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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [PATCH v2] xen/grant-table: Simplify the update to the per-vCPU maptrack freelist
From: Julien Grall <jgrall@xxxxxxxxxx>
Since XSA-228 (commit 02cbeeb62075 "gnttab: split maptrack lock
to make it fulfill its purpose again"), v->maptrack_head,
v->maptrack_tail and the content of the freelist are accessed 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 in get_maptrack_handle() when initializing
the free list of the current vCPU. 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} as wel as the
freelist.
Take the opportunity to add a comment on top of the lock definition
and explain what it protects.
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
----
Changes in v2:
- Fix typoes
- Update the commit message
- Don't use interchangeably MAPTRACK_TAIL and
INVALID_MAPTRACK_HANDLE
---
xen/common/grant_table.c | 66 ++++++++++++++++------------------------
xen/include/xen/sched.h | 8 ++++-
2 files changed, 34 insertions(+), 40 deletions(-)
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index ab30e2e8cfb6..fab77ab9ccb8 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -543,33 +543,27 @@ 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) )
+ {
+ spin_unlock(&v->maptrack_freelist_lock);
+ return INVALID_MAPTRACK_HANDLE;
+ }
- 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 = maptrack_entry(t, head).ref;
+ if ( unlikely(next == MAPTRACK_TAIL) )
+ head = INVALID_MAPTRACK_HANDLE;
+ else
+ v->maptrack_head = next;
spin_unlock(&v->maptrack_freelist_lock);
@@ -623,7 +617,7 @@ put_maptrack_handle(
{
struct domain *currd = current->domain;
struct vcpu *v;
- unsigned int prev_tail, cur_tail;
+ unsigned int tail;
/* 1. Set entry to be a tail. */
maptrack_entry(t, handle).ref = MAPTRACK_TAIL;
@@ -633,14 +627,11 @@ 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 );
+ 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);
+ maptrack_entry(t, tail).ref = handle;
spin_unlock(&v->maptrack_freelist_lock);
}
@@ -650,7 +641,7 @@ 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 +677,7 @@ 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);
@@ -707,7 +698,7 @@ get_maptrack_handle(
new_mt[i].vcpu = curr->vcpu_id;
}
- /* Set tail directly if this is the first page for this VCPU. */
+ /* Set tail directly if this is the first page for the local vCPU. */
if ( curr->maptrack_tail == MAPTRACK_TAIL )
curr->maptrack_tail = handle + MAPTRACK_PER_PAGE - 1;
@@ -716,13 +707,10 @@ 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..6c52ba2af019 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -255,7 +255,13 @@ struct vcpu
/* VCPU paused by system controller. */
int controller_pause_count;
- /* Grant table map tracking. */
+ /*
+ * Grant table map tracking. The lock maptrack_freelist_lock
+ * protects to:
+ * - The entries in the freelist
+ * - maptrack_head
+ * - maptrack_tail
+ */
spinlock_t maptrack_freelist_lock;
unsigned int maptrack_head;
unsigned int maptrack_tail;
--
2.17.1
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