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Re: [PATCH 7/7] xen/evtchn: read producer index only once

On 08/02/2021 10:59, Jürgen Groß wrote:
On 08.02.21 11:51, Jan Beulich wrote:
Yes, but I don't see an urgent need to fix that, as there would
be thousands of accesses in the kernel needing a fix. A compiler
tearing a naturally aligned access into multiple memory accesses
would be rejected as buggy from the kernel community IMO.

I would not be so sure. From lwn [1]:

"In the Linux kernel, tearing of plain C-language loads has been observed even given properly aligned and machine-word-sized loads.)"

And for store tearing:

"Note that this tearing can happen even on properly aligned and machine-word-sized accesses, and in this particular case, even for volatile stores. Some might argue that this behavior constitutes a bug in the compiler, but either way it illustrates the perceived value of store tearing from a compiler-writer viewpoint. [...] But for properly aligned machine-sized stores, WRITE_ONCE() will prevent store tearing."


[1] https://lwn.net/Articles/793253/#Load%20Tearing


Julien Grall



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