Re: [PATCH] fs: fix a data race in i_size_write/i_size_read

[Qian Cai]

> On Feb 18, 2020, at 11:52 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 
> If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up
> being split, the kernel is FUBAR anyway.  Details, please - how could that
> end up happening?

My understanding is the compiler might decide to split the load into saying two 4-byte loads. Then, we might have,

Load1
Store
Load2

where the load value could be a garbage. Also, Marco (the KCSAN maintainer) who knew more of compiler than me mentioned that there is no guarantee that the store will not be split either. Thus, the WRITE_ONCE().