MDEV-24142 preparation: Add srw_mutex and srw_lock::u_lock()

The PERFORMANCE_SCHEMA insists on distinguishing read-update-write
locks from read-write locks, so we must add
template<bool support_u_lock> in rd_lock() and wr_lock() operations.

rd_lock::read_trylock(): Add template<bool prioritize_updater=false>
which is used by the srw_lock_low::read_lock() loop. As long as
an UPDATE lock has already been granted to some thread, we will grant
subsequent READ lock requests even if a waiting WRITE lock request
exists. This will be necessary to be compatible with existing usage
pattern of InnoDB rw_lock_t where the holder of SX-latch (which we
will rename to UPDATE latch) may acquire an additional S-latch
on the same object. For normal read-write locks without update operations
this should make no difference at all, because the rw_lock::UPDATER
flag would never be set.

storage/innobase/include/rw_lock.h

@@ -20,7 +20,7 @@ this program; if not, write to the Free Software Foundation, Inc.,
 #include <atomic>
 #include "my_dbug.h"
 
-/** Simple read-write lock based on std::atomic */
+/** Simple read-update-write lock based on std::atomic */
 class rw_lock
 {
   /** The lock word */
@@ -35,6 +35,8 @@ class rw_lock
   static constexpr uint32_t WRITER_WAITING= 1U << 30;
   /** Flag to indicate that write_lock() or write_lock_wait() is pending */
   static constexpr uint32_t WRITER_PENDING= WRITER | WRITER_WAITING;
+  /** Flag to indicate that an update lock exists */
+  static constexpr uint32_t UPDATER= 1U << 29;
 
   /** Start waiting for an exclusive lock.
   @return current value of the lock word */
@@ -45,13 +47,14 @@ class rw_lock
   @return whether the exclusive lock was acquired */
   bool write_lock_wait_try(uint32_t &l)
   {
-    l= WRITER_WAITING;
     return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
                                         std::memory_order_relaxed);
   }
   /** Try to acquire a shared lock.
+  @tparam prioritize_updater   whether to ignore WRITER_WAITING for UPDATER
   @param l the value of the lock word
   @return whether the lock was acquired */
+  template<bool prioritize_updater= false>
   bool read_trylock(uint32_t &l)
   {
     l= UNLOCKED;
@@ -59,16 +62,53 @@ class rw_lock
                                          std::memory_order_relaxed))
     {
       DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
-      if (l & WRITER_PENDING)
+      DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
+      if (prioritize_updater
+          ? (WRITER & l) || ((WRITER_WAITING | UPDATER) & l) == WRITER_WAITING
+          : (WRITER_PENDING & l))
         return false;
     }
     return true;
   }
+  /** Try to acquire an update lock.
+  @param l the value of the lock word
+  @return whether the lock was acquired */
+  bool update_trylock(uint32_t &l)
+  {
+    l= UNLOCKED;
+    while (!lock.compare_exchange_strong(l, l | UPDATER,
+                                         std::memory_order_acquire,
+                                         std::memory_order_relaxed))
+    {
+      DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
+      DBUG_ASSERT((~(WRITER_PENDING | UPDATER) & l) < UPDATER);
+      if ((WRITER_PENDING | UPDATER) & l)
+        return false;
+    }
+    return true;
+  }
+  /** Try to upgrade an update lock to an exclusive lock.
+  @return whether the update lock was upgraded to exclusive */
+  bool upgrade_trylock()
+  {
+    auto l= UPDATER;
+    while (!lock.compare_exchange_strong(l, l ^ (WRITER | UPDATER),
+                                         std::memory_order_acquire,
+                                         std::memory_order_relaxed))
+    {
+      DBUG_ASSERT(!(~l & (UPDATER - 1)));
+      DBUG_ASSERT(((WRITER | UPDATER) & l) == UPDATER);
+      if (~(WRITER_WAITING | UPDATER) & l)
+        return false;
+    }
+    DBUG_ASSERT((l & ~WRITER_WAITING) == UPDATER);
+    return true;
+  }
   /** Wait for an exclusive lock.
   @return whether the exclusive lock was acquired */
   bool write_lock_poll()
   {
-    uint32_t l;
+    auto l= WRITER_WAITING;
     if (write_lock_wait_try(l))
       return true;
     if (!(l & WRITER_WAITING))
@@ -88,15 +128,26 @@ class rw_lock
   bool read_unlock()
   {
     auto l= lock.fetch_sub(1, std::memory_order_release);
-    DBUG_ASSERT(~WRITER_PENDING & l); /* at least one read lock */
+    DBUG_ASSERT(~(WRITER_PENDING | UPDATER) & l); /* at least one read lock */
     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()
+  {
+    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()
   {
-    IF_DBUG_ASSERT(auto l=,) lock.fetch_sub(WRITER, std::memory_order_release);
-    DBUG_ASSERT(l & WRITER); /* the write lock must have existed */
+    IF_DBUG_ASSERT(auto l=,)
+    lock.fetch_and(~WRITER, std::memory_order_release);
+    /* the write lock must have existed */
+    DBUG_ASSERT((l & (WRITER | UPDATER)) == WRITER);
   }
   /** Try to acquire a shared lock.
   @return whether the lock was acquired */
@@ -113,6 +164,9 @@ class rw_lock
   /** @return whether an exclusive lock is being held by any thread */
   bool is_write_locked() const
   { return !!(lock.load(std::memory_order_relaxed) & WRITER); }
+  /** @return whether an update lock is being held by any thread */
+  bool is_update_locked() const
+  { return !!(lock.load(std::memory_order_relaxed) & UPDATER); }
   /** @return whether a shared lock is being held by any thread */
   bool is_read_locked() const
   {

storage/innobase/include/srw_lock.h

@@ -25,6 +25,22 @@ this program; if not, write to the Free Software Foundation, Inc.,
 # define SRW_LOCK_DUMMY /* Use dummy implementation for debugging purposes */
 #endif
 
+#ifdef SRW_LOCK_DUMMY
+/** An exclusive-only variant of srw_lock */
+class srw_mutex
+{
+  pthread_mutex_t lock;
+public:
+  void init() { pthread_mutex_init(&lock, nullptr); }
+  void destroy() { pthread_mutex_destroy(&lock); }
+  void wr_lock() { pthread_mutex_lock(&lock); }
+  void wr_unlock() { pthread_mutex_unlock(&lock); }
+  bool wr_lock_try() { return !pthread_mutex_trylock(&lock); }
+};
+#else
+# define srw_mutex srw_lock_low
+#endif
+
 #include "rw_lock.h"
 
 /** Slim reader-writer lock with no recursion */
@@ -43,8 +59,12 @@ class srw_lock_low final : private rw_lock
   /** Wait for a read lock.
   @param l lock word from a failed read_trylock() */
   void read_lock(uint32_t l);
-  /** Wait for a write lock after a failed write_trylock() */
-  void write_lock();
+  /** Wait for an update lock.
+  @param l lock word from a failed update_trylock() */
+  void update_lock(uint32_t l);
+  /** Wait for a write lock after a failed write_trylock() or upgrade_trylock()
+  @param holding_u  whether we already hold u_lock() */
+  void write_lock(bool holding_u);
   /** Wait for signal
   @param l lock word from a failed acquisition */
   inline void writer_wait(uint32_t l);
@@ -65,9 +85,15 @@ class srw_lock_low final : private rw_lock
 #endif
   bool rd_lock_try() { uint32_t l; return read_trylock(l); }
   bool wr_lock_try() { return write_trylock(); }
+  template<bool support_u_lock= false>
   void rd_lock() { uint32_t l; if (!read_trylock(l)) read_lock(l); }
-  void wr_lock() { if (!write_trylock()) write_lock(); }
+  void u_lock() { uint32_t l; if (!update_trylock(l)) update_lock(l); }
+  bool u_lock_try() { uint32_t l; return update_trylock(l); }
+  void u_wr_upgrade() { if (!upgrade_trylock()) write_lock(true); }
+  template<bool support_u_lock= false>
+  void wr_lock() { if (!write_trylock()) write_lock(false); }
   void rd_unlock();
+  void u_unlock();
   void wr_unlock();
 };
 
@@ -87,8 +113,12 @@ class srw_lock
   srw_lock_low lock;
   PSI_rwlock *pfs_psi;
 
+  template<bool support_u_lock>
   ATTRIBUTE_NOINLINE void psi_rd_lock(const char *file, unsigned line);
+  template<bool support_u_lock>
   ATTRIBUTE_NOINLINE void psi_wr_lock(const char *file, unsigned line);
+  ATTRIBUTE_NOINLINE void psi_u_lock(const char *file, unsigned line);
+  ATTRIBUTE_NOINLINE void psi_u_wr_upgrade(const char *file, unsigned line);
 public:
   void init(mysql_pfs_key_t key)
   {
@@ -104,10 +134,11 @@ class srw_lock
     }
     lock.destroy();
   }
+  template<bool support_u_lock= false>
   void rd_lock(const char *file, unsigned line)
   {
     if (psi_likely(pfs_psi != nullptr))
-      psi_rd_lock(file, line);
+      psi_rd_lock<support_u_lock>(file, line);
     else
       lock.rd_lock();
   }
@@ -117,10 +148,24 @@ class srw_lock
       PSI_RWLOCK_CALL(unlock_rwlock)(pfs_psi);
     lock.rd_unlock();
   }
+  void u_lock(const char *file, unsigned line)
+  {
+    if (psi_likely(pfs_psi != nullptr))
+      psi_u_lock(file, line);
+    else
+      lock.u_lock();
+  }
+  void u_unlock()
+  {
+    if (pfs_psi)
+      PSI_RWLOCK_CALL(unlock_rwlock)(pfs_psi);
+    lock.u_unlock();
+  }
+  template<bool support_u_lock= false>
   void wr_lock(const char *file, unsigned line)
   {
     if (psi_likely(pfs_psi != nullptr))
-      psi_wr_lock(file, line);
+      psi_wr_lock<support_u_lock>(file, line);
     else
       lock.wr_lock();
   }
@@ -130,7 +175,15 @@ class srw_lock
       PSI_RWLOCK_CALL(unlock_rwlock)(pfs_psi);
     lock.wr_unlock();
   }
+  void u_wr_upgrade(const char *file, unsigned line)
+  {
+    if (psi_likely(pfs_psi != nullptr))
+      psi_u_wr_upgrade(file, line);
+    else
+      lock.u_wr_upgrade();
+  }
   bool rd_lock_try() { return lock.rd_lock_try(); }
+  bool u_lock_try() { return lock.u_lock_try(); }
   bool wr_lock_try() { return lock.wr_lock_try(); }
 };
 #endif

storage/innobase/sync/srw_lock.cc

@@ -130,7 +130,7 @@ void srw_lock_low::read_lock(uint32_t l)
       for (auto spin= srv_n_spin_wait_rounds; spin; spin--)
       {
         ut_delay(srv_spin_wait_delay);
-        if (read_trylock(l))
+        if (read_trylock<true>(l))
           return;
         else if (l == WRITER_WAITING)
           goto wake_writer;
@@ -138,11 +138,50 @@ void srw_lock_low::read_lock(uint32_t l)
 
     readers_wait(l);
   }
-  while (!read_trylock(l));
+  while (!read_trylock<true>(l));
 }
 
-/** Wait for a write lock after a failed write_trylock() */
-void srw_lock_low::write_lock()
+/** Wait for an update lock.
+@param lock word value from a failed update_trylock() */
+void srw_lock_low::update_lock(uint32_t l)
+{
+  do
+  {
+    if (l == WRITER_WAITING)
+    {
+    wake_writer:
+#ifdef SRW_LOCK_DUMMY
+      pthread_mutex_lock(&mutex);
+      {
+        pthread_cond_signal(&cond_exclusive);
+        pthread_cond_wait(&cond_shared, &mutex);
+        l= value();
+      }
+      while (l == WRITER_WAITING);
+      pthread_mutex_unlock(&mutex);
+      continue;
+#else
+      writer_wake();
+#endif
+    }
+    else
+      for (auto spin= srv_n_spin_wait_rounds; spin; spin--)
+      {
+        ut_delay(srv_spin_wait_delay);
+        if (update_trylock(l))
+          return;
+        else if (l == WRITER_WAITING)
+          goto wake_writer;
+      }
+
+    readers_wait(l);
+  }
+  while (!update_trylock(l));
+}
+
+/** Wait for a write lock after a failed write_trylock() or upgrade_trylock()
+@param holding_u  whether we already hold u_lock() */
+void srw_lock_low::write_lock(bool holding_u)
 {
   for (;;)
   {
@@ -150,6 +189,7 @@ void srw_lock_low::write_lock()
     /* We are the first writer to be granted the lock. Spin for a while. */
     for (auto spin= srv_n_spin_wait_rounds; spin; spin--)
     {
+      l= holding_u ? WRITER_WAITING | UPDATER : WRITER_WAITING;
       if (write_lock_wait_try(l))
         return;
       if (!(l & WRITER_WAITING))
@@ -157,13 +197,22 @@ void srw_lock_low::write_lock()
       ut_delay(srv_spin_wait_delay);
     }
 
+    l= holding_u ? WRITER_WAITING | UPDATER : WRITER_WAITING;
     if (write_lock_wait_try(l))
       return;
 
     if (!(l & WRITER_WAITING))
     {
-      if (l == UNLOCKED && write_trylock())
+      switch (l) {
+      case UNLOCKED:
+        DBUG_ASSERT(!holding_u);
+        if (write_trylock())
+          return;
+        break;
+      case UPDATER:
+        if (holding_u && upgrade_trylock())
           return;
+      }
       l= write_lock_wait_start() | WRITER_WAITING;
     }
     else
@@ -175,9 +224,12 @@ void srw_lock_low::write_lock()
 
 void srw_lock_low::rd_unlock() { if (read_unlock()) writer_wake(); }
 
+void srw_lock_low::u_unlock() { if (update_unlock()) writer_wake(); }
+
 void srw_lock_low::wr_unlock() { write_unlock(); readers_wake(); }
 
 #ifdef UNIV_PFS_RWLOCK
+template<bool support_u_lock>
 void srw_lock::psi_rd_lock(const char *file, unsigned line)
 {
   PSI_rwlock_locker_state state;
@@ -185,7 +237,9 @@ void srw_lock::psi_rd_lock(const char *file, unsigned line)
   const bool nowait= lock.read_trylock(l);
   if (PSI_rwlock_locker *locker= PSI_RWLOCK_CALL(start_rwlock_rdwait)
       (&state, pfs_psi,
-       nowait ? PSI_RWLOCK_TRYREADLOCK : PSI_RWLOCK_READLOCK, file, line))
+       support_u_lock
+       ? (nowait ? PSI_RWLOCK_TRYSHAREDLOCK : PSI_RWLOCK_SHAREDLOCK)
+       : (nowait ? PSI_RWLOCK_TRYREADLOCK : PSI_RWLOCK_READLOCK), file, line))
   {
     if (!nowait)
       lock.read_lock(l);
@@ -195,13 +249,32 @@ void srw_lock::psi_rd_lock(const char *file, unsigned line)
     lock.read_lock(l);
 }
 
+template void srw_lock::psi_rd_lock<false>(const char *, unsigned);
+template void srw_lock::psi_rd_lock<true>(const char *, unsigned);
+
+void srw_lock::psi_u_lock(const char *file, unsigned line)
+{
+  PSI_rwlock_locker_state state;
+  if (PSI_rwlock_locker *locker= PSI_RWLOCK_CALL(start_rwlock_wrwait)
+      (&state, pfs_psi, PSI_RWLOCK_SHAREDEXCLUSIVELOCK, file, line))
+  {
+    lock.u_lock();
+    PSI_RWLOCK_CALL(end_rwlock_rdwait)(locker, 0);
+  }
+  else
+    lock.u_lock();
+}
+
+template<bool support_u_lock>
 void srw_lock::psi_wr_lock(const char *file, unsigned line)
 {
   PSI_rwlock_locker_state state;
   const bool nowait= lock.write_trylock();
   if (PSI_rwlock_locker *locker= PSI_RWLOCK_CALL(start_rwlock_wrwait)
       (&state, pfs_psi,
-       nowait ? PSI_RWLOCK_TRYWRITELOCK : PSI_RWLOCK_WRITELOCK,
+       support_u_lock
+       ? (nowait ? PSI_RWLOCK_TRYEXCLUSIVELOCK : PSI_RWLOCK_EXCLUSIVELOCK)
+       : (nowait ? PSI_RWLOCK_TRYWRITELOCK : PSI_RWLOCK_WRITELOCK),
        file, line))
   {
     if (!nowait)
@@ -211,4 +284,24 @@ void srw_lock::psi_wr_lock(const char *file, unsigned line)
   else if (!nowait)
     lock.wr_lock();
 }
+
+template void srw_lock::psi_wr_lock<false>(const char *, unsigned);
+template void srw_lock::psi_wr_lock<true>(const char *, unsigned);
+
+void srw_lock::psi_u_wr_upgrade(const char *file, unsigned line)
+{
+  PSI_rwlock_locker_state state;
+  const bool nowait= lock.upgrade_trylock();
+  if (PSI_rwlock_locker *locker= PSI_RWLOCK_CALL(start_rwlock_wrwait)
+      (&state, pfs_psi,
+       nowait ? PSI_RWLOCK_TRYEXCLUSIVELOCK : PSI_RWLOCK_EXCLUSIVELOCK,
+       file, line))
+  {
+    if (!nowait)
+      lock.write_lock(true);
+    PSI_RWLOCK_CALL(end_rwlock_rdwait)(locker, 0);
+  }
+  else if (!nowait)
+    lock.write_lock(true);
+}
 #endif /* UNIV_PFS_RWLOCK */