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

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Date:	Sat, 31 Oct 2015 09:47:36 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Yang Shi <yang.shi@...aro.org>, ast@...nel.org,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	netdev@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] bpf: convert hashtab lock to raw lock

On Fri, 30 Oct 2015 17:03:58 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> 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.

The error is that in -rt, you called a mutex and not a spin lock while
atomic.

-- Steve

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