Re: [syzbot] [net?] possible deadlock in vm_insert_page

[Hillf Danton <hdanton@sina.com>]

On Sat, 28 Dec 2024 01:52:28 -0800 Boqun Feng <boqun.feng@gmail.com>
> On Fri, Dec 27, 2024 at 06:03:45PM -0800, Suren Baghdasaryan wrote:
> > On Fri, Dec 27, 2024 at 4:19 PM Hillf Danton <hdanton@sina.com> wrote:
> > > On Fri, 27 Dec 2024 04:59:22 -0800
> > > >
> > > >        CPU0                    CPU1
> > > >        ----                    ----
> > > >   lock(&po->pg_vec_lock);
> > > >                                lock(&mm->mmap_lock);
> > > >                                lock(&po->pg_vec_lock);
> > > >   lock(&vma->vm_lock->lock);
> > > >
> > > >  *** DEADLOCK ***
> > > >
> > > > 2 locks held by syz.8.396/8273:
> > > >  #0: ffff0000d6a2cc10 (&mm->mmap_lock){++++}-{4:4}, at: mmap_write_lock_killable include/linux/mmap_lock.h:122 [inline]
> > > >  #0: ffff0000d6a2cc10 (&mm->mmap_lock){++++}-{4:4}, at: vm_mmap_pgoff+0x154/0x38c mm/util.c:578
> > > >  #1: ffff0000d4aa2868 (&po->pg_vec_lock){+.+.}-{4:4}, at: packet_mmap+0x9c/0x4c8 net/packet/af_packet.c:4650
> > > >
> > > Given &mm->mmap_lock and &po->pg_vec_lock in same locking order on both sides,
> > > this deadlock report is bogus. Due to lockdep glitch?
> 
> What do you mean by "both sides"? Note that, here is the report saying

CPU0/1 in the lockdep diagram above.

> the locks that are already held by the current task, and that current
> task is going to acquire &vma->vm_lock->lock, so lockdep finds new
> dependency:

Note the current task acquires &po->pg_vec_lock after taking &mm->mmap_lock,
and it is the &mm->mmap_lock (ignored by lockdep?) that makes the report
look bogus.
> 
> 	&po->pg_vec_lock --> &vma->vm_lock->lock
> 
> and there will be a circular dependency because (see above) lockdep
> recorded a dependency chain that:
> 
> 	&vma->vm_lock->lock --> ... --> &po->pg_vec_lock