be (I think) used in the same manner as your
> rw semaphore.  The only difference is that it would require changing the
> bd_blocksize and the i_blkbits to pointers and protecting them both with
> the same srcu struct.  Then, the inode i_blkbits would also need to be
> special cased, so that we only require such handling when it is
> associated with a block device.  It got messy.

No, it couldn't be used this way.

If you do
srcu_read_lock(&srcu)
ptr1 = srcu_dereference(pointer, &srcu);
ptr2 = srcu_dereference(pointer, &srcu);
srcu_read_unlock(&srcu)

it doesn't guarantee that ptr1 == ptr2.

All that it guarantees is that when synchronize_srcu exits, there are no 
references to the old structure. But after rcu_assign_pointer and before 
synchronize_srcu exits, readers can read both old and new value of the 
pointer and it is not specified which value do they read.

> > If we wanted to use RCU, we would have to read blocksize just once and 
> > pass the value between all functions involved - that would result in a 
> > massive code change.
> 
> If we did that, we wouldn't need rcu at all, would we?

Yes, we wouldn't need RCU then.

Mikulas

> Cheers,
> Jeff
>