Date:	Tue, 17 May 2016 12:01:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	catalin.marinas@....com, dennis.chen@....com,
	jiangshanlai@...il.com, josh@...htriplett.org,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	rostedt@...dmis.org, steve.capper@....com, will.deacon@....com
Subject: Re: [PATCHv2] rcu: tree: correctly handle sparse possible CPUs

On Tue, May 17, 2016 at 11:22:10AM +0100, Mark Rutland wrote:
> In many cases in the RCU tree code, we iterate over the set of CPUs for
> a leaf node described by rcu_node::grplo and rcu_node::grphi, checking
> per-cpu data for each CPU in this range. However, if the set of possible
> CPUs is sparse, some CPUs described in this range are not possible, and
> thus no per-cpu region will have been allocated (or initialised) for
> them by the generic percpu code.
> 
> Erroneous accesses to a per-cpu area for these !possible CPUs may fault
> or may hit other data depending on the addressed generated when the
> erroneous per cpu offset is applied. In practice, both cases have been
> observed on arm64 hardware (the former being silent, but detectable with
> additional patches).
> 
> To avoid issues resulting from this, we must iterate over the set of
> *possible* cpus for a given leaf node. This patch adds new helpers to
> enable this (also unifying and simplifying some related bitmask
> manipulation logic), and moves the RCU tree code over to them.
> 
> Without this patch, running reboot at a shell can result in an oops
> like:

Very good, this one applies cleanly and I have queued it for review
and testing.

One question below, though.

							Thanx, Paul

> [ 3369.075979] Unable to handle kernel paging request at virtual address ffffff8008b21b4c
> [ 3369.083881] pgd = ffffffc3ecdda000
> [ 3369.087270] [ffffff8008b21b4c] *pgd=00000083eca48003, *pud=00000083eca48003, *pmd=0000000000000000
> [ 3369.096222] Internal error: Oops: 96000007 [#1] PREEMPT SMP
> [ 3369.101781] Modules linked in:
> [ 3369.104825] CPU: 2 PID: 1817 Comm: NetworkManager Tainted: G        W       4.6.0+ #3
> [ 3369.121239] task: ffffffc0fa13e000 ti: ffffffc3eb940000 task.ti: ffffffc3eb940000
> [ 3369.128708] PC is at sync_rcu_exp_select_cpus+0x188/0x510
> [ 3369.134094] LR is at sync_rcu_exp_select_cpus+0x104/0x510
> [ 3369.139479] pc : [<ffffff80081109a8>] lr : [<ffffff8008110924>] pstate: 200001c5
> [ 3369.146860] sp : ffffffc3eb9435a0
> [ 3369.150162] x29: ffffffc3eb9435a0 x28: ffffff8008be4f88
> [ 3369.155465] x27: ffffff8008b66c80 x26: ffffffc3eceb2600
> [ 3369.160767] x25: 0000000000000001 x24: ffffff8008be4f88
> [ 3369.166070] x23: ffffff8008b51c3c x22: ffffff8008b66c80
> [ 3369.171371] x21: 0000000000000001 x20: ffffff8008b21b40
> [ 3369.176673] x19: ffffff8008b66c80 x18: 0000000000000000
> [ 3369.181975] x17: 0000007fa951a010 x16: ffffff80086a30f0
> [ 3369.187278] x15: 0000007fa9505590 x14: 0000000000000000
> [ 3369.192580] x13: ffffff8008b51000 x12: ffffffc3eb940000
> [ 3369.197882] x11: 0000000000000006 x10: ffffff8008b51b78
> [ 3369.203184] x9 : 0000000000000001 x8 : ffffff8008be4000
> [ 3369.208486] x7 : ffffff8008b21b40 x6 : 0000000000001003
> [ 3369.213788] x5 : 0000000000000000 x4 : ffffff8008b27280
> [ 3369.219090] x3 : ffffff8008b21b4c x2 : 0000000000000001
> [ 3369.224406] x1 : 0000000000000001 x0 : 0000000000000140
> ...
> [ 3369.972257] [<ffffff80081109a8>] sync_rcu_exp_select_cpus+0x188/0x510
> [ 3369.978685] [<ffffff80081128b4>] synchronize_rcu_expedited+0x64/0xa8
> [ 3369.985026] [<ffffff80086b987c>] synchronize_net+0x24/0x30
> [ 3369.990499] [<ffffff80086ddb54>] dev_deactivate_many+0x28c/0x298
> [ 3369.996493] [<ffffff80086b6bb8>] __dev_close_many+0x60/0xd0
> [ 3370.002052] [<ffffff80086b6d48>] __dev_close+0x28/0x40
> [ 3370.007178] [<ffffff80086bf62c>] __dev_change_flags+0x8c/0x158
> [ 3370.012999] [<ffffff80086bf718>] dev_change_flags+0x20/0x60
> [ 3370.018558] [<ffffff80086cf7f0>] do_setlink+0x288/0x918
> [ 3370.023771] [<ffffff80086d0798>] rtnl_newlink+0x398/0x6a8
> [ 3370.029158] [<ffffff80086cee84>] rtnetlink_rcv_msg+0xe4/0x220
> [ 3370.034891] [<ffffff80086e274c>] netlink_rcv_skb+0xc4/0xf8
> [ 3370.040364] [<ffffff80086ced8c>] rtnetlink_rcv+0x2c/0x40
> [ 3370.045663] [<ffffff80086e1fe8>] netlink_unicast+0x160/0x238
> [ 3370.051309] [<ffffff80086e24b8>] netlink_sendmsg+0x2f0/0x358
> [ 3370.056956] [<ffffff80086a0070>] sock_sendmsg+0x18/0x30
> [ 3370.062168] [<ffffff80086a21cc>] ___sys_sendmsg+0x26c/0x280
> [ 3370.067728] [<ffffff80086a30ac>] __sys_sendmsg+0x44/0x88
> [ 3370.073027] [<ffffff80086a3100>] SyS_sendmsg+0x10/0x20
> [ 3370.078153] [<ffffff8008085e70>] el0_svc_naked+0x24/0x28
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Reported-by: Dennis Chen <dennis.chen@....com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Steve Capper <steve.capper@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  kernel/rcu/tree.c        | 19 +++++++++----------
>  kernel/rcu/tree.h        | 18 ++++++++++++++++++
>  kernel/rcu/tree_exp.h    | 12 ++++--------
>  kernel/rcu/tree_plugin.h |  5 +++--
>  4 files changed, 34 insertions(+), 20 deletions(-)
> 
> Since v1 [1]:
>  * rebase to the -rcu rcu/dev branch.
>  * replace all occurences missed by v1.
>  * s/CPUS/CPUs/ in commit message. Gah.
> 
> Paul, I've given this a spin on arm64 and I've build-tested for x86.
> Things look fine, though this hasn't seen a thorough beating.
> 
> Mark.
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index afdcb7b..65ee19e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1279,15 +1279,16 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
>  static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
>  {
>  	int cpu;
> +	unsigned long bit;
>  	unsigned long flags;
>  	struct rcu_node *rnp;
> 
>  	rcu_for_each_leaf_node(rsp, rnp) {
>  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>  		if (rnp->qsmask != 0) {
> -			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
> -				if (rnp->qsmask & (1UL << cpu))
> -					dump_cpu_task(rnp->grplo + cpu);
> +			for_each_leaf_node_possible_cpu_bit(rnp, cpu, bit)
> +				if (rnp->qsmask & bit)
> +					dump_cpu_task(cpu);
>  		}
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	}
> @@ -1316,6 +1317,7 @@ static void rcu_stall_kick_kthreads(struct rcu_state *rsp)
>  static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>  {
>  	int cpu;
> +	unsigned long bit;
>  	long delta;
>  	unsigned long flags;
>  	unsigned long gpa;
> @@ -1353,10 +1355,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>  		ndetected += rcu_print_task_stall(rnp);
>  		if (rnp->qsmask != 0) {
> -			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
> -				if (rnp->qsmask & (1UL << cpu)) {
> -					print_cpu_stall_info(rsp,
> -							     rnp->grplo + cpu);
> +			for_each_leaf_node_possible_cpu_bit(rnp, cpu, bit)
> +				if (rnp->qsmask & bit) {
> +					print_cpu_stall_info(rsp, cpu);
>  					ndetected++;
>  				}
>  		}
> @@ -2908,9 +2909,7 @@ static void force_qs_rnp(struct rcu_state *rsp,
>  				continue;
>  			}
>  		}
> -		cpu = rnp->grplo;
> -		bit = 1;
> -		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
> +		for_each_leaf_node_possible_cpu_bit(rnp, cpu, bit) {
>  			if ((rnp->qsmask & bit) != 0) {
>  				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
>  					mask |= bit;
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index e3959f5..f73444f 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -281,6 +281,24 @@ struct rcu_node {
>  	     (rnp) < &(rsp)->node[rcu_num_nodes]; (rnp)++)
> 
>  /*
> + * Iterate over all possible CPUs in a leaf RCU node.
> + */
> +#define for_each_leaf_node_possible_cpu(rnp, cpu) \
> +	for ((cpu) = rnp->grplo; \
> +	     cpu <= rnp->grphi; \
> +	     cpu = cpumask_next((cpu), cpu_possible_mask))

What if the rnp->grplo corresponds to a non-existent CPU?  I admit that
this is perhaps unlikely given the usual power-of-two defaults and
pwoer-of-two computer-system structure, but...

Would something like this handle that possibility?

+#define for_each_leaf_node_possible_cpu(rnp, cpu) \
+	for ((cpu) = cpumask_next(rnp->grplo - 1, cpu_possible_mask); \
+	     cpu <= rnp->grphi; \
+	     cpu = cpumask_next((cpu), cpu_possible_mask))

Or maybe like this, with less duplicated code but very strange style:

+#define for_each_leaf_node_possible_cpu(rnp, cpu) \
+	for ((cpu) = rnp->grplo - 1; \
+	     cpu = cpumask_next((cpu), cpu_possible_mask), cpu <= rnp->grphi; 1)

The first one is probably far better, assuming that it works, but I could
not resist inflicting the second one on you.  ;-)

> +
> +/*
> + * Iterate over all possible CPUs in a leaf RCU node, at each step providing a
> + * bit for comparison against rcu_node bitmasks.
> + */
> +#define for_each_leaf_node_possible_cpu_bit(rnp, cpu, bit) \
> +	for ((cpu) = rnp->grplo, (bit) = 1; \
> +	     cpu <= rnp->grphi; \
> +	     cpu = cpumask_next((cpu), cpu_possible_mask), \
> +		   (bit) = 1UL << (cpu - rnp->grplo))

Same question here.

> +
> +/*
>   * Union to allow "aggregate OR" operation on the need for a quiescent
>   * state by the normal and expedited grace periods.
>   */
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 00a02a2..6655a44 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -356,7 +356,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> 
>  		/* Each pass checks a CPU for identity, offline, and idle. */
>  		mask_ofl_test = 0;
> -		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
> +		for_each_leaf_node_possible_cpu(rnp, cpu) {
>  			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
>  			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> 
> @@ -376,8 +376,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> 
>  		/* IPI the remaining CPUs for expedited quiescent state. */
> -		mask = 1;
> -		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
> +		for_each_leaf_node_possible_cpu_bit(rnp, cpu, mask) {
>  			if (!(mask_ofl_ipi & mask))
>  				continue;
>  retry_ipi:
> @@ -440,8 +439,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
>  		ndetected = 0;
>  		rcu_for_each_leaf_node(rsp, rnp) {
>  			ndetected += rcu_print_task_exp_stall(rnp);
> -			mask = 1;
> -			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
> +			for_each_leaf_node_possible_cpu_bit(rnp, cpu, mask) {
>  				struct rcu_data *rdp;
> 
>  				if (!(rnp->expmask & mask))
> @@ -453,7 +451,6 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
>  					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
>  					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
>  			}
> -			mask <<= 1;
>  		}
>  		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
>  			jiffies - jiffies_start, rsp->expedited_sequence,
> @@ -473,8 +470,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
>  			pr_cont("\n");
>  		}
>  		rcu_for_each_leaf_node(rsp, rnp) {
> -			mask = 1;
> -			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
> +			for_each_leaf_node_possible_cpu_bit(rnp, cpu, mask) {
>  				if (!(rnp->expmask & mask))
>  					continue;
>  				dump_cpu_task(cpu);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 02a9197..fd6b0f7 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1157,6 +1157,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
>  {
>  	struct task_struct *t = rnp->boost_kthread_task;
>  	unsigned long mask = rcu_rnp_online_cpus(rnp);
> +	unsigned long bit;
>  	cpumask_var_t cm;
>  	int cpu;
> 
> @@ -1164,8 +1165,8 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
>  		return;
>  	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
>  		return;
> -	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
> -		if ((mask & 0x1) && cpu != outgoingcpu)
> +	for_each_leaf_node_cpu_bit(rnp, cpu, bit)
> +		if ((mask & bit) && cpu != outgoingcpu)
>  			cpumask_set_cpu(cpu, cm);
>  	if (cpumask_weight(cm) == 0)
>  		cpumask_setall(cm);
> -- 
> 1.9.1
> 

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