MDEV-26467 Unnecessary compare-and-swap loop in srw_mutex

srw_mutex::wait_and_lock(): In the spin loop, we will try to poll
for non-conflicting lock word state by reads, avoiding any writes.
We invoke explicit std::atomic_thread_fence(std::memory_order_acquire)
before returning. The individual operations on the lock word
can use memory_order_relaxed.

srw_mutex::lock: Document that the value for a single writer is
HOLDER+1 instead of HOLDER.

srw_mutex::wr_lock_try(), srw_mutex::wr_unlock(): Adjust the value
of the lock word of a single writer from HOLDER to HOLDER+1.

storage/innobase/include/srw_lock.h

@@ -36,7 +36,8 @@ class srw_mutex final
 /** Futex-based mutex */
 class srw_mutex final
 {
-  /** The lock word, containing HOLDER and a count of waiters */
+  /** The lock word, containing HOLDER + 1 if the lock is being held,
+  plus the number of waiters */
   std::atomic<uint32_t> lock;
   /** Identifies that the lock is being held */
   static constexpr uint32_t HOLDER= 1U << 31;
@@ -62,7 +63,7 @@ class srw_mutex final
   bool wr_lock_try()
   {
     uint32_t lk= 0;
-    return lock.compare_exchange_strong(lk, HOLDER,
+    return lock.compare_exchange_strong(lk, HOLDER + 1,
                                         std::memory_order_acquire,
                                         std::memory_order_relaxed);
   }
@@ -70,8 +71,8 @@ class srw_mutex final
   void wr_lock() { if (!wr_lock_try()) wait_and_lock(); }
   void wr_unlock()
   {
-    const uint32_t lk= lock.fetch_and(~HOLDER, std::memory_order_release);
-    if (lk != HOLDER)
+    const uint32_t lk= lock.fetch_sub(HOLDER + 1, std::memory_order_release);
+    if (lk != HOLDER + 1)
     {
       DBUG_ASSERT(lk & HOLDER);
       wake();

storage/innobase/sync/srw_lock.cc

@@ -233,33 +233,39 @@ void ssux_lock_low::wake() { SRW_FUTEX(&readers, WAKE, 1); }
 void srw_mutex::wait_and_lock()
 {
   uint32_t lk= 1 + lock.fetch_add(1, std::memory_order_relaxed);
-  for (auto spin= srv_n_spin_wait_rounds; spin; spin--)
+  for (auto spin= srv_n_spin_wait_rounds;;)
   {
-    lk&= ~HOLDER;
-    DBUG_ASSERT(lk);
-    while (!lock.compare_exchange_weak(lk, HOLDER | (lk - 1),
-                                       std::memory_order_acquire,
-                                       std::memory_order_relaxed))
-      if (lk & HOLDER)
-        goto occupied;
-    return;
-occupied:
+    DBUG_ASSERT(~HOLDER & lk);
+    if (lk & HOLDER)
+      lk= lock.load(std::memory_order_relaxed);
+    else
+    {
+      lk= lock.fetch_or(HOLDER, std::memory_order_relaxed);
+      if (!(lk & HOLDER))
+        goto acquired;
+    }
     ut_delay(srv_spin_wait_delay);
+    if (!--spin)
+      break;
   }
 
-  for (;;)
+  for (;; wait(lk))
   {
-    lk= lock.load(std::memory_order_relaxed);
-    while (!(lk & HOLDER))
+    if (lk & HOLDER)
     {
+      lk= lock.load(std::memory_order_relaxed);
+      if (lk & HOLDER)
+        continue;
+    }
+    lk= lock.fetch_or(HOLDER, std::memory_order_relaxed);
+    if (!(lk & HOLDER))
+    {
+acquired:
       DBUG_ASSERT(lk);
-      if (lock.compare_exchange_weak(lk, HOLDER | (lk - 1),
-                                     std::memory_order_acquire,
-                                     std::memory_order_relaxed))
-        return;
+      std::atomic_thread_fence(std::memory_order_acquire);
+      return;
     }
     DBUG_ASSERT(lk > HOLDER);
-    wait(lk);
   }
 }