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MDEV-26010 fixup: Use acquire/release memory order

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In commit 5f22511e35 we depend on
Total Store Ordering. For correct operation on ISAs that implement
weaker memory ordering, we must explicitly use release/acquire stores
and loads on buf_page_t::oldest_modification_ to prevent a race condition
when buf_page_t::list does not happen to be on the same cache line.

buf_page_t::clear_oldest_modification(): Assert that the block is
not in buf_pool.flush_list, and use std::memory_order_release.

buf_page_t::oldest_modification_acquire(): Read oldest_modification_
with std::memory_order_acquire. In this way, if the return value is 0,
the caller may safely assume that it will not observe the buf_page_t
as being in buf_pool.flush_list, even if it is not holding
buf_pool.flush_list_mutex.

buf_flush_relocate_on_flush_list(), buf_LRU_free_page():
Invoke buf_page_t::oldest_modification_acquire().
This commit is contained in:
Marko Mäkelä 2021-06-26 11:16:40 +03:00
commit 759deaa0a2
5 changed files with 52 additions and 23 deletions
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39
storage/innobase/include/buf0buf.h
View file

@ -771,7 +771,7 @@ private:
1 if no modifications are pending, but the block is in buf_pool.flush_list;
2 if modifications are pending, but the block is not in buf_pool.flush_list
(because id().space() is the temporary tablespace). */
Atomic_counter<lsn_t> oldest_modification_;
Atomic_relaxed<lsn_t> oldest_modification_;
/** type of pending I/O operation; protected by buf_pool.mutex
if in_LRU_list */
@ -937,11 +937,18 @@ public:
inline void set_corrupt_id();
/** @return the log sequence number of the oldest pending modification
@retval 0 if the block is not in buf_pool.flush_list
@retval 0 if the block is being removed from (or not in) buf_pool.flush_list
@retval 1 if the block is in buf_pool.flush_list but not modified
@retval 2 if the block belongs to the temporary tablespace and
has unwritten changes */
lsn_t oldest_modification() const { return oldest_modification_; }
/** @return the log sequence number of the oldest pending modification,
@retval 0 if the block is definitely not in buf_pool.flush_list
@retval 1 if the block is in buf_pool.flush_list but not modified
@retval 2 if the block belongs to the temporary tablespace and
has unwritten changes */
lsn_t oldest_modification_acquire() const
{ return oldest_modification_.load(std::memory_order_acquire); }
/** Set oldest_modification when adding to buf_pool.flush_list */
inline void set_oldest_modification(lsn_t lsn);
/** Clear oldest_modification after removing from buf_pool.flush_list */
@ -963,7 +970,8 @@ public:
void free_file_page()
{
ut_ad(state() == BUF_BLOCK_REMOVE_HASH);
ut_d(oldest_modification_= 0); /* for buf_LRU_block_free_non_file_page() */
/* buf_LRU_block_free_non_file_page() asserts !oldest_modification() */
ut_d(oldest_modification_= 0;)
set_corrupt_id();
ut_d(set_state(BUF_BLOCK_MEMORY));
}
@ -2221,7 +2229,8 @@ inline void buf_page_t::set_corrupt_id()
break;
case 2:
ut_ad(fsp_is_system_temporary(id().space()));
ut_d(oldest_modification_= 0); /* for buf_LRU_block_free_non_file_page() */
/* buf_LRU_block_free_non_file_page() asserts !oldest_modification() */
ut_d(oldest_modification_= 0;)
break;
default:
ut_ad("block is dirty" == 0);
@ -2258,7 +2267,12 @@ inline void buf_page_t::clear_oldest_modification()
ut_ad(state == BUF_BLOCK_FILE_PAGE || state == BUF_BLOCK_ZIP_PAGE ||
state == BUF_BLOCK_REMOVE_HASH);
ut_ad(oldest_modification());
oldest_modification_= 0;
ut_ad(!list.prev);
ut_ad(!list.next);
/* We must use release memory order to guarantee that callers of
oldest_modification_acquire() will observe the block as
being detached from buf_pool.flush_list, after reading the value 0. */
oldest_modification_.store(0, std::memory_order_release);
}
/** Note that a block is no longer dirty, while not removing
@ -2268,8 +2282,19 @@ inline void buf_page_t::clear_oldest_modification(bool temporary)
mysql_mutex_assert_not_owner(&buf_pool.flush_list_mutex);
ut_ad(temporary == fsp_is_system_temporary(id().space()));
ut_ad(io_fix_ == BUF_IO_WRITE);
ut_ad(temporary ? oldest_modification() == 2 : oldest_modification() > 2);
oldest_modification_= !temporary;
if (temporary)
{
ut_ad(oldest_modification() == 2);
oldest_modification_= 0;
}
else
{
/* We use release memory order to guarantee that callers of
oldest_modification_acquire() will observe the block as
being detached from buf_pool.flush_list, after reading the value 0. */
ut_ad(oldest_modification() > 2);
oldest_modification_.store(1, std::memory_order_release);
}
}
/** @return whether the block is modified and ready for flushing */