Re: WARNING in enqueue_task_dl

[Daniel Wagner <dwagner@suse.de>]

On Tue, Jul 23, 2019 at 09:45:16PM -0700, Eric Biggers wrote:
> On Thu, Feb 07, 2019 at 10:35:04AM +0100, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> > On Mon, Jan 7, 2019 at 5:19 PM Daniel Bristot de Oliveira
> > <bristot@redhat.com> wrote:
> > >
> > > On 11/19/18 4:32 PM, Juri Lelli wrote:
> > > > From 9326fd2b20269cffef7290bdc5b8173460d3c870 Mon Sep 17 00:00:00 2001
> > > > From: Juri Lelli <juri.lelli@redhat.com>
> > > > Date: Mon, 19 Nov 2018 16:04:42 +0100
> > > > Subject: [PATCH] sched/core: Fix PI boosting between RT and DEADLINE
> > > >
> > > > syzbot reported the following warning:
> > > >
> > > >  WARNING: CPU: 1 PID: 6351 at kernel/sched/deadline.c:628
> > > >  enqueue_task_dl+0x22da/0x38a0 kernel/sched/deadline.c:1504
> > > >  PM: Basic memory bitmaps freed
> > > >  Kernel panic - not syncing: panic_on_warn set ...
> > > >  CPU: 1 PID: 6351 Comm: syz-executor0 Not tainted 4.20.0-rc2+ #338
> > > >  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > >  Google 01/01/2011
> > > >  Call Trace:
> > > >    __dump_stack lib/dump_stack.c:77 [inline]
> > > >    dump_stack+0x244/0x39d lib/dump_stack.c:113
> > > >    panic+0x2ad/0x55c kernel/panic.c:188
> > > >    __warn.cold.8+0x20/0x45 kernel/panic.c:540
> > > >    report_bug+0x254/0x2d0 lib/bug.c:186
> > > >    fixup_bug arch/x86/kernel/traps.c:178 [inline]
> > > >    do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
> > > >    do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:290
> > > >    invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:969
> > > >  RIP: 0010:enqueue_task_dl+0x22da/0x38a0 kernel/sched/deadline.c:1504
> > > >  Code: ff 48 8b 8d c8 fe ff ff 48 c1 e6 2a 4c 8b 9d d0 fe ff ff 8b 95 d8 fe
> > > >  ff ff 48 8b 85 e0 fe ff ff e9 16 e4 ff ff e8 16 d0 ea ff <0f> 0b e9 17 f1
> > > >  ff ff 48 8b bd e8 fe ff ff 4c 89 95 c8 fe ff ff 48
> > > >  RSP: 0018:ffff8881ba39fa18 EFLAGS: 00010002
> > > >  RAX: 0000000000000000 RBX: ffff8881b9d6c000 RCX: ffff8881b9d6c278
> > > >  RDX: ffff8881b9d6c03c RSI: 0000000000000002 RDI: ffff8881daf2d710
> > > >  RBP: ffff8881ba39fb78 R08: 0000000000000001 R09: ffff8881daf00000
> > > >  R10: 0000001a4d4f1987 R11: ffff8881daf2db3b R12: 1ffff11037473f4e
> > > >  R13: ffff8881b9d6c2cc R14: ffff8881daf2ccc0 R15: ffff8881daf2ccc0
> > > >    enqueue_task+0x184/0x390 kernel/sched/core.c:730
> > > >    __sched_setscheduler+0xe99/0x2190 kernel/sched/core.c:4336
> > > >    sched_setattr kernel/sched/core.c:4394 [inline]
> > > >    __do_sys_sched_setattr kernel/sched/core.c:4570 [inline]
> > > >    __se_sys_sched_setattr kernel/sched/core.c:4549 [inline]
> > > >    __x64_sys_sched_setattr+0x1b2/0x2f0 kernel/sched/core.c:4549
> > > >    do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> > > >    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > >  RIP: 0033:0x457569
> > > >  Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
> > > >  48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
> > > >  ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > > >  RSP: 002b:00007f05ce0a2c78 EFLAGS: 00000246 ORIG_RAX: 000000000000013a
> > > >  RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457569
> > > >  RDX: 0000000000000000 RSI: 0000000020000000 RDI: 0000000000000000
> > > >  RBP: 000000000072bfa0 R08: 0000000000000000 R09: 0000000000000000
> > > >  R10: 0000000000000000 R11: 0000000000000246 R12: 00007f05ce0a36d4
> > > >  R13: 00000000004c369f R14: 00000000004d5730 R15: 00000000ffffffff
> > > >
> > > > At deadline.c:628 we have:
> > > >
> > > >  623 static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se)
> > > >  624 {
> > > >  625  struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> > > >  626  struct rq *rq = rq_of_dl_rq(dl_rq);
> > > >  627
> > > >  628  WARN_ON(dl_se->dl_boosted);
> > > >  629  WARN_ON(dl_time_before(rq_clock(rq), dl_se->deadline));
> > > >         [...]
> > > >      }
> > > >
> > > > Which means that setup_new_dl_entity() has been called on a task
> > > > currently boosted. This shouldn't happen though, as setup_new_
> > > > dl_entity() is only called when the 'dynamic' deadline of the new entity
> > > > is in the past w.r.t. rq_clock and boosted tasks shouldn't verify this
> > > > condition.
> > > >
> > > > Digging through PI code I noticed that what above might in fact happen
> > > > if an RT tasks blocks on an rt_mutex hold by a DEADLINE task. In the
> > > > first branch of boosting conditions we check only if a pi_task 'dynamic'
> > > > deadline is earlier than mutex holder's and in this case we set mutex
> > > > holder to be dl_boosted. However, since RT 'dynamic' deadlines are only
> > > > initialized if such tasks get boosted at some point (or if they become
> > > > DEADLINE of course), in general RT 'dynamic' deadlines are usually equal
> > > > to 0 and this verifies the aforementioned condition.
> > > >
> > > > Fix it by checking that the potential donor task is actually (even if
> > > > temporary because in turn boosted) running at DEADLINE priority before
> > > > using its 'dynamic' deadline value.
> > > >
> > > > Reported-by: syzbot+119ba87189432ead09b4@syzkaller.appspotmail.com
> > > > Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> > >
> > > Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> > 
> > What happened with this patch? I still don't see it in linux-next.
> > 
> > There is a number of reproducers that involve sched_setattr and lead
> > to dead machines on syzbot:
> > https://syzkaller.appspot.com/bug?id=0b210638616bb68109e9642158d4c0072770ae1c
> > 
> 
> Ping.  Patch is not applied, and this WARNING is still being hit.
> 
> Also note the bisection result:
> 
> 	commit 7c80cfc99b7bfdc92cee26f8008859f326f4a37f
> 	Author: Peter Zijlstra <peterz@infradead.org>
> 	Date: Sat May 6 14:03:17 2017 +0000
> 
> 	  sched/fair: Clean up calc_cfs_shares()

I've tested this patch against 5.8-rc1. Without the fix, after around 2 hours
the warning was triggered by the reproducer. With the patch, it survived
roughly 12 hours without the warning.

Tested-by: Daniel Wagner <dwagner@suse.de>