BUG#21051: RESET QUERY CACHE very slow when query_cache_type=0
There were two problems: RESET QUERY CACHE took a long time to complete and other threads were blocked during this time. The patch does three things: 1 fixes a bug with improper use of test-lock-test_again technique. AKA Double-Checked Locking is applicable here only in few places. 2 Somewhat improves performance of RESET QUERY CACHE. Do my_hash_reset() instead of deleting elements one by one. Note however that the slowdown also happens when inserting into sorted list of free blocks, should be rewritten using balanced tree. 3 Makes RESET QUERY CACHE non-blocking. The patch adjusts the locking protocol of the query cache in the following way: it introduces a flag flush_in_progress, which is set when Query_cache::flush_cache() is in progress. This call sets the flag on enter, and then releases the lock. Every other call is able to acquire the lock, but does nothing if flush_in_progress is set (as if the query cache is disabled). The only exception is the concurrent calls to Query_cache::flush_cache(), that are blocked until the flush is over. When leaving Query_cache::flush_cache(), the lock is acquired and the flag is reset, and one thread waiting on Query_cache::flush_cache() (if any) is notified that it may proceed.
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