genirq: Free irq_desc with rcu

The new VMD device driver needs to iterate over a list of
"demultiplexing" interrupts. Protecting that list with a lock is not
possible because the list is also required in code pathes which hold
irq descriptor lock. Therefor the demultiplexing interrupt handler
would create a lock inversion scenario if it calls a demux handler
with the list protection lock held.

A solution for this is to free the irq descriptor via RCU, so the
list can be walked with rcu read lock held.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>

diff --git a/include/linux/irqdesc.h b/include/linux/irqdesc.h
index a587a33..dcca77c 100644 (file)
--- a/include/linux/irqdesc.h
+++ b/include/linux/irqdesc.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_IRQDESC_H
 #define _LINUX_IRQDESC_H
 
+#include <linux/rcupdate.h>
+
 /*
  * Core internal functions to deal with irq descriptors
  */
@@ -40,6 +42,7 @@ struct pt_regs;
  *                     IRQF_NO_SUSPEND set
  * @force_resume_depth:        number of irqactions on a irq descriptor with
  *                     IRQF_FORCE_RESUME set
+ * @rcu:               rcu head for delayed free
  * @dir:               /proc/irq/ procfs entry
  * @name:              flow handler name for /proc/interrupts output
  */
@@ -82,6 +85,9 @@ struct irq_desc {
 #ifdef CONFIG_PROC_FS
        struct proc_dir_entry   *dir;
 #endif
+#ifdef CONFIG_SPARSE_IRQ
+       struct rcu_head         rcu;
+#endif
        int                     parent_irq;
        struct module           *owner;
        const char              *name;

diff --git a/kernel/irq/irqdesc.c b/kernel/irq/irqdesc.c
index 239e2ae..0409da0 100644 (file)
--- a/kernel/irq/irqdesc.c
+++ b/kernel/irq/irqdesc.c
@@ -159,6 +159,7 @@ static struct irq_desc *alloc_desc(int irq, int node, struct module *owner)
 
        raw_spin_lock_init(&desc->lock);
        lockdep_set_class(&desc->lock, &irq_desc_lock_class);
+       init_rcu_head(&desc->rcu);
 
        desc_set_defaults(irq, desc, node, owner);
 
@@ -171,6 +172,15 @@ err_desc:
        return NULL;
 }
 
+static void delayed_free_desc(struct rcu_head *rhp)
+{
+       struct irq_desc *desc = container_of(rhp, struct irq_desc, rcu);
+
+       free_masks(desc);
+       free_percpu(desc->kstat_irqs);
+       kfree(desc);
+}
+
 static void free_desc(unsigned int irq)
 {
        struct irq_desc *desc = irq_to_desc(irq);
@@ -187,9 +197,12 @@ static void free_desc(unsigned int irq)
        delete_irq_desc(irq);
        mutex_unlock(&sparse_irq_lock);
 
-       free_masks(desc);
-       free_percpu(desc->kstat_irqs);
-       kfree(desc);
+       /*
+        * We free the descriptor, masks and stat fields via RCU. That
+        * allows demultiplex interrupts to do rcu based management of
+        * the child interrupts.
+        */
+       call_rcu(&desc->rcu, delayed_free_desc);
 }
 
 static int alloc_descs(unsigned int start, unsigned int cnt, int node,