netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Message-ID: <20190125000515.jizijxz4n735gclx@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 24 Jan 2019 16:05:16 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, jakub.kicinski@...ronome.com,
        netdev@...r.kernel.org, kernel-team@...com, mingo@...hat.com,
        will.deacon@....com, jannh@...gle.com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Thu, Jan 24, 2019 at 03:42:32PM -0800, Paul E. McKenney wrote:
> 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.

Let's keep "srcu in bpf" discussion separate from bpf_spin_lock discussion.
bpf is not switching to srcu any time soon.
If/when it happens it will be only for certain prog+map types
like bpf syscall probes that need to be able to do copy_from_user
from bpf prog.