Re: [PATCH v3] rcu-tasks: Fix access non-existent percpu rtpcp variable in rcu_tasks_need_gpcb()

From: Neeraj upadhyay
Date: Mon Jul 15 2024 - 04:51:52 EST

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In reply to: Z qiang: "Re: [PATCH v3] rcu-tasks: Fix access non-existent percpu rtpcp variable in rcu_tasks_need_gpcb()"
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On Mon, Jul 15, 2024 at 1:40 PM Z qiang <qiang.zhang1211@xxxxxxxxx> wrote:
>
> >
> > For kernels built with CONFIG_FORCE_NR_CPUS=y, the nr_cpu_ids is
> > defined as NR_CPUS instead of the number of possible cpus, this
> > will cause the following system panic:
> >
> > smpboot: Allowing 4 CPUs, 0 hotplug CPUs
> > ...
> > setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:512 nr_node_ids:1
> > ...
> > BUG: unable to handle page fault for address: ffffffff9911c8c8
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > Oops: 0000 [#1] PREEMPT SMP PTI
> > CPU: 0 PID: 15 Comm: rcu_tasks_trace Tainted: G W
> > 6.6.21 #1 5dc7acf91a5e8e9ac9dcfc35bee0245691283ea6
> > RIP: 0010:rcu_tasks_need_gpcb+0x25d/0x2c0
> > RSP: 0018:ffffa371c00a3e60 EFLAGS: 00010082
> > CR2: ffffffff9911c8c8 CR3: 000000040fa20005 CR4: 00000000001706f0
> > Call Trace:
> > <TASK>
> > ? __die+0x23/0x80
> > ? page_fault_oops+0xa4/0x180
> > ? exc_page_fault+0x152/0x180
> > ? asm_exc_page_fault+0x26/0x40
> > ? rcu_tasks_need_gpcb+0x25d/0x2c0
> > ? __pfx_rcu_tasks_kthread+0x40/0x40
> > rcu_tasks_one_gp+0x69/0x180
> > rcu_tasks_kthread+0x94/0xc0
> > kthread+0xe8/0x140
> > ? __pfx_kthread+0x40/0x40
> > ret_from_fork+0x34/0x80
> > ? __pfx_kthread+0x40/0x40
> > ret_from_fork_asm+0x1b/0x80
> > </TASK>
> >
> > Considering that there may be holes in the CPU numbers, use the
> > maximum possible cpu number, instead of nr_cpu_ids, for configuring
> > enqueue and dequeue limits.
> >
> > Closes: https://lore.kernel.org/linux-input/CALMA0xaTSMN+p4xUXkzrtR5r6k7hgoswcaXx7baR_z9r5jjskw@xxxxxxxxxxxxxx/T/#u
> > Reported-by: Zhixu Liu <zhixu.liu@xxxxxxxxx>
> > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > ---
> > kernel/rcu/tasks.h | 80 +++++++++++++++++++++++++++++-----------------
> > 1 file changed, 51 insertions(+), 29 deletions(-)
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 2b1d6abf3ba3..12d63ce84cc9 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -49,6 +49,7 @@ struct rcu_tasks_percpu {
> > struct list_head rtp_blkd_tasks;
> > struct list_head rtp_exit_list;
> > int cpu;
> > + int index;
> > struct rcu_tasks *rtpp;
> > };
> >
> > @@ -110,6 +111,7 @@ struct rcu_tasks {
> > call_rcu_func_t call_func;
> > unsigned int wait_state;
> > struct rcu_tasks_percpu __percpu *rtpcpu;
> > + struct rcu_tasks_percpu **rtpcp_array;
> > int percpu_enqueue_shift;
> > int percpu_enqueue_lim;
> > int percpu_dequeue_lim;
> > @@ -182,6 +184,8 @@ module_param(rcu_task_collapse_lim, int, 0444);
> > static int rcu_task_lazy_lim __read_mostly = 32;
> > module_param(rcu_task_lazy_lim, int, 0444);
> >
> > +static int rcu_task_cpu_ids;
> > +
> > /* RCU tasks grace-period state for debugging. */
> > #define RTGS_INIT 0
> > #define RTGS_WAIT_WAIT_CBS 1
> > @@ -245,6 +249,8 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> > int cpu;
> > int lim;
> > int shift;
> > + int maxcpu;
> > + int index = 0;
> >
> > if (rcu_task_enqueue_lim < 0) {
> > rcu_task_enqueue_lim = 1;
> > @@ -254,14 +260,9 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> > }
> > lim = rcu_task_enqueue_lim;
> >
> > - if (lim > nr_cpu_ids)
> > - lim = nr_cpu_ids;
> > - shift = ilog2(nr_cpu_ids / lim);
> > - if (((nr_cpu_ids - 1) >> shift) >= lim)
> > - shift++;
> > - WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
> > - WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
> > - smp_store_release(&rtp->percpu_enqueue_lim, lim);
> > + rtp->rtpcp_array = kcalloc(num_possible_cpus(), sizeof(struct rcu_tasks_percpu *), GFP_KERNEL);
> > + BUG_ON(!rtp->rtpcp_array);
> > +
> > for_each_possible_cpu(cpu) {
> > struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> >
> > @@ -273,14 +274,29 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> > INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
> > rtpcp->cpu = cpu;
> > rtpcp->rtpp = rtp;
> > + rtpcp->index = index;
> > + rtp->rtpcp_array[index] = rtpcp;
> > + index++;
> > if (!rtpcp->rtp_blkd_tasks.next)
> > INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
> > if (!rtpcp->rtp_exit_list.next)
> > INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
> > + maxcpu = cpu;
> > }
> >
> > - pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d.\n", rtp->name,
> > - data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim), rcu_task_cb_adjust);
> > + rcu_task_cpu_ids = maxcpu + 1;
> > + if (lim > rcu_task_cpu_ids)
> > + lim = rcu_task_cpu_ids;
> > + shift = ilog2(rcu_task_cpu_ids / lim);
> > + if (((rcu_task_cpu_ids - 1) >> shift) >= lim)
> > + shift++;
> > + WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
> > + WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
> > + smp_store_release(&rtp->percpu_enqueue_lim, lim);
>
> It seems that smp_store_release does not need, the
> WRITE_ONCE(rtp->percpu_enqueue_lim, lim)
> is enough, cblist_init_generic() is invoke in early boot, at this
> time, no other tasks access these
> percpu_*lim.
>

Yes, it makes sense to me. Also, the corresponding acquire has moved to
->percpu_dequeue_lim. So, this is something which can be done and
evaluated as a separate change.

- Neeraj

> Thanks
> Zqiang
>
>
> > +
> > + pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d rcu_task_cpu_ids=%d.\n",
> > + rtp->name, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim),
> > + rcu_task_cb_adjust, rcu_task_cpu_ids);
> > }
> >
> > // Compute wakeup time for lazy callback timer.
> > @@ -348,7 +364,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
> > rtpcp->rtp_n_lock_retries = 0;
> > }
> > if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
> > - READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
> > + READ_ONCE(rtp->percpu_enqueue_lim) != rcu_task_cpu_ids)
> > needadjust = true; // Defer adjustment to avoid deadlock.
> > }
> > // Queuing callbacks before initialization not yet supported.
> > @@ -368,10 +384,10 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
> > raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
> > if (unlikely(needadjust)) {
> > raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
> > - if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
> > + if (rtp->percpu_enqueue_lim != rcu_task_cpu_ids) {
> > WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
> > - WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
> > - smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
> > + WRITE_ONCE(rtp->percpu_dequeue_lim, rcu_task_cpu_ids);
> > + smp_store_release(&rtp->percpu_enqueue_lim, rcu_task_cpu_ids);
> > pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
> > }
> > raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
> > @@ -444,6 +460,8 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
> >
> > dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
> > for (cpu = 0; cpu < dequeue_limit; cpu++) {
> > + if (!cpu_possible(cpu))
> > + continue;
> > struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> >
> > /* Advance and accelerate any new callbacks. */
> > @@ -481,7 +499,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
> > if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
> > raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
> > if (rtp->percpu_enqueue_lim > 1) {
> > - WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
> > + WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(rcu_task_cpu_ids));
> > smp_store_release(&rtp->percpu_enqueue_lim, 1);
> > rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
> > gpdone = false;
> > @@ -496,7 +514,9 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
> > pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
> > }
> > if (rtp->percpu_dequeue_lim == 1) {
> > - for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
> > + for (cpu = rtp->percpu_dequeue_lim; cpu < rcu_task_cpu_ids; cpu++) {
> > + if (!cpu_possible(cpu))
> > + continue;
> > struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> >
> > WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
> > @@ -511,30 +531,32 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
> > // Advance callbacks and invoke any that are ready.
> > static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
> > {
> > - int cpu;
> > - int cpunext;
> > int cpuwq;
> > unsigned long flags;
> > int len;
> > + int index;
> > struct rcu_head *rhp;
> > struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
> > struct rcu_tasks_percpu *rtpcp_next;
> >
> > - cpu = rtpcp->cpu;
> > - cpunext = cpu * 2 + 1;
> > - if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
> > - rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
> > - cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
> > - queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
> > - cpunext++;
> > - if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
> > - rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
> > - cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
> > + index = rtpcp->index * 2 + 1;
> > + if (index < num_possible_cpus()) {
> > + rtpcp_next = rtp->rtpcp_array[index];
> > + if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
> > + cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
> > queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
> > + index++;
> > + if (index < num_possible_cpus()) {
> > + rtpcp_next = rtp->rtpcp_array[index];
> > + if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
> > + cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
> > + queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
> > + }
> > + }
> > }
> > }
> >
> > - if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
> > + if (rcu_segcblist_empty(&rtpcp->cblist))
> > return;
> > raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
> > rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
> > --
> > 2.17.1
> >

Next message: Michael S. Tsirkin: "Re: linux-next: build warning after merge of the vhost tree"
Previous message: Bartosz Golaszewski: "[GIT PULL] gpio updates for v6.11-rc1"
In reply to: Z qiang: "Re: [PATCH v3] rcu-tasks: Fix access non-existent percpu rtpcp variable in rcu_tasks_need_gpcb()"
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