ipc/sem.c: fix complex_count vs. simple op race

Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a
race:

sem_lock has a fast path that allows parallel simple operations.
There are two reasons why a simple operation cannot run in parallel:
 - a non-simple operations is ongoing (sma->sem_perm.lock held)
 - a complex operation is sleeping (sma->complex_count != 0)

As both facts are stored independently, a thread can bypass the current
checks by sleeping in the right positions.  See below for more details
(or kernel bugzilla 105651).

The patch fixes that by creating one variable (complex_mode)
that tracks both reasons why parallel operations are not possible.

The patch also updates stale documentation regarding the locking.

With regards to stable kernels:
The patch is required for all kernels that include the
commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") (3.10?)

The alternative is to revert the patch that introduced the race.

The patch is safe for backporting, i.e. it makes no assumptions
about memory barriers in spin_unlock_wait().

Background:
Here is the race of the current implementation:

Thread A: (simple op)
- does the first "sma->complex_count == 0" test

Thread B: (complex op)
- does sem_lock(): This includes an array scan. But the scan can't
  find Thread A, because Thread A does not own sem->lock yet.
- the thread does the operation, increases complex_count,
  drops sem_lock, sleeps

Thread A:
- spin_lock(&sem->lock), spin_is_locked(sma->sem_perm.lock)
- sleeps before the complex_count test

Thread C: (complex op)
- does sem_lock (no array scan, complex_count==1)
- wakes up Thread B.
- decrements complex_count

Thread A:
- does the complex_count test

Bug:
Now both thread A and thread C operate on the same array, without
any synchronization.

Fixes: 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()")
Link: http://lkml.kernel.org/r/1469123695-5661-1-git-send-email-manfred@colorfullife.com
Reported-by: <felixh@informatik.uni-bremen.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: <1vier1@web.de>
Cc: <stable@vger.kernel.org>	[3.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 83 insertions, 55 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 7c9d4f7683c0..5e318c5f749d 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -162,14 +162,21 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
 
 /*
  * Locking:
+ * a) global sem_lock() for read/write
  *      sem_undo.id_next,
  *      sem_array.complex_count,
- *      sem_array.pending{_alter,_cont},
- *      sem_array.sem_undo: global sem_lock() for read/write
- *      sem_undo.proc_next: only "current" is allowed to read/write that field.
+ *      sem_array.complex_mode
+ *      sem_array.pending{_alter,_const},
+ *      sem_array.sem_undo
  *
+ * b) global or semaphore sem_lock() for read/write:
  *      sem_array.sem_base[i].pending_{const,alter}:
- *              global or semaphore sem_lock() for read/write
+ *      sem_array.complex_mode (for read)
+ *
+ * c) special:
+ *      sem_undo_list.list_proc:
+ *      * undo_list->lock for write
+ *      * rcu for read
  */
 
 #define sc_semmsl       sem_ctls[0]
@@ -260,30 +267,61 @@ static void sem_rcu_free(struct rcu_head *head)
 }
 
 /*
- * Wait until all currently ongoing simple ops have completed.
+ * Enter the mode suitable for non-simple operations:
  * Caller must own sem_perm.lock.
- * New simple ops cannot start, because simple ops first check
- * that sem_perm.lock is free.
- * that a) sem_perm.lock is free and b) complex_count is 0.
  */
-static void sem_wait_array(struct sem_array *sma)
+static void complexmode_enter(struct sem_array *sma)
 {
         int i;
         struct sem *sem;
 
-        if (sma->complex_count)  {
-                /* The thread that increased sma->complex_count waited on
-                 * all sem->lock locks. Thus we don't need to wait again.
-                 */
+        if (sma->complex_mode)  {
+                /* We are already in complex_mode. Nothing to do */
                 return;
         }
 
+        /* We need a full barrier after seting complex_mode:
+         * The write to complex_mode must be visible
+         * before we read the first sem->lock spinlock state.
+         */
+        smp_store_mb(sma->complex_mode, true);
+
         for (i = 0; i < sma->sem_nsems; i++) {
                 sem = sma->sem_base + i;
                 spin_unlock_wait(&sem->lock);
         }
+        /*
+         * spin_unlock_wait() is not a memory barriers, it is only a
+         * control barrier. The code must pair with spin_unlock(&sem->lock),
+         * thus just the control barrier is insufficient.
+         *
+         * smp_rmb() is sufficient, as writes cannot pass the control barrier.
+         */
+        smp_rmb();
+}
+
+/*
+ * Try to leave the mode that disallows simple operations:
+ * Caller must own sem_perm.lock.
+ */
+static void complexmode_tryleave(struct sem_array *sma)
+{
+        if (sma->complex_count)  {
+                /* Complex ops are sleeping.
+                 * We must stay in complex mode
+                 */
+                return;
+        }
+        /*
+         * Immediately after setting complex_mode to false,
+         * a simple op can start. Thus: all memory writes
+         * performed by the current operation must be visible
+         * before we set complex_mode to false.
+         */
+        smp_store_release(&sma->complex_mode, false);
 }
 
+#define SEM_GLOBAL_LOCK (-1)
 /*
  * If the request contains only one semaphore operation, and there are
  * no complex transactions pending, lock only the semaphore involved.
@@ -300,56 +338,42 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
                 /* Complex operation - acquire a full lock */
                 ipc_lock_object(&sma->sem_perm);
 
-                /* And wait until all simple ops that are processed
-                 * right now have dropped their locks.
-                 */
-                sem_wait_array(sma);
-                return -1;
+                /* Prevent parallel simple ops */
+                complexmode_enter(sma);
+                return SEM_GLOBAL_LOCK;
         }
 
         /*
          * Only one semaphore affected - try to optimize locking.
-         * The rules are:
-         * - optimized locking is possible if no complex operation
-         *   is either enqueued or processed right now.
-         * - The test for enqueued complex ops is simple:
-         *      sma->complex_count != 0
-         * - Testing for complex ops that are processed right now is
-         *   a bit more difficult. Complex ops acquire the full lock
-         *   and first wait that the running simple ops have completed.
-         *   (see above)
-         *   Thus: If we own a simple lock and the global lock is free
-         *      and complex_count is now 0, then it will stay 0 and
-         *      thus just locking sem->lock is sufficient.
+         * Optimized locking is possible if no complex operation
+         * is either enqueued or processed right now.
+         *
+         * Both facts are tracked by complex_mode.
          */
         sem = sma->sem_base + sops->sem_num;
 
-        if (sma->complex_count == 0) {
+        /*
+         * Initial check for complex_mode. Just an optimization,
+         * no locking, no memory barrier.
+         */
+        if (!sma->complex_mode) {
                 /*
                  * It appears that no complex operation is around.
                  * Acquire the per-semaphore lock.
                  */
                 spin_lock(&sem->lock);
 
-                /* Then check that the global lock is free */
-                if (!spin_is_locked(&sma->sem_perm.lock)) {
-                        /*
-                         * We need a memory barrier with acquire semantics,
-                         * otherwise we can race with another thread that does:
-                         *      complex_count++;
-                         *      spin_unlock(sem_perm.lock);
-                         */
-                        smp_acquire__after_ctrl_dep();
+                /*
+                 * See 51d7d5205d33
+                 * ("powerpc: Add smp_mb() to arch_spin_is_locked()"):
+                 * A full barrier is required: the write of sem->lock
+                 * must be visible before the read is executed
+                 */
+                smp_mb();
 
-                        /*
-                         * Now repeat the test of complex_count:
-                         * It can't change anymore until we drop sem->lock.
-                         * Thus: if is now 0, then it will stay 0.
-                         */
-                        if (sma->complex_count == 0) {
-                                /* fast path successful! */
-                                return sops->sem_num;
-                        }
+                if (!smp_load_acquire(&sma->complex_mode)) {
+                        /* fast path successful! */
+                        return sops->sem_num;
                 }
                 spin_unlock(&sem->lock);
         }
@@ -369,15 +393,16 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
                 /* Not a false alarm, thus complete the sequence for a
                  * full lock.
                  */
-                sem_wait_array(sma);
-                return -1;
+                complexmode_enter(sma);
+                return SEM_GLOBAL_LOCK;
         }
 }
 
 static inline void sem_unlock(struct sem_array *sma, int locknum)
 {
-        if (locknum == -1) {
+        if (locknum == SEM_GLOBAL_LOCK) {
                 unmerge_queues(sma);
+                complexmode_tryleave(sma);
                 ipc_unlock_object(&sma->sem_perm);
         } else {
                 struct sem *sem = sma->sem_base + locknum;
@@ -529,6 +554,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
         }
 
         sma->complex_count = 0;
+        sma->complex_mode = true; /* dropped by sem_unlock below */
         INIT_LIST_HEAD(&sma->pending_alter);
         INIT_LIST_HEAD(&sma->pending_const);
         INIT_LIST_HEAD(&sma->list_id);
@@ -2184,10 +2210,10 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
         /*
          * The proc interface isn't aware of sem_lock(), it calls
          * ipc_lock_object() directly (in sysvipc_find_ipc).
-         * In order to stay compatible with sem_lock(), we must wait until
-         * all simple semop() calls have left their critical regions.
+         * In order to stay compatible with sem_lock(), we must
+         * enter / leave complex_mode.
          */
-        sem_wait_array(sma);
+        complexmode_enter(sma);
 
         sem_otime = get_semotime(sma);
 
@@ -2204,6 +2230,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
                    sem_otime,
                    sma->sem_ctime);
 
+        complexmode_tryleave(sma);
+
         return 0;
 }
 #endif