This commit adds example code for heuristic lockless reads, based loosely
on the sem_lock() and sem_unlock() functions.
Reported-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
.../Documentation/access-marking.txt | 94 +++++++++++++++++++
1 file changed, 94 insertions(+)
diff --git a/tools/memory-model/Documentation/access-marking.txt b/tools/memory-model/Documentation/access-marking.txt
index 58bff26198767..be7d507997cf8 100644
--- a/tools/memory-model/Documentation/access-marking.txt
+++ b/tools/memory-model/Documentation/access-marking.txt
@@ -319,6 +319,100 @@ 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)
+ {
+ bool gf = true;
+
+ /* IMPORTANT: Heuristic plus spin_lock()! */
+ 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);
+ /* Lock held, thus global flag cannot change. */
+ if (!global_flag) {
+ spin_lock(&fp->f_lock);
+ spin_unlock(&global_lock);
+ gf = false;
+ }
+ do_something(fp);
+ if (fg)
+ spin_unlock(&global_lock);
+ else
+ spin_lock(&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. */
+ spin_lock(&fp->f_lock);
+ spin_unlock(&fp->f_lock);
+ }
+ }
+
+ void end_global(void)
+ {
+ smp_store_release(&global_flag, false);
+ /* Pre-existing global lock acquisitions will recheck. */
+ 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 global_lock, which prevents global_flag from changing.
+If this recheck finds that global_flag is now false, the acquisition
+of ->f_lock prior to the release of global_lock will result in any subsequent
+begin_global() invocation waiting to acquire ->f_lock.
+
+On the other hand, if the value read from global_flag is false, then
+global_flag, then rechecking under ->f_lock combined with synchronization
+with begin_global() guarantees than any erroneous read will cause the
+do_something_locked() function's first do_something() invocation to happen
+before begin_global() returns. The combination of the smp_load_acquire()
+in do_something_locked() and the smp_store_release() in end_global()
+guarantees that either the do_something_locked() function's first
+do_something() invocation happens after the call to end_global() or that
+do_something_locked() acquires global_lock() and rechecks under the lock.
+
+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 locking each
+and every foo structure.
+
+
Lockless Reads and Writes
-------------------------
--
2.31.1.189.g2e36527f23
