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

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Date:	Mon, 2 Nov 2015 09:59:27 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 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@...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.
> 
> I see. The patch makes sense then.
> Would be good to document this peculiarity of spin_lock.

I'm working on a document.

Thanks,

	tglx
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