MDEV-23399 fixup: Interleaved doublewrite batches

Author: Vladislav Vaintroub

storage/innobase/buf/buf0dblwr.cc

@@ -53,8 +53,8 @@ inline buf_block_t *buf_dblwr_trx_sys_get(mtr_t *mtr)
 @param header   doublewrite page header in the TRX_SYS page */
 inline void buf_dblwr_t::init(const byte *header)
 {
-  ut_ad(!first_free);
-  ut_ad(!reserved);
+  ut_ad(!active_slot->first_free);
+  ut_ad(!active_slot->reserved);
   ut_ad(!batch_running);
 
   mysql_mutex_init(buf_dblwr_mutex_key, &mutex, nullptr);
@@ -63,10 +63,14 @@ inline void buf_dblwr_t::init(const byte *header)
   block2= page_id_t(0, mach_read_from_4(header + TRX_SYS_DOUBLEWRITE_BLOCK2));
 
   const uint32_t buf_size= 2 * block_size();
-  write_buf= static_cast<byte*>(aligned_malloc(buf_size << srv_page_size_shift,
-                                               srv_page_size));
-  buf_block_arr= static_cast<element*>
+  for (int i= 0; i < 2; i++)
+  {
+    slots[i].write_buf= static_cast<byte*>
+      (aligned_malloc(buf_size << srv_page_size_shift, srv_page_size));
+    slots[i].buf_block_arr= static_cast<element*>
       (ut_zalloc_nokey(buf_size * sizeof(element)));
+  }
+  active_slot= &slots[0];
 }
 
 /** Create or restore the doublewrite buffer in the TRX_SYS page.
@@ -272,6 +276,7 @@ dberr_t buf_dblwr_t::init_or_load_pages(pfs_os_file_t file, const char *path)
                      TRX_SYS_DOUBLEWRITE + read_buf) !=
     TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED_N;
 
+  auto write_buf= active_slot->write_buf;
   /* Read the pages from the doublewrite buffer to memory */
   err= os_file_read(IORequestRead, file, write_buf,
                     block1.page_no() << srv_page_size_shift,
@@ -443,16 +448,20 @@ void buf_dblwr_t::close()
     return;
 
   /* Free the double write data structures. */
-  ut_ad(!reserved);
-  ut_ad(!first_free);
+  ut_ad(!active_slot->reserved);
+  ut_ad(!active_slot->first_free);
   ut_ad(!batch_running);
 
   mysql_cond_destroy(&cond);
-  aligned_free(write_buf);
-  ut_free(buf_block_arr);
+  for (int i= 0; i < 2; i++)
+  {
+    aligned_free(slots[i].write_buf);
+    ut_free(slots[i].buf_block_arr);
+  }
   mysql_mutex_destroy(&mutex);
 
   memset((void*) this, 0, sizeof *this);
+  active_slot= &slots[0];
 }
 
 /** Update the doublewrite buffer on write completion. */
@@ -466,10 +475,11 @@ void buf_dblwr_t::write_completed()
   mysql_mutex_lock(&mutex);
 
   ut_ad(batch_running);
-  ut_ad(reserved);
-  ut_ad(reserved <= first_free);
+  slot *flush_slot= active_slot == &slots[0] ? &slots[1] : &slots[0];
+  ut_ad(flush_slot->reserved);
+  ut_ad(flush_slot->reserved <= flush_slot->first_free);
 
-  if (!--reserved)
+  if (!--flush_slot->reserved)
   {
     mysql_mutex_unlock(&mutex);
     /* This will finish the batch. Sync data files to the disk. */
@@ -477,7 +487,7 @@ void buf_dblwr_t::write_completed()
     mysql_mutex_lock(&mutex);
 
     /* We can now reuse the doublewrite memory buffer: */
-    first_free= 0;
+    flush_slot->first_free= 0;
     batch_running= false;
     mysql_cond_broadcast(&cond);
   }
@@ -552,25 +562,30 @@ bool buf_dblwr_t::flush_buffered_writes(const ulint size)
 
   for (;;)
   {
-    if (!first_free)
+    if (!active_slot->first_free)
       return false;
     if (!batch_running)
       break;
     mysql_cond_wait(&cond, &mutex);
   }
 
-  ut_ad(reserved == first_free);
-  /* Disallow anyone else to post to doublewrite buffer or to
-  start another batch of flushing. */
+  ut_ad(active_slot->reserved == active_slot->first_free);
+
+  /* Disallow anyone else to start another batch of flushing. */
+  slot *flush_slot= active_slot;
+  /* Switch the active slot */
+  active_slot= active_slot == &slots[0] ? &slots[1] : &slots[0];
+  ut_a(active_slot->first_free == 0);
   batch_running= true;
-  const ulint old_first_free= first_free;
+  const ulint old_first_free= flush_slot->first_free;
+  auto write_buf= flush_slot->write_buf;
 
   /* Now safe to release the mutex. */
   mysql_mutex_unlock(&mutex);
 #ifdef UNIV_DEBUG
   for (ulint len2= 0, i= 0; i < old_first_free; len2 += srv_page_size, i++)
   {
-    buf_page_t *bpage= buf_block_arr[i].request.bpage;
+    buf_page_t *bpage= flush_slot->buf_block_arr[i].request.bpage;
 
     if (bpage->zip.data)
       /* No simple validate for ROW_FORMAT=COMPRESSED pages exists. */
@@ -602,7 +617,7 @@ bool buf_dblwr_t::flush_buffered_writes(const ulint size)
   }
 
   /* increment the doublewrite flushed pages counter */
-  srv_stats.dblwr_pages_written.add(first_free);
+  srv_stats.dblwr_pages_written.add(flush_slot->first_free);
   srv_stats.dblwr_writes.inc();
 
   /* Now flush the doublewrite buffer data to disk */
@@ -612,20 +627,13 @@ bool buf_dblwr_t::flush_buffered_writes(const ulint size)
   and in recovery we will find them in the doublewrite buffer
   blocks. Next do the writes to the intended positions. */
 
-  /* Up to this point old_first_free == first_free because we have set
-  the batch_running flag disallowing any other thread to post any
-  request but we can't safely access first_free in the loop below.
-  This is so because it is possible that after we are done with the
-  last iteration and before we terminate the loop, the batch gets
-  finished in the IO helper thread and another thread posts a new
-  batch setting first_free to a higher value.  If this happens and we
-  are using first_free in the loop termination condition then we'll
-  end up dispatching the same block twice from two different
-  threads. */
-  ut_ad(old_first_free == first_free);
+
+  ut_ad(active_slot != flush_slot);
+  ut_ad(flush_slot->first_free == old_first_free);
+
   for (ulint i= 0; i < old_first_free; i++)
   {
-    auto e= buf_block_arr[i];
+    auto e= flush_slot->buf_block_arr[i];
     buf_page_t* bpage= e.request.bpage;
     ut_ad(bpage->in_file());
 
@@ -696,18 +704,15 @@ void buf_dblwr_t::add_to_batch(fil_space_t *space, const IORequest &request,
 
   for (;;)
   {
-    while (batch_running)
-      mysql_cond_wait(&cond, &mutex);
-
-    ut_ad(first_free <= buf_size);
-    if (first_free != buf_size)
+    ut_ad(active_slot->first_free <= buf_size);
+    if (active_slot->first_free != buf_size)
       break;
 
     if (flush_buffered_writes(buf_size / 2))
       mysql_mutex_lock(&mutex);
   }
 
-  byte *p= write_buf + srv_page_size * first_free;
+  byte *p= active_slot->write_buf + srv_page_size * active_slot->first_free;
 
   /* We request frame here to get correct buffer in case of
   encryption and/or page compression */
@@ -715,11 +720,13 @@ void buf_dblwr_t::add_to_batch(fil_space_t *space, const IORequest &request,
 
   memcpy_aligned<OS_FILE_LOG_BLOCK_SIZE>(p, frame, size);
   ut_ad(!request.bpage->zip_size() || request.bpage->zip_size() == size);
-  ut_ad(reserved == first_free);
-  ut_ad(reserved < buf_size);
-  new (buf_block_arr + first_free++) element{space, request, size};
-  reserved= first_free;
-
-  if (first_free != buf_size || !flush_buffered_writes(buf_size / 2))
+  ut_ad(active_slot->reserved == active_slot->first_free);
+  ut_ad(active_slot->reserved < buf_size);
+  new (active_slot->buf_block_arr + active_slot->first_free++)
+    element{space, request, size};
+  active_slot->reserved= active_slot->first_free;
+
+  if (active_slot->first_free != buf_size ||
+      !flush_buffered_writes(buf_size / 2))
     mysql_mutex_unlock(&mutex);
 }

storage/innobase/include/buf0dblwr.h

@@ -32,6 +32,29 @@ Created 2011/12/19 Inaam Rana
 /** Doublewrite control struct */
 class buf_dblwr_t
 {
+  struct element
+  {
+    /** tablespace */
+    fil_space_t *space;
+    /** asynchronous write request */
+    IORequest request;
+    /** payload size in bytes */
+    size_t size;
+  };
+
+  struct slot
+  {
+    /** first free position in write_buf measured in units of
+     * srv_page_size */
+    ulint first_free;
+    /** number of slots reserved for the current write batch */
+    ulint reserved;
+    /** the doublewrite buffer, aligned to srv_page_size */
+    byte* write_buf;
+    /** buffer blocks to be written via write_buf */
+    element* buf_block_arr;
+  };
+
   /** the page number of the first doublewrite block (block_size() pages) */
   page_id_t block1= page_id_t(0, 0);
   /** the page number of the second doublewrite block (block_size() pages) */
@@ -43,25 +66,10 @@ class buf_dblwr_t
   mysql_cond_t cond;
   /** whether a batch is being written from the doublewrite buffer */
   bool batch_running;
-  /** first free position in write_buf measured in units of srv_page_size */
-  ulint first_free;
-  /** number of slots reserved for the current write batch */
-  ulint reserved;
-  /** the doublewrite buffer, aligned to srv_page_size */
-  byte *write_buf;
 
-  struct element
-  {
-    /** tablespace */
-    fil_space_t *space;
-    /** asynchronous write request */
-    IORequest request;
-    /** payload size in bytes */
-    size_t size;
-  };
+  slot slots[2];
+  slot *active_slot=&slots[0];
 
-  /** buffer blocks to be written via write_buf */
-  element *buf_block_arr;
 
   /** Initialize the doublewrite buffer data structure.
   @param header   doublewrite page header in the TRX_SYS page */