Re: [PATCH] bpf: convert hashtab lock to raw lock

[Shi, Yang]

On 11/2/2015 12:59 AM, Thomas Gleixner wrote:
> On Sun, 1 Nov 2015, Alexei Starovoitov wrote:
> > On Sat, Oct 31, 2015 at 09:47:36AM -0400, Steven Rostedt wrote:
> > > On Fri, 30 Oct 2015 17:03:58 -0700
> > > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > > On Fri, Oct 30, 2015 at 03:16:26PM -0700, Yang Shi wrote:
> > > > > When running bpf samples on rt kernel, it reports the below warning:
> > > > >
> > > > > BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
> > > > > in_atomic(): 1, irqs_disabled(): 128, pid: 477, name: ping
> > > > > Preemption disabled at:[<ffff80000017db58>] kprobe_perf_func+0x30/0x228
> > > > ...
> > > > > diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> > > > > index 83c209d..972b76b 100644
> > > > > --- a/kernel/bpf/hashtab.c
> > > > > +++ b/kernel/bpf/hashtab.c
> > > > > @@ -17,7 +17,7 @@
> > > > >   struct bpf_htab {
> > > > >   	struct bpf_map map;
> > > > >   	struct hlist_head *buckets;
> > > > > -	spinlock_t lock;
> > > > > +	raw_spinlock_t lock;
> > > >
> > > > How do we address such things in general?
> > > > I bet there are tons of places around the kernel that
> > > > call spin_lock from atomic.
> > > > I'd hate to lose the benefits of lockdep of non-raw spin_lock
> > > > just to make rt happy.
> > >
> > > You wont lose any benefits of lockdep. Lockdep still checks
> > > raw_spin_lock(). The only difference between raw_spin_lock and
> > > spin_lock is that in -rt spin_lock turns into an rt_mutex() and
> > > raw_spin_lock stays a spin lock.
> >
> > I see. The patch makes sense then.
> > Would be good to document this peculiarity of spin_lock.
>
> I'm working on a document.

Thanks Steven and Thomas for your elaboration and comment.

Yang

> Thanks,
>
> 	tglx

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