[bug-report] task info hung problem in fb_deferred_io_work()

From: Harshit Mogalapalli
Date: Thu Apr 18 2024 - 08:40:43 EST

Hi,

While fuzzing 5.15.y kernel with Syzkaller, we noticed a INFO: task hung bug in fb_deferred_io_work()


This is in 5.15.149 tag, and this is introduced by a set of commits:

2655757a3f10 fbdev: flush deferred IO before closing
15492bab7393 fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()
87b9802ca824 fbdev: Fix invalid page access after closing deferred I/O devices
9a95fc04261f fbdev: Rename pagelist to pagereflist for deferred I/O
186b89659c4c fbdev: Track deferred-I/O pages in pageref struct
e79b2b2aadef fbdev: defio: fix the pagelist corruption
0616b00a31d6 fbdev: Don't sort deferred-I/O pages by default
5d3aff76a316 fbdev/defio: Early-out if page is already enlisted

I then tried if it could be reproducible on upstream kernel as well, and it did. below is the call trace with latest 6.9-rc4 based kernel. I have enabled the LOCKDEP to get locking related information.

INFO: task kworker/0:3:1077 blocked for more than 122 seconds.
Not tainted 6.9.0-rc4+ #54
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/0:3 state:D stack:0 pid:1077 tgid:1077 ppid:2 flags:0x00004000
Workqueue: events fb_deferred_io_work
Call Trace:
<TASK>
__schedule (kernel/sched/core.c:5409 kernel/sched/core.c:6746)
? __pfx___schedule (kernel/sched/core.c:6617)
? schedule (kernel/sched/core.c:6804 kernel/sched/core.c:6837)
? __pfx___lock_release (kernel/locking/lockdep.c:5406)
? schedule (kernel/sched/core.c:6804 kernel/sched/core.c:6837)
schedule (./arch/x86/include/asm/preempt.h:84 kernel/sched/core.c:6824 kernel/sched/core.c:6838)
io_schedule (kernel/sched/core.c:9019 kernel/sched/core.c:9045)
folio_wait_bit_common (mm/filemap.c:1272)
? __pfx_folio_wait_bit_common (mm/filemap.c:1206)
? __pfx___mutex_lock (kernel/locking/mutex.c:751)
? lock_acquire.part.0 (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5756)
? __pfx_wake_page_function (mm/filemap.c:1091)
? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5719)
fb_deferred_io_work (./include/linux/pagemap.h:1069 drivers/video/fbdev/core/fb_defio.c:248)
? lock_acquire (./include/trace/events/lock.h:24 kernel/locking/lockdep.c:5725)
process_one_work (kernel/workqueue.c:3254)
? __pfx_lock_acquire.part.0 (kernel/locking/lockdep.c:5719)
? __pfx_process_one_work (kernel/workqueue.c:3156)
? assign_work (kernel/workqueue.c:1210)
worker_thread (kernel/workqueue.c:3329 kernel/workqueue.c:3416)
? __kthread_parkme (kernel/kthread.c:293)
? __pfx_worker_thread (kernel/workqueue.c:3362)
kthread (kernel/kthread.c:388)
? __pfx_kthread (kernel/kthread.c:341)
ret_from_fork (arch/x86/kernel/process.c:147)
? __pfx_kthread (kernel/kthread.c:341)
ret_from_fork_asm (arch/x86/entry/entry_64.S:257)
</TASK>
INFO: task r:2664 blocked for more than 123 seconds.
Not tainted 6.9.0-rc4+ #54
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:r state:D stack:0 pid:2664 tgid:2664 ppid:2618 flags:0x00000002
Call Trace:
<TASK>
__schedule (kernel/sched/core.c:5409 kernel/sched/core.c:6746)
? __pfx___schedule (kernel/sched/core.c:6617)
? schedule (kernel/sched/core.c:6804 kernel/sched/core.c:6837)
? __pfx___lock_release (kernel/locking/lockdep.c:5406)
? schedule (kernel/sched/core.c:6804 kernel/sched/core.c:6837)
schedule (./arch/x86/include/asm/preempt.h:84 kernel/sched/core.c:6824 kernel/sched/core.c:6838)
schedule_timeout (kernel/time/timer.c:2559)
? __pfx_schedule_timeout (kernel/time/timer.c:2544)
? mark_lock.part.0 (./arch/x86/include/asm/bitops.h:227 /arch/x86/include/asm/bitops.h:239 /include/asm-generic/bitops/instrumented-non-atomic.h:142 kernel/locking/lockdep.c:228 kernel/locking/lockdep.c:4656)
? mark_held_locks (kernel/locking/lockdep.c:4274)
? lockdep_hardirqs_on_prepare.part.0 (kernel/locking/lockdep.c:4300 kernel/locking/lockdep.c:4359)
__wait_for_common (kernel/sched/completion.c:95 kernel/sched/completion.c:116)
? __pfx_schedule_timeout (kernel/time/timer.c:2544)
? __pfx___wait_for_common (kernel/sched/completion.c:110)
? start_flush_work (kernel/workqueue.c:4192)
__flush_work (kernel/workqueue.c:4207)
? __pfx___flush_work (kernel/workqueue.c:4195)
? __pfx_wq_barrier_func (kernel/workqueue.c:3742)
? flush_delayed_work (kernel/workqueue.c:4249)
? __pfx_fb_release (drivers/video/fbdev/core/fb_chrdev.c:375)
fb_deferred_io_lastclose (drivers/video/fbdev/core/fb_defio.c:321)
? __pfx_fb_release (drivers/video/fbdev/core/fb_chrdev.c:375)
fb_deferred_io_release (drivers/video/fbdev/core/fb_defio.c:333)
fb_release (drivers/video/fbdev/core/fb_chrdev.c:383)
? __pfx_fb_release (drivers/video/fbdev/core/fb_chrdev.c:375)
__fput (fs/file_table.c:422)
task_work_run (kernel/task_work.c:180 (discriminator 1))
? __pfx_task_work_run (kernel/task_work.c:148)
? do_raw_spin_unlock (./arch/x86/include/asm/paravirt.h:589 /arch/x86/include/asm/qspinlock.h:57 kernel/locking/spinlock_debug.c:142)
do_exit (kernel/exit.c:879)
? do_raw_spin_lock (./arch/x86/include/asm/atomic.h:115 /include/linux/atomic/atomic-arch-fallback.h:2170 /include/linux/atomic/atomic-instrumented.h:1302 /include/asm-generic/qspinlock.h:111 kernel/locking/spinlock_debug.c:116)
? __pfx_do_exit (kernel/exit.c:819)
? __pfx_do_raw_spin_lock (kernel/locking/spinlock_debug.c:114)
do_group_exit (kernel/exit.c:1008)
__x64_sys_exit_group (kernel/exit.c:1036)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f9cfe71e5f1
RSP: 002b:00007ffd636aec08 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 00007f9cfe7fc510 RCX: 00007f9cfe71e5f1
RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffffffffff80 R09: 0000000000000000
R10: 0000000000000020 R11: 0000000000000246 R12: 00007f9cfe7fc510
R13: 0000000000000000 R14: 00007f9cfe7fc9e8 R15: 00007f9cfe7fca00
</TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/50:
#0: ffffffff950219c0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks (./include/linux/rcupdate.h:329 (discriminator 1) /include/linux/rcupdate.h:781 (discriminator 1) kernel/locking/lockdep.c:6614 (discriminator 1))
3 locks held by kworker/0:3/1077:
#0: ffff88810005ad48 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work (kernel/workqueue.c:3229)
#1: ffff8881080dfd98 ((work_completion)(&(&info->deferred_work)->work)){+.+.}-{0:0}, at: process_one_work (kernel/workqueue.c:3230)
#2: ffff88811416db38 (&fbdefio->lock){+.+.}-{3:3}, at: fb_deferred_io_work (drivers/video/fbdev/core/fb_defio.c:246)
1 lock held by r/2664:
#0: ffff8881141fe078 (&fb_info->lock){+.+.}-{3:3}, at: fb_release (drivers/video/fbdev/core/fb_chrdev.c:380)



Syzkaller initially generated a long reproducer, Vegard simplified it to the one below

C repro:

#define _GNU_SOURCE
#include <sys/mman.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

int main(void)
{
int fd = open("/dev/fb0", O_RDWR, 0);

char *ptr = (char *) 0x20000000ul;

mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0xff000);
*ptr = 0;

mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0);
*ptr = 0;

return 0;
}

This needs to be run as root to see the hang in 122 seconds.

238 static void fb_deferred_io_work(struct work_struct *work)
239 {
240 struct fb_info *info = container_of(work, struct fb_info, deferred_work.work);
241 struct fb_deferred_io_pageref *pageref, *next;
242 struct fb_deferred_io *fbdefio = info->fbdefio;
243
244 /* here we mkclean the pages, then do all deferred IO */
245 mutex_lock(&fbdefio->lock);
246 list_for_each_entry(pageref, &fbdefio->pagereflist, list) {
247 struct page *cur = pageref->page;
248 lock_page(cur);
249 page_mkclean(cur);
250 unlock_page(cur);
251 }

It is stalling on lock_page() in fb_deferred_io_work()

I will be happy to try any patches and report back.


Thanks,
Harshit