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

[Jeff Moyer]

Mikulas Patocka <mpatocka at redhat.com> writes:

> On Tue, 25 Sep 2012, Jeff Moyer wrote:
>
>> Jeff Moyer <jmoyer at redhat.com> writes:
>> 
>> > Mikulas Patocka <mpatocka at 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.

> 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?

Cheers,
Jeff