Messages in this thread |  | | | Date | Mon, 6 Jun 2011 09:28:35 -0500 | | Subject | Change in functionality of futex() system call. | | From | David Oliver <> |
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Hello,
The functionality of the futex() system call appears to have changed between versions 2.6.18 and 2.6.32.28.
Specifically, performing a FUTEX_WAIT on a read-only mapped location results in an EFAULT. Although other operations, such as FUTEX_WAKE, are only meaningful for writable locations, FUTEX_WAIT is useful for processes with read-only access to a memory-mapped file.
The code below illustrates the changed behavior (each of the EXPECT operations succeed on the older kernel, the ASSERTs pass in each case), assuming the file /tmp/futex_test exists and contains int(42).
With the older kernel, the syscall() suspends until another process changes the file and issues a FUTEX_WAKE, whereas the new behavior is for an EFAULT error, independent of the file contents.
Let me know if you need further clarification.
Cheers!
David Oliver.
#include <errno.h> #include <fcntl.h> #include <stdint.h> typedef uint32_t u32; // for futex.h #include <linux/futex.h> #include <sys/mman.h> #include <sys/syscall.h> #include <unistd.h> #include "gtest/gtest.h" // test framework to illustrate issue.
TEST(Futex, futex_in_read_only_file_is_ok) { int fd = open("/tmp/futex_test", O_RDONLY); ASSERT_GE(fd, 0); int* futex = static_cast<int *>(mmap(0, sizeof(int), PROT_READ, MAP_SHARED, fd, 0)); ASSERT_NE((int *)(0), futex); int rc = syscall(SYS_futex, futex, FUTEX_WAIT, 42, 0, 0, 0);
EXPECT_NE(-1, rc); // fails. if (rc == -1) { EXPECT_NE(errno, EFAULT); // fails. } }
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