[Xen-devel] [PATCH v9 08/19] qspinlock: Make a new qnode structure to support virtualization

[Waiman Long <Waiman.Long@xxxxxx>]

In order to support additional virtualization features like unfair lock
and para-virtualized spinlock, it is necessary to store additional
CPU specific data into the queue node structure. As a result, a new
qnode structure is created and the mcs_spinlock structure is now part
of the new structure.

It is also necessary to expand arch_mcs_spin_lock_contended() to the
underlying while loop as additional code will need to be inserted
into the loop.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 kernel/locking/qspinlock.c |   36 +++++++++++++++++++++++-------------
 1 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 80fe9ee..994358c 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -61,12 +61,21 @@
 #include "mcs_spinlock.h"
 
 /*
+ * To have additional features for better virtualization support, it is
+ * necessary to store additional data in the queue node structure. So
+ * a new queue node structure will have to be defined and used here.
+ */
+struct qnode {
+       struct mcs_spinlock mcs;
+};
+
+/*
  * Per-CPU queue node structures; we can never have more than 4 nested
  * contexts: task, softirq, hardirq, nmi.
  *
  * Exactly fits one cacheline.
  */
-static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[4]);
+static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[4]);
 
 /*
  * We must be able to distinguish between no-tail and the tail at 0:0,
@@ -83,12 +92,12 @@ static inline u32 encode_tail(int cpu, int idx)
        return tail;
 }
 
-static inline struct mcs_spinlock *decode_tail(u32 tail)
+static inline struct qnode *decode_tail(u32 tail)
 {
        int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
        int idx = (tail &  _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
 
-       return per_cpu_ptr(&mcs_nodes[idx], cpu);
+       return per_cpu_ptr(&qnodes[idx], cpu);
 }
 
 #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
@@ -343,7 +352,7 @@ static inline int trylock_pending(struct qspinlock *lock, 
u32 *pval)
  */
 void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 {
-       struct mcs_spinlock *prev, *next, *node;
+       struct qnode *prev, *next, *node;
        u32 old, tail;
        int idx;
 
@@ -352,13 +361,13 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
        if (trylock_pending(lock, &val))
                return; /* Lock acquired */
 
-       node = this_cpu_ptr(&mcs_nodes[0]);
-       idx = node->count++;
+       node = this_cpu_ptr(&qnodes[0]);
+       idx = node->mcs.count++;
        tail = encode_tail(smp_processor_id(), idx);
 
        node += idx;
-       node->locked = 0;
-       node->next = NULL;
+       node->mcs.locked = 0;
+       node->mcs.next = NULL;
 
        /*
         * We touched a (possibly) cold cacheline; attempt the trylock once
@@ -381,9 +390,10 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
         */
        if (old & _Q_TAIL_MASK) {
                prev = decode_tail(old);
-               ACCESS_ONCE(prev->next) = node;
+               ACCESS_ONCE(prev->mcs.next) = (struct mcs_spinlock *)node;
 
-               arch_mcs_spin_lock_contended(&node->locked);
+               while (!smp_load_acquire(&node->mcs.locked))
+                       arch_mutex_cpu_relax();
        }
 
        /*
@@ -423,15 +433,15 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
        /*
         * contended path; wait for next, release.
         */
-       while (!(next = ACCESS_ONCE(node->next)))
+       while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
                arch_mutex_cpu_relax();
 
-       arch_mcs_spin_unlock_contended(&next->locked);
+       arch_mcs_spin_unlock_contended(&next->mcs.locked);
 
 release:
        /*
         * release the node
         */
-       this_cpu_dec(mcs_nodes[0].count);
+       this_cpu_dec(qnodes[0].mcs.count);
 }
 EXPORT_SYMBOL(queue_spin_lock_slowpath);
-- 
1.7.1


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