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Date: Tue, 14 Jul 2015 10:39:12 -0400
From: Boris Ostrovsky <boris.ostrovsky@...cle.com>
To: Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>
CC: Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Peter Anvin <hpa@...or.com>, xiao jin <jin.xiao@...el.com>,
Joerg Roedel <jroedel@...e.de>, Borislav Petkov <bp@...e.de>,
Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during
cpu up/down
On 07/05/2015 01:12 PM, Thomas Gleixner wrote:
> When a cpu goes up some architectures (e.g. x86) have to walk the irq
> space to set up the vector space for the cpu. While this needs extra
> protection at the architecture level we can avoid a few race
> conditions by preventing the concurrent allocation/free of irq
> descriptors and the associated data.
>
> When a cpu goes down it moves the interrupts which are targeted to
> this cpu away by reassigning the affinities. While this happens
> interrupts can be allocated and freed, which opens a can of race
> conditions in the code which reassignes the affinities because
> interrupt descriptors might be freed underneath.
>
> Example:
>
> CPU1 CPU2
> cpu_up/down
> irq_desc = irq_to_desc(irq);
> remove_from_radix_tree(desc);
> raw_spin_lock(&desc->lock);
> free(desc);
>
> We could protect the irq descriptors with RCU, but that would require
> a full tree change of all accesses to interrupt descriptors. But
> fortunately these kind of race conditions are rather limited to a few
> things like cpu hotplug. The normal setup/teardown is very well
> serialized. So the simpler and obvious solution is:
>
> Prevent allocation and freeing of interrupt descriptors accross cpu
> hotplug.
This breaks Xen guests that allocate interrupt descriptors in .cpu_up().
Any chance this locking can be moved into arch code? Otherwise we will
need to have something like arch_post_cpu_up() after the lock is released.
(The patch doesn't appear to have any side effects for the down path
since Xen guests deallocate descriptors in __cpu_die()).
-boris
>
> Signed-off-by: Thomas Gleixner<tglx@...utronix.de>
> ---
> include/linux/irqdesc.h | 7 ++++++-
> kernel/cpu.c | 21 ++++++++++++++++++++-
> kernel/irq/internals.h | 4 ----
> 3 files changed, 26 insertions(+), 6 deletions(-)
>
> Index: tip/include/linux/irqdesc.h
> ===================================================================
> --- tip.orig/include/linux/irqdesc.h
> +++ tip/include/linux/irqdesc.h
> @@ -90,7 +90,12 @@ struct irq_desc {
> const char *name;
> } ____cacheline_internodealigned_in_smp;
>
> -#ifndef CONFIG_SPARSE_IRQ
> +#ifdef CONFIG_SPARSE_IRQ
> +extern void irq_lock_sparse(void);
> +extern void irq_unlock_sparse(void);
> +#else
> +static inline void irq_lock_sparse(void) { }
> +static inline void irq_unlock_sparse(void) { }
> extern struct irq_desc irq_desc[NR_IRQS];
> #endif
>
> Index: tip/kernel/cpu.c
> ===================================================================
> --- tip.orig/kernel/cpu.c
> +++ tip/kernel/cpu.c
> @@ -392,13 +392,19 @@ static int __ref _cpu_down(unsigned int
> smpboot_park_threads(cpu);
>
> /*
> - * So now all preempt/rcu users must observe !cpu_active().
> + * Prevent irq alloc/free while the dying cpu reorganizes the
> + * interrupt affinities.
> */
> + irq_lock_sparse();
>
> + /*
> + * So now all preempt/rcu users must observe !cpu_active().
> + */
> err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
> if (err) {
> /* CPU didn't die: tell everyone. Can't complain. */
> cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
> + irq_unlock_sparse();
> goto out_release;
> }
> BUG_ON(cpu_online(cpu));
> @@ -415,6 +421,9 @@ static int __ref _cpu_down(unsigned int
> smp_mb(); /* Read from cpu_dead_idle before __cpu_die(). */
> per_cpu(cpu_dead_idle, cpu) = false;
>
> + /* Interrupts are moved away from the dying cpu, reenable alloc/free */
> + irq_unlock_sparse();
> +
> hotplug_cpu__broadcast_tick_pull(cpu);
> /* This actually kills the CPU. */
> __cpu_die(cpu);
> @@ -517,8 +526,18 @@ static int _cpu_up(unsigned int cpu, int
> goto out_notify;
> }
>
> + /*
> + * Some architectures have to walk the irq descriptors to
> + * setup the vector space for the cpu which comes online.
> + * Prevent irq alloc/free across the bringup.
> + */
> + irq_lock_sparse();
> +
> /* Arch-specific enabling code. */
> ret = __cpu_up(cpu, idle);
> +
> + irq_unlock_sparse();
> +
> if (ret != 0)
> goto out_notify;
> BUG_ON(!cpu_online(cpu));
> Index: tip/kernel/irq/internals.h
> ===================================================================
> --- tip.orig/kernel/irq/internals.h
> +++ tip/kernel/irq/internals.h
> @@ -76,12 +76,8 @@ extern void unmask_threaded_irq(struct i
>
> #ifdef CONFIG_SPARSE_IRQ
> static inline void irq_mark_irq(unsigned int irq) { }
> -extern void irq_lock_sparse(void);
> -extern void irq_unlock_sparse(void);
> #else
> extern void irq_mark_irq(unsigned int irq);
> -static inline void irq_lock_sparse(void) { }
> -static inline void irq_unlock_sparse(void) { }
> #endif
>
> extern void init_kstat_irqs(struct irq_desc *desc, int node, int nr);
>
>
> --
--
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