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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 24 Jan 2019 23:42:35 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x0ONgYDL25362526 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 24 Jan 2019 23:42:34 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2A221B2065; Thu, 24 Jan 2019 23:42:34 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D7DE5B2066; Thu, 24 Jan 2019 23:42:33 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.203.30]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 24 Jan 2019 23:42:33 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id AB97E16C64CE; Thu, 24 Jan 2019 15:42:32 -0800 (PST) Date: Thu, 24 Jan 2019 15:42:32 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Alexei Starovoitov , 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 Reply-To: paulmck@linux.ibm.com References: <20190124041403.2100609-1-ast@kernel.org> <20190124041403.2100609-2-ast@kernel.org> <20190124180109.GA27771@hirez.programming.kicks-ass.net> <20190124185652.GB17767@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190124185652.GB17767@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19012423-0068-0000-0000-000003883E30 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010471; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000277; SDB=6.01151300; UDB=6.00600050; IPR=6.00931640; MB=3.00025277; MTD=3.00000008; XFM=3.00000015; UTC=2019-01-24 23:42:37 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19012423-0069-0000-0000-00004741D108 Message-Id: <20190124234232.GY4240@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-01-24_15:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=299 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901240161 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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? >