detector" that doesn't introduce bug #56715 "Concurrent
transactions + FLUSH result in sporadical unwarranted
deadlock errors".
Deadlock could have occurred when workload containing a mix
of DML, DDL and FLUSH TABLES statements affecting the same
set of tables was executed in a heavily concurrent environment.
This deadlock occurred when several connections tried to
perform deadlock detection in the metadata locking subsystem.
The first connection started traversing wait-for graph,
encountered a sub-graph representing a wait for flush, acquired
LOCK_open and dived into sub-graph inspection. Then it
encountered sub-graph corresponding to wait for metadata lock
and blocked while trying to acquire a rd-lock on
MDL_lock::m_rwlock, since some,other thread had a wr-lock on it.
When this wr-lock was released it could have happened (if there
was another pending wr-lock against this rwlock) that the rd-lock
from the first connection was left unsatisfied but at the same
time the new rd-lock request from the second connection sneaked
in and was satisfied (for this to be possible the second
rd-request should come exactly after the wr-lock is released but
before pending the wr-lock manages to grab rwlock, which is
possible both on Linux and in our own rwlock implementation).
If this second connection continued traversing the wait-for graph
and encountered a sub-graph representing a wait for flush it tried
to acquire LOCK_open and thus the deadlock was created.
The previous patch tried to workaround this problem by not
allowing the deadlock detector to lock LOCK_open mutex if
some other thread doing deadlock detection already owns it
and current search depth is greater than 0. Instead deadlock
was reported. As a result it has introduced bug #56715.
This patch solves this problem in a different way.
It introduces a new rw_pr_lock_t implementation to be used
by MDL subsystem instead of one based on Linux rwlocks or
our own rwlock implementation. This new implementation
never allows situation in which an rwlock is rd-locked and
there is a blocked pending rd-lock. Thus the situation which
has caused this bug becomes impossible with this implementation.
Due to fact that this implementation is optimized for
wr-lock/unlock scenario which is most common in the MDL
subsystem it doesn't introduce noticeable performance
regressions in sysbench tests. Moreover it significantly
improves situation for POINT_SELECT test when many
connections are used.
No test case is provided as this bug is very hard to repeat
in MTR environment but is repeatable with the help of RQG
tests.
This patch also doesn't include a test for bug #56715
"Concurrent transactions + FLUSH result in sporadical
unwarranted deadlock errors" as it takes too much time to
be run as part of normal test-suite runs.
config.h.cmake:
We no longer need to check for presence of
pthread_rwlockattr_setkind_np as we no longer
use Linux-specific implementation of rw_pr_lock_t
which uses this function.
configure.cmake:
We no longer need to check for presence of
pthread_rwlockattr_setkind_np as we no longer
use Linux-specific implementation of rw_pr_lock_t
which uses this function.
configure.in:
We no longer need to check for presence of
pthread_rwlockattr_setkind_np as we no longer
use Linux-specific implementation of rw_pr_lock_t
which uses this function.
include/my_pthread.h:
Introduced new implementation of rw_pr_lock_t.
Since it never allows situation in which rwlock is rd-locked
and there is a blocked pending rd-lock it is not affected by
bug #56405 "Deadlock in the MDL deadlock detector".
This implementation is also optimized for wr-lock/unlock
scenario which is most common in MDL subsystem. So it doesn't
introduce noticiable performance regressions in sysbench tests
(compared to old Linux-specific implementation). Moreover it
significantly improves situation for POINT_SELECT test when
many connections are used.
As part of this change removed try-lock part of API for
this type of lock. It is not used in our code and it would
be hard to implement correctly within constraints of new
implementation.
Finally, removed support of preferring readers from
my_rw_lock_t implementation as the only user of this
feature was old rw_pr_lock_t implementation.
include/mysql/psi/mysql_thread.h:
Removed try-lock part of prlock API.
It is not used in our code and it would be hard
to implement correctly within constraints of new
prlock implementation.
mysys/thr_rwlock.c:
Introduced new implementation of rw_pr_lock_t.
Since it never allows situation in which rwlock is rd-locked
and there is a blocked pending rd-lock it is not affected by
bug #56405 "Deadlock in the MDL deadlock detector".
This implementation is also optimized for wr-lock/unlock
scenario which is most common in MDL subsystem. So it doesn't
introduce noticiable performance regressions in sysbench tests
(compared to old Linux-specific implementation). Moreover it
significantly improves situation for POINT_SELECT test when
many connections are used.
Also removed support of preferring readers from
my_rw_lock_t implementation as the only user of this
feature was old rw_pr_lock_t implementation.
Implement a few simple asserts in my_rwlock_t locks.
include/my_pthread.h:
Declare two simple assert functions.
include/mysql/psi/mysql_thread.h:
Add wrappers for new assert functions.
mysys/thr_rwlock.c:
Add asserts.
sql/sql_base.cc:
Silence a compiler warning for the case when
SAFE_MUTEX is not ON.
This change is for performance optimization.
Fixed the performance schema instrumentation interface as follows:
- simplified mysql_unlock_mutex()
- simplified mysql_unlock_rwlock()
- simplified mysql_cond_signal()
- simplified mysql_cond_broadcast()
Changed the get_thread_XXX_locker apis to have one extra parameter,
to provide memory to the instrumentation implementation.
This API change allows to use memory provided by the caller,
to avoid having to use thread local storage.
Using this extra parameter will be done in a separate fix,
this change is for the interface only.
Adjusted all the code and unit tests accordingly.
Essentially, the problem is that safemalloc is excruciatingly
slow as it checks all allocated blocks for overrun at each
memory management primitive, yielding a almost exponential
slowdown for the memory management functions (malloc, realloc,
free). The overrun check basically consists of verifying some
bytes of a block for certain magic keys, which catches some
simple forms of overrun. Another minor problem is violation
of aliasing rules and that its own internal list of blocks
is prone to corruption.
Another issue with safemalloc is rather the maintenance cost
as the tool has a significant impact on the server code.
Given the magnitude of memory debuggers available nowadays,
especially those that are provided with the platform malloc
implementation, maintenance of a in-house and largely obsolete
memory debugger becomes a burden that is not worth the effort
due to its slowness and lack of support for detecting more
common forms of heap corruption.
Since there are third-party tools that can provide the same
functionality at a lower or comparable performance cost, the
solution is to simply remove safemalloc. Third-party tools
can provide the same functionality at a lower or comparable
performance cost.
The removal of safemalloc also allows a simplification of the
malloc wrappers, removing quite a bit of kludge: redefinition
of my_malloc, my_free and the removal of the unused second
argument of my_free. Since free() always check whether the
supplied pointer is null, redudant checks are also removed.
Also, this patch adds unit testing for my_malloc and moves
my_realloc implementation into the same file as the other
memory allocation primitives.
client/mysqldump.c:
Pass my_free directly as its signature is compatible with the
callback type -- which wasn't the case for free_table_ent.
This patch:
- Moves all definitions from the mysql_priv.h file into
header files for the component where the variable is
defined
- Creates header files if the component lacks one
- Eliminates all include directives from mysql_priv.h
- Eliminates all circular include cycles
- Rename time.cc to sql_time.cc
- Rename mysql_priv.h to sql_priv.h
This is a fix specific for HPUX,
for which the compiler does not resolve properly dependencies
involving unused inline functions.
(See existing comments in mysql_thread.h)
In include/mysql/psi/mysql_thread.h,
the instrumentation helpers for mysql_prlock_*
uses the pr lock apis.
These apis are implemented in mysys/thr_rwlock.c,
which is not linked to client code.
As a result, the code does not link in libmysql_r, on HPUX.
The fix is to cut dependencies explicitely,
by introducing -DDISABLE_MYSQL_RWLOCK_H,
when building client code.
Before this fix, the performance schema instrumentation
in mdl.h / mdl.cc was incomplete, causing:
- build warnings,
- no data collection for the performance schema
This fix:
- added instrumentation helpers for the new preferred
reader read write lock, mysql_prlock_*
- implemented completely the performance schema
instrumentation of mdl.h / mdl.cc
