Fix a crash when block device is read and block size is changed at the same time
 

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.

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?

For now, my preference is to get the full patch set in. I will continue
to investigate the performance impact of the data structure size changes
that I've been seeing.

So, for the four patches:

Acked-by: Jeff Moyer <jmoyer@redhat.com>

Jens, can you have a look at the patch set? We are seeing problem
reports of this in the wild[1][2].

Cheers,
Jeff

[1] https://bugzilla.redhat.com/show_bug.cgi?id=824107
[2] https://bugzilla.redhat.com/show_bug.cgi?id=812129

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Fix a crash when block device is read and block size is changed at the same time
 

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.

> 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

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