On Wed, Dec 16, 2020 at 5:16 PM Paul E. McKenney <paul...@kernel.org> wrote: > > On Wed, Dec 16, 2020 at 10:52:06AM +0100, Daniel Vetter wrote: > > On Wed, Dec 16, 2020 at 2:14 AM syzbot > > <syzbot+972b924c988834e86...@syzkaller.appspotmail.com> wrote: > > > > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: 94801e5c Merge tag 'pinctrl-v5.10-3' of > > > git://git.kernel.o.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=130558c5500000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=ee8a1012a5314210 > > > dashboard link: > > > https://syzkaller.appspot.com/bug?extid=972b924c988834e868b2 > > > compiler: gcc (GCC) 10.1.0-syz 20200507 > > > userspace arch: i386 > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the > > > commit: > > > Reported-by: syzbot+972b924c988834e86...@syzkaller.appspotmail.com > > > > > > ============================= > > > WARNING: suspicious RCU usage > > > 5.10.0-rc7-syzkaller #0 Not tainted > > > ----------------------------- > > > kernel/sched/core.c:7270 Illegal context switch in RCU-sched read-side > > > critical section! > > > > > > other info that might help us debug this: > > > > > > > > > rcu_scheduler_active = 2, debug_locks = 0 > > > 7 locks held by syz-executor.1/9232: > > > #0: ffffffff8b328c60 (console_lock){+.+.}-{0:0}, at: > > > do_fb_ioctl+0x2e4/0x690 drivers/video/fbdev/core/fbmem.c:1106 > > > #1: ffff888041bd4078 (&fb_info->lock){+.+.}-{3:3}, at: lock_fb_info > > > include/linux/fb.h:636 [inline] > > > #1: ffff888041bd4078 (&fb_info->lock){+.+.}-{3:3}, at: > > > do_fb_ioctl+0x2ee/0x690 drivers/video/fbdev/core/fbmem.c:1107 > > > #2: ffff888041adca78 (&helper->lock){+.+.}-{3:3}, at: > > > drm_fb_helper_pan_display+0xce/0x970 drivers/gpu/drm/drm_fb_helper.c:1448 > > > #3: ffff8880159f01b8 (&dev->master_mutex){+.+.}-{3:3}, at: > > > drm_master_internal_acquire+0x1d/0x70 drivers/gpu/drm/drm_auth.c:407 > > > #4: ffff888041adc898 (&client->modeset_mutex){+.+.}-{3:3}, at: > > > drm_client_modeset_commit_locked+0x44/0x580 > > > drivers/gpu/drm/drm_client_modeset.c:1143 > > > #5: ffffc90001c07730 (crtc_ww_class_acquire){+.+.}-{0:0}, at: > > > drm_client_modeset_commit_atomic+0xb7/0x7c0 > > > drivers/gpu/drm/drm_client_modeset.c:981 > > > #6: ffff888015986108 (crtc_ww_class_mutex){+.+.}-{3:3}, at: > > > ww_mutex_lock_slow include/linux/ww_mutex.h:287 [inline] > > > #6: ffff888015986108 (crtc_ww_class_mutex){+.+.}-{3:3}, at: > > > modeset_lock+0x31c/0x650 drivers/gpu/drm/drm_modeset_lock.c:260 > > > > Given that we managed to take all these locks without upsetting anyone > > the rcu section is very deep down. And looking at the backtrace below > > I just couldn't find anything. > > > > Best I can think of is that an interrupt of some sort leaked an rcu > > section, and we got shot here. But I'd assume the rcu debugging would > > catch this? Backtrace of the start of that rcu read side section would > > be really useful here, but I'm not seeing that in the logs. There's > > more stuff there, but it's just the usual "everything falls apart" > > stuff of little value to understanding how we got there. > > In my experience, lockdep will indeed complain if an interrupt handler > returns while in an RCU read-side critical section. > > > Adding some rcu people for more insights on what could have gone wrong here. > > -Daniel > > > > > stack backtrace: > > > CPU: 1 PID: 9232 Comm: syz-executor.1 Not tainted 5.10.0-rc7-syzkaller #0 > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS > > > rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014 > > > Call Trace: > > > __dump_stack lib/dump_stack.c:77 [inline] > > > dump_stack+0x107/0x163 lib/dump_stack.c:118 > > > ___might_sleep+0x25d/0x2b0 kernel/sched/core.c:7270 > > > __mutex_lock_common kernel/locking/mutex.c:935 [inline] > > > __ww_mutex_lock.constprop.0+0xa9/0x2cc0 kernel/locking/mutex.c:1111 > > > ww_mutex_lock+0x3d/0x170 kernel/locking/mutex.c:1190 > > Acquiring a mutex while under the influence of rcu_read_lock() will > definitely get you this lockdep complaint, and rightfully so. > > If you need to acquire a mutex with RCU-like protection, one approach > is to use SRCU. But usually this indicates (as you suspected) that > someone forgot to invoke rcu_read_unlock(). > > One way to locate this is to enlist the aid of lockdep. You can do this > by putting something like this in the callers: > > RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) || > lock_is_held(&rcu_lock_map) || > lock_is_held(&rcu_sched_lock_map), > "We are in an RCU read-side critical section"); > > This will get you a lockdep complaint much like the one above if the > caller is in any sort of RCU read-side critical section. You can push > this up the call stack one level at a time or just sprinkle it up the > stack in one go. > > The complaint is specifically about RCU-sched, so you could focus on > that using this instead: > > RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map), > "We are in an RCU-sched read-side critical section"); > > This of course assumes that this is reproducible. :-/ > > But even if it isn't reproducible, for example, if the mutex is only > acquired occasionally, these RCU_LOCKDEP_WARN() calls can be used to > check assumptions about state.
There is another recent claim of a false "suspicious RCU usage": https://lore.kernel.org/lkml/20201216205536.gx2...@casper.infradead.org/ Can this be wrong accounting by lock debugging? > Thanx, Paul > > > > modeset_lock+0x392/0x650 drivers/gpu/drm/drm_modeset_lock.c:263 > > > drm_modeset_lock drivers/gpu/drm/drm_modeset_lock.c:342 [inline] > > > drm_modeset_lock+0x50/0x90 drivers/gpu/drm/drm_modeset_lock.c:338 > > > drm_atomic_get_plane_state+0x19d/0x510 drivers/gpu/drm/drm_atomic.c:481 > > > drm_client_modeset_commit_atomic+0x225/0x7c0 > > > drivers/gpu/drm/drm_client_modeset.c:994 > > > drm_client_modeset_commit_locked+0x145/0x580 > > > drivers/gpu/drm/drm_client_modeset.c:1145 > > > pan_display_atomic drivers/gpu/drm/drm_fb_helper.c:1395 [inline] > > > drm_fb_helper_pan_display+0x28b/0x970 > > > drivers/gpu/drm/drm_fb_helper.c:1455 > > > fb_pan_display+0x2f7/0x6c0 drivers/video/fbdev/core/fbmem.c:925 > > > fb_set_var+0x57f/0xda0 drivers/video/fbdev/core/fbmem.c:1043 > > > do_fb_ioctl+0x2f9/0x690 drivers/video/fbdev/core/fbmem.c:1108 > > > fb_compat_ioctl+0x17c/0xaf0 drivers/video/fbdev/core/fbmem.c:1315 > > > __do_compat_sys_ioctl+0x1d3/0x230 fs/ioctl.c:842 > > > do_syscall_32_irqs_on arch/x86/entry/common.c:78 [inline] > > > __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:137 > > > do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:160 > > > entry_SYSENTER_compat_after_hwframe+0x4d/0x5c > > > RIP: 0023:0xf7fd8549 > > > Code: 03 74 c0 01 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 > > > 10 08 03 74 d8 01 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 > > > 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90 > > > RSP: 002b:00000000f55d20bc EFLAGS: 00000296 ORIG_RAX: 0000000000000036 > > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000004601 > > > RDX: 0000000020000240 RSI: 0000000000000000 RDI: 0000000000000000 > > > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 > > > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 > > > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 > > > detected fb_set_par error, error code: -16 > > > > > > > > > --- > > > This report is generated by a bot. It may contain errors. > > > See https://goo.gl/tpsmEJ for more information about syzbot. > > > syzbot engineers can be reached at syzkal...@googlegroups.com. > > > > > > syzbot will keep track of this issue. See: > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
