Re: net/atm: warning in alloc_tx/__might_sleep

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Jan 11, 2017 at 11:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> 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.

I think a nested loop is quite unnecessary, probably due to the code itself
is pretty old. The alloc_tx() is in the outer loop, the alloc_skb() is
in the inner
loop, both seem to wait for a successful GFP allocation. The inner one
is even more unnecessary.

Of course, I am not surprised MM may already have a mechanism to do
the similar logic.

There maybe some reason ATM needs such a logic, although other proto
could handle skb allocation failure quite well in ->sendmsg().