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Date:   Wed, 7 Mar 2018 14:37:30 +0000
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Andrea Parri <parri.andrea@gmail.com>
Cc:     Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@gmail.com>,
        Jason Wessel <jason.wessel@windriver.com>
Subject: Re: [PATCH] Documentation/locking: Document the semantics of
 spin_is_locked()
Message-ID: <20180307143730.y7hoo3vjbogx6gmr@holly.lan>
References: <1519814372-19941-1-git-send-email-parri.andrea@gmail.com>
 <20180228105631.GA7681@arm.com>
 <20180228112403.GA32228@andrea>
 <20180228113456.GC7681@arm.com>
 <20180228121523.GA354@andrea>
 <20180307131341.GA28486@andrea>
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On Wed, Mar 07, 2018 at 02:13:41PM +0100, Andrea Parri wrote:
> On Wed, Feb 28, 2018 at 01:15:23PM +0100, Andrea Parri wrote:
> > On Wed, Feb 28, 2018 at 11:34:56AM +0000, Will Deacon wrote:
> 
> [...]
> 
> >> only if there's some evidence that you've looked at the callsites
> >> and determined that they won't break.
> 
> I looked at the callsites for {,raw_}spin_is_locked() (reported below):
>
> In most cases (40+), these primitives are used within BUG_ON/WARN_ON or
> the like; a handful of other cases using these with no concurrency, for
> checking "self-lock", or for heuristics.
> 
> I confirm that the "ipc/sem.c" case, mentioned in the arm64 and powerpc
> commits adding smp_mb() to their arch_spin_is_locked(), disappeared.
> 
> And that the "debug_core" case seems to be the only case requiring some
> thoughts: my understanding (but I Cc the KGDB maintainers, so that they
> can correct me, or provide other details) is that KGDB does not rely on
> implicit barriers in raw_spin_is_locked().
> 
> (This seems instead to rely on barriers in the IPIs sending/handling, in
>  part., kgdb_roundup_cpus, kgdb_nmicallback; yes, these barriers are not
>  documented, but I've discussed with Daniel, Jason about the eventuality
>  of adding such documentations/inline comments.)

Indeed.

Whilst responding to queries from Andrea I certainly saw opportunities 
to clean things up... and the result of those clean ups would actually 
be the removal of both calls to raw_spin_is_locked(). Nevertheless, for
now lets deal with the code as it is:

The calls to raw_spin_is_locked() within debug-core will pretty much always 
be from cores that did not take the lock because the code is triggered
once we have selected a master and are rounding up the other cpus. Thus
we do have to analyse the sequencing here.

Pretty much every architecture I looked at implements the round up 
using the IPI machinery (hardly surprising; this is obvious way to 
implement it). I think this provides the required barriers implicitly
so the is-this-round-up code will correctly observe the locks to be 
locked when triggered via an IPI.

It is more difficult to describe the analysis if the is-this-a-round-up
code is spuriously triggered before the IPI but so far I've not come up 
with anything worse than a benign race (which exists even with barriers).
The round up code will eventually figure out it has spuriously tried to
park and will exit without altering important state. The return value of 
kgdb_nmicallback() does change in this case but no architecture cares 
about that[1].


Daniel


[1] So one of the clean ups I alluded to above is therefore to remove 
    the return code ;-) .


> (N.B. I have _not_ tested any of these observations, say by removing the
>  smp_mb() from your implementation; so, you know...)
> 
>   Andrea
> 
> 
> ./mm/khugepaged.c:1222:						VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));
> ./mm/khugepaged.c:1663:						VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));
> ./mm/swap.c:828:						VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&lruvec_pgdat(lruvec)->lru_lock));
> ./security/apparmor/file.c:497:					old = rcu_dereference_protected(fctx->label, spin_is_locked(&fctx->lock));
> ./net/netfilter/ipset/ip_set_hash_gen.h:18:			__ipset_dereference_protected(p, spin_is_locked(&(set)->lock))
> ./fs/ocfs2/dlmglue.c:760:					mlog_bug_on_msg(spin_is_locked(&res->l_lock),
> ./fs/ocfs2/inode.c:1194:					mlog_bug_on_msg(spin_is_locked(&oi->ip_lock),
> ./fs/userfaultfd.c:156:						VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
> ./fs/userfaultfd.c:158:						VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
> ./fs/userfaultfd.c:160:						VM_BUG_ON(spin_is_locked(&ctx->event_wqh.lock));
> ./fs/userfaultfd.c:162:						VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
> ./fs/userfaultfd.c:919:						VM_BUG_ON(!spin_is_locked(&wqh->lock));
> ./virt/kvm/arm/vgic/vgic.c:192:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&irq->irq_lock));
> ./virt/kvm/arm/vgic/vgic.c:269:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&vgic_cpu->ap_list_lock));
> ./virt/kvm/arm/vgic/vgic.c:307:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&irq->irq_lock));
> ./virt/kvm/arm/vgic/vgic.c:663:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&irq->irq_lock));
> ./virt/kvm/arm/vgic/vgic.c:694:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&vgic_cpu->ap_list_lock));
> ./virt/kvm/arm/vgic/vgic.c:715:					DEBUG_SPINLOCK_BUG_ON(!spin_is_locked(&vgic_cpu->ap_list_lock));
> ./virt/kvm/kvm_main.c:3934:					WARN_ON(raw_spin_is_locked(&kvm_count_lock));
> ./kernel/debug/debug_core.c:527:				if (!raw_spin_is_locked(&dbg_slave_lock))
> ./kernel/debug/debug_core.c:755:				raw_spin_is_locked(&dbg_master_lock)) {
> ./kernel/locking/spinlock_debug.c:98:				SPIN_BUG_ON(!raw_spin_is_locked(lock), lock, "already unlocked");
> ./kernel/locking/mutex-debug.c:39:				SMP_DEBUG_LOCKS_WARN_ON(!spin_is_locked(&lock->wait_lock));
> ./kernel/locking/mutex-debug.c:54:				SMP_DEBUG_LOCKS_WARN_ON(!spin_is_locked(&lock->wait_lock));
> ./kernel/futex.c:1368:						if (WARN_ON_SMP(!q->lock_ptr || !spin_is_locked(q->lock_ptr))
> ./kernel/printk/printk_safe.c:281:				if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) { // same cpu (printk in nmi)
> ./kernel/printk/printk_safe.c:314:	    			raw_spin_is_locked(&logbuf_lock)) { // same cpu (printk in nmi)
> ./include/net/sock.h:1529:					return !sk->sk_lock.owned && !spin_is_locked(&sk->sk_lock.slock); // returns in BUG_ON/WARN_ON_ONCE
> ./arch/x86/pci/i386.c:62:					WARN_ON_SMP(!spin_is_locked(&pcibios_fwaddrmap_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3446:			printk("cryptocop_completed_jobs_lock %d\n", spin_is_locked(&cryptocop_completed_jobs_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3447:			printk("cryptocop_job_queue_lock %d\n", spin_is_locked(&cryptocop_job_queue_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3448:			printk("descr_pool_lock %d\n", spin_is_locked(&descr_pool_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3449:			printk("cryptocop_sessions_lock %d\n", spin_is_locked(cryptocop_sessions_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3450:			printk("running_job_lock %d\n", spin_is_locked(running_job_lock));
> ./arch/cris/arch-v32/drivers/cryptocop.c:3451:			printk("cryptocop_process_lock %d\n", spin_is_locked(cryptocop_process_lock));
> ./arch/parisc/kernel/firmware.c:208:        			if (spin_is_locked(&pdc_lock)) // self-lock: if (is_locked) unlock(pdc_lock)
> ./drivers/staging/irda/drivers/sir_dev.c:637:			if(spin_is_locked(&dev->tx_lock)) { 	// for debug
> ./drivers/staging/lustre/lustre/osc/osc_cl_internal.h:189:	return spin_is_locked(&obj->oo_lock); // for assert
> ./drivers/tty/serial/sn_console.c:891:				if (spin_is_locked(&port->sc_port.lock)) { // single lock
> ./drivers/tty/serial/sn_console.c:908:				if (!spin_is_locked(&port->sc_port.lock) // single lock
> ./drivers/misc/sgi-xp/xpc_channel.c:31:				DBUG_ON(!spin_is_locked(&ch->lock));
> ./drivers/misc/sgi-xp/xpc_channel.c:85:				DBUG_ON(!spin_is_locked(&ch->lock));
> ./drivers/misc/sgi-xp/xpc_channel.c:761:			DBUG_ON(!spin_is_locked(&ch->lock));
> ./drivers/misc/sgi-xp/xpc_sn2.c:1674:				DBUG_ON(!spin_is_locked(&ch->lock));
> ./drivers/misc/sgi-xp/xpc_uv.c:1186:				DBUG_ON(!spin_is_locked(&ch->lock));
> ./drivers/net/ethernet/smsc/smsc911x.h:70:			WARN_ON_SMP(!spin_is_locked(&pdata->mac_lock))
> ./drivers/net/ethernet/intel/igbvf/mbx.c:267:			WARN_ON_ONCE(!spin_is_locked(&hw->mbx_lock));
> ./drivers/net/ethernet/intel/igbvf/mbx.c:305:			WARN_ON_ONCE(!spin_is_locked(&hw->mbx_lock));
> ./drivers/net/ethernet/intel/i40e/i40e_main.c:1527:		WARN(!spin_is_locked(&vsi->mac_filter_hash_lock),
> ./drivers/net/wireless/zydas/zd1211rw/zd_mac.c:238:		ZD_ASSERT(!spin_is_locked(&mac->lock));
> ./drivers/scsi/fnic/fnic_scsi.c:184:				int sh_locked = spin_is_locked(host->host_lock); // self-lock: if (!is_locked) lock(host_lock)
> ./drivers/scsi/snic/snic_scsi.c:2004:				SNIC_BUG_ON(!spin_is_locked(io_lock));
> ./drivers/scsi/snic/snic_scsi.c:2607:				SNIC_BUG_ON(!spin_is_locked(io_lock));
> ./drivers/atm/nicstar.c:2692:					if (spin_is_locked(&card->int_lock)) {	// optimization ("Probably it isn't worth spinning")
> ./drivers/hv/hv_balloon.c:644:					WARN_ON_ONCE(!spin_is_locked(&dm_device.ha_lock));