This commit adds example code for heuristic lockless reads, based loosely
on the sem_lock() and sem_unlock() functions.
[ paulmck: Apply Alan Stern and Manfred Spraul feedback. ]
Reported-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/tools/memory-model/Documentation/access-marking.txt b/tools/memory-model/Documentation/access-marking.txt
index 58bff26198767..d96fe20ed582a 100644
--- a/tools/memory-model/Documentation/access-marking.txt
+++ b/tools/memory-model/Documentation/access-marking.txt
@@ -319,6 +319,99 @@ of the ASSERT_EXCLUSIVE_WRITER() is to allow KCSAN to check for a buggy
concurrent lockless write.
+Lock-Protected Writes With Heuristic Lockless Reads
+---------------------------------------------------
+
+For another example, suppose that the code can normally make use of
+a per-data-structure lock, but there are times when a global lock
+is required. These times are indicated via a global flag. The code
+might look as follows, and is based loosely on nf_conntrack_lock(),
+nf_conntrack_all_lock(), and nf_conntrack_all_unlock():
+
+ bool global_flag;
+ DEFINE_SPINLOCK(global_lock);
+ struct foo {
+ spinlock_t f_lock;
+ int f_data;
+ };
+
+ /* All foo structures are in the following array. */
+ int nfoo;
+ struct foo *foo_array;
+
+ void do_something_locked(struct foo *fp)
+ {
+ /* This works even if data_race() returns nonsense. */
+ if (!data_race(global_flag)) {
+ spin_lock(&fp->f_lock);
+ if (!smp_load_acquire(&global_flag)) {
+ do_something(fp);
+ spin_unlock(&fp->f_lock);
+ return;
+ }
+ spin_unlock(&fp->f_lock);
+ }
+ spin_lock(&global_lock);
+ /* global_lock held, thus global flag cannot be set. */
+ spin_lock(&fp->f_lock);
+ spin_unlock(&global_lock);
+ /*
+ * global_flag might be set here, but begin_global()
+ * will wait for ->f_lock to be released.
+ */
+ do_something(fp);
+ spin_unlock(&fp->f_lock);
+ }
+
+ void begin_global(void)
+ {
+ int i;
+
+ spin_lock(&global_lock);
+ WRITE_ONCE(global_flag, true);
+ for (i = 0; i < nfoo; i++) {
+ /*
+ * Wait for pre-existing local locks. One at
+ * a time to avoid lockdep limitations.
+ */
+ spin_lock(&fp->f_lock);
+ spin_unlock(&fp->f_lock);
+ }
+ }
+
+ void end_global(void)
+ {
+ smp_store_release(&global_flag, false);
+ spin_unlock(&global_lock);
+ }
+
+All code paths leading from the do_something_locked() function's first
+read from global_flag acquire a lock, so endless load fusing cannot
+happen.
+
+If the value read from global_flag is true, then global_flag is
+rechecked while holding ->f_lock, which, if global_flag is now false,
+prevents begin_global() from completing. It is therefore safe to invoke
+do_something().
+
+Otherwise, if either value read from global_flag is true, then after
+global_lock is acquired global_flag must be false. The acquisition of
+->f_lock will prevent any call to begin_global() from returning, which
+means that it is safe to release global_lock and invoke do_something().
+
+For this to work, only those foo structures in foo_array[] may be passed
+to do_something_locked(). The reason for this is that the synchronization
+with begin_global() relies on momentarily holding the lock of each and
+every foo structure.
+
+The smp_load_acquire() and smp_store_release() are required because
+changes to a foo structure between calls to begin_global() and
+end_global() are carried out without holding that structure's ->f_lock.
+The smp_load_acquire() and smp_store_release() ensure that the next
+invocation of do_something() from do_something_locked() will see those
+changes.
+
+
Lockless Reads and Writes
-------------------------
