MDEV-24167 fixup: Wake up all update_lock() in u_unlock()

It turns out that the hang that was fixed in
commit 43d3dad11468bf45d2098ebca70002c49104f298
for the SRW_LOCK_DUMMY implementation is also possible in the futex
implementation. We have observed hangs of ssux_lock_low::u_unlock()
on Windows where the undesirable value is rw_lock::UPDATER, in the
test mariabackup.xb_compressed_encrypted.

The exact sequence of events to the hang is not known, but
it seems that u_unlock() had better always wake up one thread.
Possibly, the case involves multiple blocked u_unlock().

On a busy server, the hang might be 'rescued' by a subsequent
lock acquisition and release that is executed by another thread.

rw_lock::update_unlock(): Change the return type to void.

ssux_lock_low::u_unlock(): Always invoke readers_wake() [sic],
to wake up any pending update_lock() or write_lock().
On futex implementation, this will wake up all waiters.
On SRW_LOCK_DUMMY, writer_wake() and readers_wake() do the same
thing: wake up one write_lock(), or all update_lock() waiters.

2 files changed, 5 insertions, 11 deletions

diff --git a/storage/innobase/include/rw_lock.h b/storage/innobase/include/rw_lock.h
index ac01c28f346..cae71e8f6c7 100644
--- a/storage/innobase/include/rw_lock.h
+++ b/storage/innobase/include/rw_lock.h
@@ -132,14 +132,13 @@ public:
     DBUG_ASSERT(!(l & WRITER)); /* no write lock must have existed */
     return (~WRITER_PENDING & l) == 1;
   }
-  /** Release an update lock.
-  @return whether any writers may have to be woken up */
-  bool update_unlock()
+  /** Release an update lock */
+  void update_unlock()
   {
-    auto l= lock.fetch_and(~UPDATER, std::memory_order_release);
+    IF_DBUG_ASSERT(auto l=,)
+    lock.fetch_and(~UPDATER, std::memory_order_release);
     /* the update lock must have existed */
     DBUG_ASSERT((l & (WRITER | UPDATER)) == UPDATER);
-    return !(~(WRITER_PENDING | UPDATER) & l);
   }
   /** Release an exclusive lock */
   void write_unlock()
diff --git a/storage/innobase/sync/srw_lock.cc b/storage/innobase/sync/srw_lock.cc
index 17260ebb318..45bf7a29c00 100644
--- a/storage/innobase/sync/srw_lock.cc
+++ b/storage/innobase/sync/srw_lock.cc
@@ -233,13 +233,8 @@ void ssux_lock_low::rd_unlock() { if (read_unlock()) writer_wake(); }
 
 void ssux_lock_low::u_unlock()
 {
-#ifdef SRW_LOCK_DUMMY
   update_unlock();
-  writer_wake(); /* Wake up either write_lock() or update_lock() */
-#else
-  if (update_unlock())
-    writer_wake(); /* Wake up one waiter (hopefully the writer) */
-#endif
+  readers_wake(); /* Wake up all write_lock(), update_lock() */
 }
 
 void ssux_lock_low::wr_unlock() { write_unlock(); readers_wake(); }