9d34cee80b
MDL_wait::m_wait_status has to be reset to EMPTY state before going
into waiting routines. There're three options to get it done:
1. Before MDL_lock::rw_lock critical section of
MDL_context::acquire_lock(). In this case MDL_context is not exposed
neither via MDL_lock::m_waiting nor has it MDL_context::m_waiting_for
set.
Cons: m_wait.reset_status() brings unwanted overhead when lock can
be served immediately.
2. Current solution. Within MDL_lock::rw_lock critical section of
MDL_context::acquire_lock(). MDL_context::m_waiting_for is not yet set
however MDL_context was already exposed via MDL_lock::m_waiting list.
The latter is not a problem since we're still holding exclusive lock
on MDL_lock::rw_lock.
Cons: increases critical section size for no good reason.
3. Whenever MDL_wait is created and after wait in
MDL_context::acquire_lock() is completed. At this point MDL_context
is not exposed via MDL_lock::m_waiting anymore and
MDL_context::m_waiting_for is reset.
Cons: none, it is just plain beauty.
Now MDL_wait::m_wait_status is manipulated as following:
EMPTY - set whenever MDL_wait object is created and after each wait
GRANTED - can be set by a thread that releases incompatible lock
VICTIM - can be set either by owner thread or by concurrent higher
priority thread during deadlock detection
TIMEOUT - always set by owner thread
KILLED - always set by owner thread
This is part of broader cleanup, which aims to make large part of
MDL_lock members private. It is needed to simplify further work on
MDEV-19749 - MDL scalability regression after backup locks.
Code status:
ci.appveyor.com
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