The Linux 5.2+ "Register Corruption" Bug / Golang Issue Was A One-Line Kernel Caching Issue
Written by Michael Larabel in Linux Kernel on 28 November 2019 at 12:19 AM EST. 14 Comments
LINUX KERNEL --
Coming together just over a day ago was the Google folks working on Golang figuring out many Go issues stemmed from a bug on the Linux 5.2 kernel and newer that was worked out to be some sort of a register corruption issue. That issue is now sorted out and fortunately it's a one-line kernel fix and boils down to being a caching issue.

Kernel developer Sebastian Andrzej Siewior figured out the bug -- since confirmed to address both the C test case and the Golang issues -- from caching access to the fpu_fpregs_owner_ctx context. The context was being cached but as the kernel deferred loading the FPU registers on return to userland, fpu_fpregs_owner_ctx could change during preemption and shouldn't be cached, per the patch devised to fix the issue.

Simply by passing fpu_fpregs_owner_ctx to this_cpu_read() instead of this_cpu_read_stable(), where as the latter allows the value to be cached rather than making GCC load the per-CPU variable each time it's accessed, fixes the issue of Linux 5.2+ built by GCC9 leading to the problems hitting Golang and other bugs.

This one-line change is confirmed to resolve the problem so will hopefully be rolling out to the kernel stable series shortly.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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