Zack's Kernel News
Zack's Kernel News
Chronicler Zack Brown reports on the latest news, views, dilemmas, and developments within the Linux kernel community.
Bug Hunt
Compiler bugs are rough on the Linux kernel because the kernel needs to compile on as many different systems as possible. The more constraints Linux imposes on the toolchain, the more difficult it becomes for absolutely anyone to build a kernel on their strange and unpredictable hardware setup. Traditionally, Linux makes a big effort to avoid losing compatibility with any version of the GCC compiler.
Fengguang Wu recently reported a memory paging error in the kernel and used git bisect to trace the problem to a patch by Peter Zijlstra. Peter had implemented an optimization suggested by Linus Torvalds to use the asm goto instruction in the modify_and_test() functions. In theory, asm goto was cleaner than the previous implementation, which needed an extra hardware register.
All seemed well until Fengguang reported that surprising memory error. Peter wasn't able to reproduce the bug on his system and couldn't see anything wrong with the patch itself. He suggested Fengguang test whether other compilers produced the same problem. Apparently, Fengguang had compiled the broken kernel under GCC version 4.8.1; he also tried the earlier GCC 4.6.1 but was able to reproduce the breakage.
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