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Commit65d8fc777f("futex: Remove requirement for lock_page() in get_futex_key()") removed an unnecessary lock_page() with the side-effect that page->mapping needed to be treated very carefully. Two defensive warnings were added in case any assumption was missed and the first warning assumed a correct application would not alter a mapping backing a futex key. Since merging, it has not triggered for any unexpected case but Mark Rutland reported the following bug triggering due to the first warning. kernel BUG at kernel/futex.c:679! Internal error: Oops - BUG: 0 [#1] PREEMPT SMP Modules linked in: CPU: 0 PID: 3695 Comm: syz-executor1 Not tainted 4.13.0-rc3-00020-g307fec773ba3 #3 Hardware name: linux,dummy-virt (DT) task: ffff80001e271780 task.stack: ffff000010908000 PC is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679 LR is at get_futex_key+0x6a4/0xcf0 kernel/futex.c:679 pc : [<ffff00000821ac14>] lr : [<ffff00000821ac14>] pstate:80000145The fact that it's a bug instead of a warning was due to an unrelated arm64 problem, but the warning itself triggered because the underlying mapping changed. This is an application issue but from a kernel perspective it's a recoverable situation and the warning is unnecessary so this patch removes the warning. The warning may potentially be triggered with the following test program from Mark although it may be necessary to adjust NR_FUTEX_THREADS to be a value smaller than the number of CPUs in the system. #include <linux/futex.h> #include <pthread.h> #include <stdio.h> #include <stdlib.h> #include <sys/mman.h> #include <sys/syscall.h> #include <sys/time.h> #include <unistd.h> #define NR_FUTEX_THREADS 16 pthread_t threads[NR_FUTEX_THREADS]; void *mem; #define MEM_PROT (PROT_READ | PROT_WRITE) #define MEM_SIZE 65536 static int futex_wrapper(int *uaddr, int op, int val, const struct timespec *timeout, int *uaddr2, int val3) { syscall(SYS_futex, uaddr, op, val, timeout, uaddr2, val3); } void *poll_futex(void *unused) { for (;;) { futex_wrapper(mem, FUTEX_CMP_REQUEUE_PI, 1, NULL, mem + 4, 1); } } int main(int argc, char *argv[]) { int i; mem = mmap(NULL, MEM_SIZE, MEM_PROT, MAP_SHARED | MAP_ANONYMOUS, -1, 0); printf("Mapping @ %p\n", mem); printf("Creating futex threads...\n"); for (i = 0; i < NR_FUTEX_THREADS; i++) pthread_create(&threads[i], NULL, poll_futex, NULL); printf("Flipping mapping...\n"); for (;;) { mmap(mem, MEM_SIZE, MEM_PROT, MAP_FIXED | MAP_SHARED | MAP_ANONYMOUS, -1, 0); } return 0; } Reported-and-tested-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Mel Gorman <mgorman@suse.de> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: stable@vger.kernel.org # 4.7+ Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.