| Message ID | 20200304191104.2796501-1-yhs@fb.com |
|---|---|
| State | Accepted |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | bpf: Fix deadlock with rq_lock in bpf_send_signal() | expand |
On Wed, Mar 04, 2020 at 11:11:04AM -0800, Yonghong Song wrote: > > - if (in_nmi()) { > + /* Delay sending signal if irq is disabled. Otherwise, > + * we risk deadlock with rq_lock. > + */ This comment read in isolation is confusing. It's not clear how irq has anything to do with rq_lock and why deadlock is possible. But commit log has very nice summary, so I've just deleted that comment and applied to bpf tree. The patch 2 did reproduce the lockdep splat for me. Which was great. Running test_progs few times before and after I noticed that older send_signal tests are flaky and fail from time to time. It's unrelated to this patch, but please take a look.
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index 19e793aa441a..55a69b53054e 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -732,7 +732,10 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type) if (unlikely(!nmi_uaccess_okay())) return -EPERM; - if (in_nmi()) { + /* Delay sending signal if irq is disabled. Otherwise, + * we risk deadlock with rq_lock. + */ + if (irqs_disabled()) { /* Do an early check on signal validity. Otherwise, * the error is lost in deferred irq_work. */
When experimenting with bpf_send_signal() helper in our production environment (5.2 based), we experienced a deadlock in NMI mode: #5 [ffffc9002219f770] queued_spin_lock_slowpath at ffffffff8110be24 #6 [ffffc9002219f770] _raw_spin_lock_irqsave at ffffffff81a43012 #7 [ffffc9002219f780] try_to_wake_up at ffffffff810e7ecd #8 [ffffc9002219f7e0] signal_wake_up_state at ffffffff810c7b55 #9 [ffffc9002219f7f0] __send_signal at ffffffff810c8602 #10 [ffffc9002219f830] do_send_sig_info at ffffffff810ca31a #11 [ffffc9002219f868] bpf_send_signal at ffffffff8119d227 #12 [ffffc9002219f988] bpf_overflow_handler at ffffffff811d4140 #13 [ffffc9002219f9e0] __perf_event_overflow at ffffffff811d68cf #14 [ffffc9002219fa10] perf_swevent_overflow at ffffffff811d6a09 #15 [ffffc9002219fa38] ___perf_sw_event at ffffffff811e0f47 #16 [ffffc9002219fc30] __schedule at ffffffff81a3e04d #17 [ffffc9002219fc90] schedule at ffffffff81a3e219 #18 [ffffc9002219fca0] futex_wait_queue_me at ffffffff8113d1b9 #19 [ffffc9002219fcd8] futex_wait at ffffffff8113e529 #20 [ffffc9002219fdf0] do_futex at ffffffff8113ffbc #21 [ffffc9002219fec0] __x64_sys_futex at ffffffff81140d1c #22 [ffffc9002219ff38] do_syscall_64 at ffffffff81002602 #23 [ffffc9002219ff50] entry_SYSCALL_64_after_hwframe at ffffffff81c00068 The above call stack is actually very similar to an issue reported by Commit eac9153f2b58 ("bpf/stackmap: Fix deadlock with rq_lock in bpf_get_stack()") by Song Liu. The only difference is bpf_send_signal() helper instead of bpf_get_stack() helper. The above deadlock is triggered with a perf_sw_event. Similar to Commit eac9153f2b58, the below almost identical reproducer used tracepoint point sched/sched_switch so the issue can be easily caught. /* stress_test.c */ #include <stdio.h> #include <stdlib.h> #include <sys/mman.h> #include <pthread.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #define THREAD_COUNT 1000 char *filename; void *worker(void *p) { void *ptr; int fd; char *pptr; fd = open(filename, O_RDONLY); if (fd < 0) return NULL; while (1) { struct timespec ts = {0, 1000 + rand() % 2000}; ptr = mmap(NULL, 4096 * 64, PROT_READ, MAP_PRIVATE, fd, 0); usleep(1); if (ptr == MAP_FAILED) { printf("failed to mmap\n"); break; } munmap(ptr, 4096 * 64); usleep(1); pptr = malloc(1); usleep(1); pptr[0] = 1; usleep(1); free(pptr); usleep(1); nanosleep(&ts, NULL); } close(fd); return NULL; } int main(int argc, char *argv[]) { void *ptr; int i; pthread_t threads[THREAD_COUNT]; if (argc < 2) return 0; filename = argv[1]; for (i = 0; i < THREAD_COUNT; i++) { if (pthread_create(threads + i, NULL, worker, NULL)) { fprintf(stderr, "Error creating thread\n"); return 0; } } for (i = 0; i < THREAD_COUNT; i++) pthread_join(threads[i], NULL); return 0; } and the following command: 1. run `stress_test /bin/ls` in one windown 2. hack bcc trace.py with the following change: --- a/tools/trace.py +++ b/tools/trace.py @@ -513,6 +513,7 @@ BPF_PERF_OUTPUT(%s); __data.tgid = __tgid; __data.pid = __pid; bpf_get_current_comm(&__data.comm, sizeof(__data.comm)); + bpf_send_signal(10); %s %s %s.perf_submit(%s, &__data, sizeof(__data)); 3. in a different window run ./trace.py -p $(pidof stress_test) t:sched:sched_switch The deadlock can be reproduced in our production system. Similar to Song's fix, the fix is to delay sending signal if irqs is disabled to avoid deadlocks involving with rq_lock. With this change, my above stress-test in our production system won't cause deadlock any more. I also implemented a scale-down version of reproducer in the selftest (a subsequent commit). With latest bpf-next, it complains for the following potential deadlock. [ 32.832450] -> #1 (&p->pi_lock){-.-.}: [ 32.833100] _raw_spin_lock_irqsave+0x44/0x80 [ 32.833696] task_rq_lock+0x2c/0xa0 [ 32.834182] task_sched_runtime+0x59/0xd0 [ 32.834721] thread_group_cputime+0x250/0x270 [ 32.835304] thread_group_cputime_adjusted+0x2e/0x70 [ 32.835959] do_task_stat+0x8a7/0xb80 [ 32.836461] proc_single_show+0x51/0xb0 ... [ 32.839512] -> #0 (&(&sighand->siglock)->rlock){....}: [ 32.840275] __lock_acquire+0x1358/0x1a20 [ 32.840826] lock_acquire+0xc7/0x1d0 [ 32.841309] _raw_spin_lock_irqsave+0x44/0x80 [ 32.841916] __lock_task_sighand+0x79/0x160 [ 32.842465] do_send_sig_info+0x35/0x90 [ 32.842977] bpf_send_signal+0xa/0x10 [ 32.843464] bpf_prog_bc13ed9e4d3163e3_send_signal_tp_sched+0x465/0x1000 [ 32.844301] trace_call_bpf+0x115/0x270 [ 32.844809] perf_trace_run_bpf_submit+0x4a/0xc0 [ 32.845411] perf_trace_sched_switch+0x10f/0x180 [ 32.846014] __schedule+0x45d/0x880 [ 32.846483] schedule+0x5f/0xd0 ... [ 32.853148] Chain exists of: [ 32.853148] &(&sighand->siglock)->rlock --> &p->pi_lock --> &rq->lock [ 32.853148] [ 32.854451] Possible unsafe locking scenario: [ 32.854451] [ 32.855173] CPU0 CPU1 [ 32.855745] ---- ---- [ 32.856278] lock(&rq->lock); [ 32.856671] lock(&p->pi_lock); [ 32.857332] lock(&rq->lock); [ 32.857999] lock(&(&sighand->siglock)->rlock); Deadlock happens on CPU0 when it tries to acquire &sighand->siglock but it has been held by CPU1 and CPU1 tries to grab &rq->lock and cannot get it. This is not exactly the callstack in our production environment, but sympotom is similar and both locks are using spin_lock_irqsave() to acquire the lock, and both involves rq_lock. The fix to delay sending signal when irq is disabled also fixed this issue. Cc: Song Liu <songliubraving@fb.com> Signed-off-by: Yonghong Song <yhs@fb.com> --- kernel/trace/bpf_trace.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)