Re: net/atm: warning in alloc_tx/__might_sleep

[Michal Hocko]

On Wed 11-01-17 20:45:25, Michal Hocko wrote:
> On Wed 11-01-17 09:37:06, Chas Williams wrote:
> > On Mon, 2017-01-09 at 18:20 +0100, Andrey Konovalov wrote:
> > > Hi!
> > > 
> > > I've got the following error report while running the syzkaller fuzzer.
> > > 
> > > On commit a121103c922847ba5010819a3f250f1f7fc84ab8 (4.10-rc3).
> > > 
> > > A reproducer is attached.
> > > 
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 0 PID: 4114 at kernel/sched/core.c:7737 __might_sleep+0x149/0x1a0
> > > do not call blocking ops when !TASK_RUNNING; state=1 set at
> > > [<ffffffff813fcb22>] prepare_to_wait+0x182/0x530
> > > Modules linked in:
> > > CPU: 0 PID: 4114 Comm: a.out Not tainted 4.10.0-rc3+ #59
> > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > > Call Trace:
> > >  __dump_stack lib/dump_stack.c:15
> > >  dump_stack+0x292/0x398 lib/dump_stack.c:51
> > >  __warn+0x19f/0x1e0 kernel/panic.c:547
> > >  warn_slowpath_fmt+0xc5/0x110 kernel/panic.c:562
> > >  __might_sleep+0x149/0x1a0 kernel/sched/core.c:7732
> > >  slab_pre_alloc_hook mm/slab.h:408
> > >  slab_alloc_node mm/slub.c:2634
> > >  kmem_cache_alloc_node+0x14a/0x280 mm/slub.c:2744
> > >  __alloc_skb+0x10f/0x800 net/core/skbuff.c:219
> > >  alloc_skb ./include/linux/skbuff.h:926
> > >  alloc_tx net/atm/common.c:75
> > 
> > This is likely alloc_skb(..., GFP_KERNEL) in alloc_tx().  The simplest
> > fix for this would be simply to switch this GFP_ATOMIC.  See if this is
> > any better.
> > 
> > diff --git a/net/atm/common.c b/net/atm/common.c
> > index a3ca922..d84220c 100644
> > --- a/net/atm/common.c
> > +++ b/net/atm/common.c
> > @@ -72,7 +72,7 @@ static struct sk_buff *alloc_tx(struct atm_vcc *vcc, unsigned int size)
> >  			 sk_wmem_alloc_get(sk), size, sk->sk_sndbuf);
> >  		return NULL;
> >  	}
> > -	while (!(skb = alloc_skb(size, GFP_KERNEL)))
> > +	while (!(skb = alloc_skb(size, GFP_ATOMIC)))
> >  		schedule();
> >  	pr_debug("%d += %d\n", sk_wmem_alloc_get(sk), skb->truesize);
> >  	atomic_add(skb->truesize, &sk->sk_wmem_alloc);
> 
> Blee, this code is just horrendous. But the "fix" is obviously broken!
> schedule() is just a noop if you do not change the task state and what
> you are just asking for is a never failing non sleeping allocation - aka
> a busy loop in the kernel!

And btw. this while loop should be really turned into GFP_KERNEL |
__GFP_NOFAIL with and explanation why this allocation cannot possibly
fail.
-- 
Michal Hocko
SUSE Labs