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 Fri, 31 Aug 2012, Mikulas Patocka wrote:

> Hi
> 
> This is a series of patches to prevent a crash when when someone is 
> reading block device and block size is changed simultaneously. (the crash 
> is already happening in the production environment)
> 
> The first patch adds a rw-lock to struct block_device, but doesn't use the 
> lock anywhere. The reason why I submit this as a separate patch is that on 
> my computer adding an unused field to this structure affects performance 
> much more than any locking changes.
> 
> The second patch uses the rw-lock. The lock is locked for read when doing 
> I/O on the block device and it is locked for write when changing block 
> size.
> 
> The third patch converts the rw-lock to a percpu rw-lock for better 
> performance, to avoid cache line bouncing.
> 
> The fourth patch is an alternate percpu rw-lock implementation using RCU 
> by Eric Dumazet. It avoids any atomic instruction in the hot path.
> 
> Mikulas

I tested performance of patches. I created 4GB ramdisk, I initially filled 
it with zeros (so that ramdisk allocation-on-demand doesn't affect the 
results).

I ran fio to perform 8 concurrent accesses on 8 core machine (two 
Barcelona Opterons):
time fio --rw=randrw --size=4G --bs=512 --filename=/dev/ram0 --direct=1 
--name=job1 --name=job2 --name=job3 --name=job4 --name=job5 --name=job6 
--name=job7 --name=job8

The results actually show that the size of struct block_device and 
alignment of subsequent fields in struct inode have far more effect on 
result that the type of locking used. (struct inode is placed just after 
struct block_device in "struct bdev_inode" in fs/block-dev.c)

plain kernel 3.5.3: 57.9s
patch 1: 43.4s
patches 1,2: 43.7s
patches 1,2,3: 38.5s
patches 1,2,3,4: 58.6s

You can see that patch 1 improves the time by 14.5 seconds, but all that 
patch 1 does is adding an unused structure field.

Patch 3 is 4.9 seconds faster than patch 1, althogh patch 1 does no 
locking at all and patch 3 does per-cpu locking. So, the reason for the 
speedup is different sizeof of struct block_device (and subsequently, 
different alignment of struct inode), rather than locking improvement.

I would be interested if other people did performance testing of the 
patches too.

Mikulas