3.16.58-rc1 review patch. If anyone has any objections, please let me know.
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From: Mel Gorman mgorman@suse.de
commit 48fb6f4db940e92cfb16cd878cddd59ea6120d06 upstream.
Commit 65d8fc777f6d ("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: 80000145
The 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 Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Ben Hutchings ben@decadent.org.uk --- kernel/futex.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -583,13 +583,14 @@ again: * this reference was taken by ihold under the page lock * pinning the inode in place so i_lock was unnecessary. The * only way for this check to fail is if the inode was - * truncated in parallel so warn for now if this happens. + * truncated in parallel which is almost certainly an + * application bug. In such a case, just retry. * * We are not calling into get_futex_key_refs() in file-backed * cases, therefore a successful atomic_inc return below will * guarantee that get_futex_key() will still imply smp_mb(); (B). */ - if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) { + if (!atomic_inc_not_zero(&inode->i_count)) { rcu_read_unlock(); put_page(page_head);