From 60ea3a3ac3d2bcd8e85b250fdbfc46b3b9dc7085 Mon Sep 17 00:00:00 2001
From: Frediano Ziglio <frediano.ziglio@citrix.com>
Date: Mon, 24 Feb 2014 12:07:41 +0100
Subject: x86/MCE: Fix race condition in mctelem_reserve

These lines (in mctelem_reserve)

        newhead = oldhead->mcte_next;
        if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {

are racy. After you read the newhead pointer it can happen that another
flow (thread or recursive invocation) change all the list but set head
with same value. So oldhead is the same as *freelp but you are setting
a new head that could point to whatever element (even already used).

This patch use instead a bit array and atomic bit operations.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Reviewed-by: Liu Jinsong <jinsong.liu@intel.com>

---
 xen/arch/x86/cpu/mcheck/mctelem.c |   81 ++++++++++++++------------------------
 1 file changed, 30 insertions(+), 51 deletions(-)

Index: xen-4.1.6-testing/xen/arch/x86/cpu/mcheck/mctelem.c
===================================================================
--- xen-4.1.6-testing.orig/xen/arch/x86/cpu/mcheck/mctelem.c
+++ xen-4.1.6-testing/xen/arch/x86/cpu/mcheck/mctelem.c
@@ -37,24 +37,19 @@ struct mctelem_ent {
 	void *mcte_data;		/* corresponding data payload */
 };
 
-#define	MCTE_F_HOME_URGENT		0x0001U	/* free to urgent freelist */
-#define	MCTE_F_HOME_NONURGENT		0x0002U /* free to nonurgent freelist */
-#define	MCTE_F_CLASS_URGENT		0x0004U /* in use - urgent errors */
-#define	MCTE_F_CLASS_NONURGENT		0x0008U /* in use - nonurgent errors */
+#define	MCTE_F_CLASS_URGENT		0x0001U /* in use - urgent errors */
+#define	MCTE_F_CLASS_NONURGENT		0x0002U /* in use - nonurgent errors */
 #define	MCTE_F_STATE_FREE		0x0010U	/* on a freelist */
 #define	MCTE_F_STATE_UNCOMMITTED	0x0020U	/* reserved; on no list */
 #define	MCTE_F_STATE_COMMITTED		0x0040U	/* on a committed list */
 #define	MCTE_F_STATE_PROCESSING		0x0080U	/* on a processing list */
 
-#define	MCTE_F_MASK_HOME	(MCTE_F_HOME_URGENT | MCTE_F_HOME_NONURGENT)
 #define	MCTE_F_MASK_CLASS	(MCTE_F_CLASS_URGENT | MCTE_F_CLASS_NONURGENT)
 #define	MCTE_F_MASK_STATE	(MCTE_F_STATE_FREE | \
 				MCTE_F_STATE_UNCOMMITTED | \
 				MCTE_F_STATE_COMMITTED | \
 				MCTE_F_STATE_PROCESSING)
 
-#define	MCTE_HOME(tep) ((tep)->mcte_flags & MCTE_F_MASK_HOME)
-
 #define	MCTE_CLASS(tep) ((tep)->mcte_flags & MCTE_F_MASK_CLASS)
 #define	MCTE_SET_CLASS(tep, new) do { \
     (tep)->mcte_flags &= ~MCTE_F_MASK_CLASS; \
@@ -69,6 +64,8 @@ struct mctelem_ent {
 #define	MC_URGENT_NENT		10
 #define	MC_NONURGENT_NENT	20
 
+#define MC_NENT (MC_URGENT_NENT + MC_NONURGENT_NENT)
+
 #define	MC_NCLASSES		(MC_NONURGENT + 1)
 
 #define	COOKIE2MCTE(c)		((struct mctelem_ent *)(c))
@@ -77,11 +74,9 @@ struct mctelem_ent {
 static struct mc_telem_ctl {
 	/* Linked lists that thread the array members together.
 	 *
-	 * The free lists are singly-linked via mcte_next, and we allocate
-	 * from them by atomically unlinking an element from the head.
-	 * Consumed entries are returned to the head of the free list.
-	 * When an entry is reserved off the free list it is not linked
-	 * on any list until it is committed or dismissed.
+	 * The free lists is a bit array where bit 1 means free.
+	 * This as element number is quite small and is easy to
+	 * atomically allocate that way.
 	 *
 	 * The committed list grows at the head and we do not maintain a
 	 * tail pointer; insertions are performed atomically.  The head
@@ -101,7 +96,7 @@ static struct mc_telem_ctl {
 	 * we can lock it for updates.  The head of the processing list
 	 * always has the oldest telemetry, and we append (as above)
 	 * at the tail of the processing list. */
-	struct mctelem_ent *mctc_free[MC_NCLASSES];
+	DECLARE_BITMAP(mctc_free, MC_NENT);
 	struct mctelem_ent *mctc_committed[MC_NCLASSES];
 	struct mctelem_ent *mctc_processing_head[MC_NCLASSES];
 	struct mctelem_ent *mctc_processing_tail[MC_NCLASSES];
@@ -207,14 +202,14 @@ int mctelem_has_deferred(unsigned int cp
  */
 static void mctelem_free(struct mctelem_ent *tep)
 {
-	mctelem_class_t target = MCTE_HOME(tep) == MCTE_F_HOME_URGENT ?
-	    MC_URGENT : MC_NONURGENT;
-
 	BUG_ON(tep->mcte_refcnt != 0);
 	BUG_ON(MCTE_STATE(tep) != MCTE_F_STATE_FREE);
 
 	tep->mcte_prev = NULL;
-	mctelem_xchg_head(&mctctl.mctc_free[target], &tep->mcte_next, tep);
+	tep->mcte_next = NULL;
+
+	/* set free in array */
+	set_bit(tep - mctctl.mctc_elems, mctctl.mctc_free);
 }
 
 /* Increment the reference count of an entry that is not linked on to
@@ -274,34 +269,25 @@ void mctelem_init(int reqdatasz)
 	}
 
 	if ((mctctl.mctc_elems = xmalloc_array(struct mctelem_ent,
-	    MC_URGENT_NENT + MC_NONURGENT_NENT)) == NULL ||
-	    (datarr = xmalloc_bytes((MC_URGENT_NENT + MC_NONURGENT_NENT) *
-	    datasz)) == NULL) {
+	    MC_NENT)) == NULL ||
+	    (datarr = xmalloc_bytes(MC_NENT * datasz)) == NULL) {
 		if (mctctl.mctc_elems)
 			xfree(mctctl.mctc_elems);
 		printk("Allocations for MCA telemetry failed\n");
 		return;
 	}
 
-	for (i = 0; i < MC_URGENT_NENT + MC_NONURGENT_NENT; i++) {
-		struct mctelem_ent *tep, **tepp;
+	for (i = 0; i < MC_NENT; i++) {
+		struct mctelem_ent *tep;
 
 		tep = mctctl.mctc_elems + i;
 		tep->mcte_flags = MCTE_F_STATE_FREE;
 		tep->mcte_refcnt = 0;
 		tep->mcte_data = datarr + i * datasz;
 
-		if (i < MC_URGENT_NENT) {
-			tepp = &mctctl.mctc_free[MC_URGENT];
-			tep->mcte_flags |= MCTE_F_HOME_URGENT;
-		} else {
-			tepp = &mctctl.mctc_free[MC_NONURGENT];
-			tep->mcte_flags |= MCTE_F_HOME_NONURGENT;
-		}
-
-		tep->mcte_next = *tepp;
+		__set_bit(i, mctctl.mctc_free);
+		tep->mcte_next = NULL;
 		tep->mcte_prev = NULL;
-		*tepp = tep;
 	}
 }
 
@@ -310,32 +296,25 @@ static int mctelem_drop_count;
 
 /* Reserve a telemetry entry, or return NULL if none available.
  * If we return an entry then the caller must subsequently call exactly one of
- * mctelem_unreserve or mctelem_commit for that entry.
+ * mctelem_dismiss or mctelem_commit for that entry.
  */
 mctelem_cookie_t mctelem_reserve(mctelem_class_t which)
 {
-	struct mctelem_ent **freelp;
-	struct mctelem_ent *oldhead, *newhead;
-	mctelem_class_t target = (which == MC_URGENT) ?
-	    MC_URGENT : MC_NONURGENT;
+	unsigned bit;
+	unsigned start_bit = (which == MC_URGENT) ? 0 : MC_URGENT_NENT;
 
-	freelp = &mctctl.mctc_free[target];
 	for (;;) {
-		if ((oldhead = *freelp) == NULL) {
-			if (which == MC_URGENT && target == MC_URGENT) {
-				/* raid the non-urgent freelist */
-				target = MC_NONURGENT;
-				freelp = &mctctl.mctc_free[target];
-				continue;
-			} else {
-				mctelem_drop_count++;
-				return (NULL);
-			}
+		bit = find_next_bit(mctctl.mctc_free, MC_NENT, start_bit);
+
+		if (bit >= MC_NENT) {
+			mctelem_drop_count++;
+			return (NULL);
 		}
 
-		newhead = oldhead->mcte_next;
-		if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {
-			struct mctelem_ent *tep = oldhead;
+		/* try to allocate, atomically clear free bit */
+		if (test_and_clear_bit(bit, mctctl.mctc_free)) {
+			/* return element we got */
+			struct mctelem_ent *tep = mctctl.mctc_elems + bit;
 
 			mctelem_hold(tep);
 			MCTE_TRANSITION_STATE(tep, FREE, UNCOMMITTED);
