From: "Paul E. McKenney" <paulmck@linux•ibm.com>
To: Peter Zijlstra <peterz@infradead•org>
Cc: Alexei Starovoitov <ast@kernel•org>,
davem@davemloft•net, daniel@iogearbox•net,
jakub.kicinski@netronome•com, netdev@vger•kernel.org,
kernel-team@fb•com, mingo@redhat•com, will.deacon@arm•com,
jannh@google•com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock
Date: Thu, 24 Jan 2019 15:42:32 -0800 [thread overview]
Message-ID: <20190124234232.GY4240@linux.ibm.com> (raw)
In-Reply-To: <20190124185652.GB17767@hirez.programming.kicks-ass.net>
On Thu, Jan 24, 2019 at 07:56:52PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote:
> >
> > Thanks for having kernel/locking people on Cc...
> >
> > On Wed, Jan 23, 2019 at 08:13:55PM -0800, Alexei Starovoitov wrote:
> >
> > > Implementation details:
> > > - on !SMP bpf_spin_lock() becomes nop
> >
> > Because no BPF program is preemptible? I don't see any assertions or
> > even a comment that says this code is non-preemptible.
> >
> > AFAICT some of the BPF_RUN_PROG things are under rcu_read_lock() only,
> > which is not sufficient.
> >
> > > - on architectures that don't support queued_spin_lock trivial lock is used.
> > > Note that arch_spin_lock cannot be used, since not all archs agree that
> > > zero == unlocked and sizeof(arch_spinlock_t) != sizeof(__u32).
> >
> > I really don't much like direct usage of qspinlock; esp. not as a
> > surprise.
Substituting the lightweight-reader SRCU as discussed earlier would allow
use of a more generic locking primitive, for example, one that allowed
blocking, at least in cases were the context allowed this.
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
branch srcu-lr.2019.01.16a.
One advantage of a more generic locking primitive would be keeping BPF
programs independent of internal changes to spinlock primitives.
Thanx, Paul
> > Why does it matter if 0 means unlocked; that's what
> > __ARCH_SPIN_LOCK_UNLOCKED is for.
> >
> > I get the sizeof(__u32) thing, but why not key off of that?
> >
> > > Next steps:
> > > - allow bpf_spin_lock in other map types (like cgroup local storage)
> > > - introduce BPF_F_LOCK flag for bpf_map_update() syscall and helper
> > > to request kernel to grab bpf_spin_lock before rewriting the value.
> > > That will serialize access to map elements.
> >
> > So clearly this map stuff is shared between bpf proglets, otherwise
> > there would not be a need for locking. But what happens if one is from
> > task context and another from IRQ context?
> >
> > I don't see a local_irq_save()/restore() anywhere. What avoids the
> > trivial lock inversion?
>
> Also; what about BPF running from NMI context and using locks?
>
next prev parent reply other threads:[~2019-01-24 23:42 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-24 4:13 [PATCH v4 bpf-next 0/9] introduce bpf_spin_lock Alexei Starovoitov
2019-01-24 4:13 ` [PATCH v4 bpf-next 1/9] bpf: " Alexei Starovoitov
2019-01-24 18:01 ` Peter Zijlstra
2019-01-24 18:56 ` Peter Zijlstra
2019-01-24 23:42 ` Paul E. McKenney [this message]
2019-01-25 0:05 ` Alexei Starovoitov
2019-01-25 1:22 ` Paul E. McKenney
2019-01-25 1:46 ` Jann Horn
2019-01-25 2:38 ` Alexei Starovoitov
2019-01-25 4:27 ` Alexei Starovoitov
2019-01-25 4:31 ` Paul E. McKenney
2019-01-25 4:47 ` Alexei Starovoitov
2019-01-25 16:02 ` Paul E. McKenney
2019-01-25 4:11 ` Paul E. McKenney
2019-01-25 16:18 ` Jann Horn
2019-01-25 22:51 ` Paul E. McKenney
2019-01-25 23:44 ` Alexei Starovoitov
2019-01-26 0:43 ` Jann Horn
2019-01-26 0:59 ` Jann Horn
2019-01-24 23:58 ` Alexei Starovoitov
2019-01-25 0:18 ` Jann Horn
2019-01-25 2:49 ` Alexei Starovoitov
2019-01-25 2:29 ` Eric Dumazet
2019-01-25 2:34 ` Alexei Starovoitov
2019-01-25 2:44 ` Eric Dumazet
2019-01-25 2:57 ` Alexei Starovoitov
2019-01-25 8:38 ` Peter Zijlstra
2019-01-25 9:10 ` Peter Zijlstra
2019-01-25 23:42 ` Alexei Starovoitov
2019-01-28 8:24 ` Peter Zijlstra
2019-01-28 8:31 ` Peter Zijlstra
2019-01-28 8:35 ` Peter Zijlstra
2019-01-28 20:49 ` Alexei Starovoitov
2019-01-28 8:43 ` Peter Zijlstra
2019-01-28 21:37 ` Alexei Starovoitov
2019-01-29 8:59 ` Peter Zijlstra
2019-01-30 2:20 ` Alexei Starovoitov
2019-01-25 9:59 ` Peter Zijlstra
2019-01-25 10:09 ` Peter Zijlstra
2019-01-25 10:23 ` Peter Zijlstra
2019-01-26 0:17 ` bpf memory model. Was: " Alexei Starovoitov
2019-01-28 9:24 ` Peter Zijlstra
2019-01-28 21:56 ` Alexei Starovoitov
2019-01-29 9:16 ` Peter Zijlstra
2019-01-30 2:32 ` Alexei Starovoitov
2019-01-30 8:58 ` Peter Zijlstra
2019-01-30 19:36 ` Alexei Starovoitov
2019-01-30 18:11 ` Will Deacon
2019-01-30 18:36 ` Paul E. McKenney
2019-01-30 19:51 ` Alexei Starovoitov
2019-01-30 21:05 ` Paul E. McKenney
2019-01-30 22:57 ` Alexei Starovoitov
2019-01-31 14:01 ` Paul E. McKenney
2019-01-31 18:47 ` Alexei Starovoitov
2019-02-01 14:05 ` Paul E. McKenney
2019-01-30 19:50 ` Alexei Starovoitov
2019-01-24 4:13 ` [PATCH v4 bpf-next 2/9] bpf: add support for bpf_spin_lock to cgroup local storage Alexei Starovoitov
2019-01-24 4:13 ` [PATCH v4 bpf-next 3/9] tools/bpf: sync include/uapi/linux/bpf.h Alexei Starovoitov
2019-01-24 4:13 ` [PATCH v4 bpf-next 4/9] selftests/bpf: add bpf_spin_lock tests Alexei Starovoitov
2019-01-24 4:13 ` [PATCH v4 bpf-next 5/9] selftests/bpf: add bpf_spin_lock C test Alexei Starovoitov
2019-01-24 4:14 ` [PATCH v4 bpf-next 6/9] bpf: introduce BPF_F_LOCK flag Alexei Starovoitov
2019-01-24 4:14 ` [PATCH v4 bpf-next 7/9] tools/bpf: sync uapi/bpf.h Alexei Starovoitov
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