[PATCH] xen/grant-table: Simplify the update to the per-vCPU maptrack freelist

[Julien Grall <julien@xen.org>]

From: Julien Grall <jgrall@amazon.com>

Since XSA-288 (commit 02cbeeb62075 "gnttab: split maptrack lock to make
it fulfill its purpose again"), v->maptrack_head and v->maptrack_tail
are with the lock v->maptrack_freelist_lock held.

Therefore it is not necessary to update the fields using cmpxchg()
and also read them atomically.

Note that there are two cases where v->maptrack_tail is accessed without
the lock. They both happen _get_maptrack_handle() when the current vCPU
list is empty. Therefore there is no possible race.

The code is now reworked to remove any use of cmpxch() and read_atomic()
when accessing the fields v->maptrack_{head, tail}.

Take the opportunity to add a comment on top of the lock definition
and explain what it protects.

Signed-off-by: Julien Grall <jgrall@amazon.com>

----

I am not sure whether we should try to protect the remaining unprotected
access with the lock or maybe add a comment?
---
 xen/common/grant_table.c | 60 +++++++++++++++-------------------------
 xen/include/xen/sched.h  |  5 +++-
 2 files changed, 27 insertions(+), 38 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index ab30e2e8cfb6..cac9d1d73446 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -543,34 +543,26 @@ double_gt_unlock(struct grant_table *lgt, struct grant_table *rgt)
 static inline grant_handle_t
 _get_maptrack_handle(struct grant_table *t, struct vcpu *v)
 {
-    unsigned int head, next, prev_head;
+    unsigned int head, next;
 
     spin_lock(&v->maptrack_freelist_lock);
 
-    do {
-        /* No maptrack pages allocated for this VCPU yet? */
-        head = read_atomic(&v->maptrack_head);
-        if ( unlikely(head == MAPTRACK_TAIL) )
-        {
-            spin_unlock(&v->maptrack_freelist_lock);
-            return INVALID_MAPTRACK_HANDLE;
-        }
-
-        /*
-         * Always keep one entry in the free list to make it easier to
-         * add free entries to the tail.
-         */
-        next = read_atomic(&maptrack_entry(t, head).ref);
-        if ( unlikely(next == MAPTRACK_TAIL) )
-        {
-            spin_unlock(&v->maptrack_freelist_lock);
-            return INVALID_MAPTRACK_HANDLE;
-        }
+    /* No maptrack pages allocated for this VCPU yet? */
+    head = v->maptrack_head;
+    if ( unlikely(head == MAPTRACK_TAIL) )
+        goto out;
 
-        prev_head = head;
-        head = cmpxchg(&v->maptrack_head, prev_head, next);
-    } while ( head != prev_head );
+    /*
+     * Always keep one entry in the free list to make it easier to
+     * add free entries to the tail.
+     */
+    next = read_atomic(&maptrack_entry(t, head).ref);
+    if ( unlikely(next == MAPTRACK_TAIL) )
+        head = MAPTRACK_TAIL;
+    else
+        v->maptrack_head = next;
 
+out:
     spin_unlock(&v->maptrack_freelist_lock);
 
     return head;
@@ -623,7 +615,7 @@ put_maptrack_handle(
 {
     struct domain *currd = current->domain;
     struct vcpu *v;
-    unsigned int prev_tail, cur_tail;
+    unsigned int prev_tail;
 
     /* 1. Set entry to be a tail. */
     maptrack_entry(t, handle).ref = MAPTRACK_TAIL;
@@ -633,11 +625,8 @@ put_maptrack_handle(
 
     spin_lock(&v->maptrack_freelist_lock);
 
-    cur_tail = read_atomic(&v->maptrack_tail);
-    do {
-        prev_tail = cur_tail;
-        cur_tail = cmpxchg(&v->maptrack_tail, prev_tail, handle);
-    } while ( cur_tail != prev_tail );
+    prev_tail = v->maptrack_tail;
+    v->maptrack_tail = handle;
 
     /* 3. Update the old tail entry to point to the new entry. */
     write_atomic(&maptrack_entry(t, prev_tail).ref, handle);
@@ -650,7 +639,7 @@ get_maptrack_handle(
     struct grant_table *lgt)
 {
     struct vcpu          *curr = current;
-    unsigned int          i, head;
+    unsigned int          i;
     grant_handle_t        handle;
     struct grant_mapping *new_mt = NULL;
 
@@ -686,7 +675,7 @@ get_maptrack_handle(
             maptrack_entry(lgt, handle).ref = MAPTRACK_TAIL;
             curr->maptrack_tail = handle;
             if ( curr->maptrack_head == MAPTRACK_TAIL )
-                write_atomic(&curr->maptrack_head, handle);
+                curr->maptrack_head = handle;
             spin_unlock(&curr->maptrack_freelist_lock);
         }
         return steal_maptrack_handle(lgt, curr);
@@ -716,13 +705,10 @@ get_maptrack_handle(
     lgt->maptrack_limit += MAPTRACK_PER_PAGE;
 
     spin_unlock(&lgt->maptrack_lock);
-    spin_lock(&curr->maptrack_freelist_lock);
-
-    do {
-        new_mt[i - 1].ref = read_atomic(&curr->maptrack_head);
-        head = cmpxchg(&curr->maptrack_head, new_mt[i - 1].ref, handle + 1);
-    } while ( head != new_mt[i - 1].ref );
 
+    spin_lock(&curr->maptrack_freelist_lock);
+    new_mt[i - 1].ref = curr->maptrack_head;
+    curr->maptrack_head = handle + 1;
     spin_unlock(&curr->maptrack_freelist_lock);
 
     return handle;
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 3982167144c6..bd1cb08266d8 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -255,7 +255,10 @@ struct vcpu
     /* VCPU paused by system controller. */
     int              controller_pause_count;
 
-    /* Grant table map tracking. */
+    /*
+     * Grant table map tracking. The lock maptrack_freelist_lock protect
+     * the access to maptrack_head and maptrack_tail.
+     */
     spinlock_t       maptrack_freelist_lock;
     unsigned int     maptrack_head;
     unsigned int     maptrack_tail;
-- 
2.17.1