Messages in this thread | | | Subject | Re: Change in functionality of futex() system call. | From | Eric Dumazet <> | Date | Mon, 06 Jun 2011 17:23:39 +0200 |
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Le lundi 06 juin 2011 à 09:28 -0500, David Oliver a écrit : > 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. > } > } >
Right you are, this came from commit 7485d0d3758e8e6491a5 (futexes: Remove rw parameter from get_futex_key()) in 2.6.33
commit 7485d0d3758e8e6491a5c9468114e74dc050785d Author: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Date: Tue Jan 5 16:32:43 2010 +0900
futexes: Remove rw parameter from get_futex_key()
Currently, futexes have two problem:
A) The current futex code doesn't handle private file mappings properly.
get_futex_key() uses PageAnon() to distinguish file and anon, which can cause the following bad scenario:
1) thread-A call futex(private-mapping, FUTEX_WAIT), it sleeps on file mapping object. 2) thread-B writes a variable and it makes it cow. 3) thread-B calls futex(private-mapping, FUTEX_WAKE), it wakes up blocked thread on the anonymous page. (but it's nothing)
B) Current futex code doesn't handle zero page properly.
Read mode get_user_pages() can return zero page, but current futex code doesn't handle it at all. Then, zero page makes infinite loop internally.
The solution is to use write mode get_user_page() always for page lookup. It prevents the lookup of both file page of private mappings and zero page.
Performance concerns:
Probaly very little, because glibc always initialize variables for futex before to call futex(). It means glibc users never see the overhead of this patch.
Compatibility concerns:
This patch has few compatibility issues. After this patch, FUTEX_WAIT require writable access to futex variables (read-only mappings makes EFAULT). But practically it's not a problem, glibc always initalizes variables for futexes explicitly - nobody uses read-only mappings.
Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Darren Hart <dvhltc@us.ibm.com> Cc: <stable@kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Nick Piggin <npiggin@suse.de> Cc: Ulrich Drepper <drepper@gmail.com> LKML-Reference: <20100105162633.45A2.A69D9226@jp.fujitsu.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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