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

[Mikulas Patocka]

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
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