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.
Race Conditions on File Operations
Michael Kerrisk discovered a POSIX violation inside the kernel and brought it up on the mailing list. Apparently the read(), write(), and other related system calls did not perform atomic input and output actions. In his tests, Michael found that multithreaded write operations would receive conflicting file offset information, causing them to overwrite each other's file edits.
He posted some code that would reliably reproduce the problem and suggested that his test results pointed to the VFS (virtual filesystem) layer as the likely culprit. Michael also did some research, finding some very old discussions (some as many as eight years old) claiming that similar situations were not actually POSIX violations. But, Michael asserted that they really were. He said:
"Linux isn't conformant with SUSv3 and SUSv4 and isn't consistent with other implementations such as FreeBSD and Solaris. And I'm pretty sure Linux isn't consistent with UNIX since early times. (e.g., page 191 of the 1992 edition of Stevens APUE discusses the sharing of the file offset between the parent and child after fork(). Although Stevens didn't explicitly spell out the atomicity guarantee, the discussion there would be a bit nonsensical without the presumption of that guarantee.)"
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