MySQL Bugs: #62692: rw_lock_free might cause assertion failure by race condition

Bug #62692	rw_lock_free might cause assertion failure by race condition
Submitted:	12 Oct 2011 5:57	Modified:	16 Sep 2014 23:18
Reporter:	Yasufumi Kinoshita	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: InnoDB Plugin storage engine	Severity:	S3 (Non-critical)
Version:	all	OS:	Any
Assigned to:	Marko Mäkelä	CPU Architecture:	Any

Description:
The assertion code
"ut_a(lock->lock_word == X_LOCK_DECR);"
at rw_lock_free()/rw_lock_free_func()
might be race condition, thought it might be very rare or architecture dependent.

How to repeat:
Sorry, it should be very very difficult to reproduce...

Suggested fix:
"ut_a(rw_lock_lock_word_decr(lock, X_LOCK_DECR));"
is not affected by race condition.

Thank you for the problem report.

I was wrong

"ut_a(rw_lock_lock_word_decr(lock, X_LOCK_DECR) && lock->lock_word == 0);"

is equivalent to x-lock

Fixed as of the upcoming 5.6.22 and 5.7.6 releases, and here's the changelog entry:

For explicit cache coherency, a write barrier was added to the head of
"os_thread_create_func()", and a read barrier was added to before the
assertion code in "rw_lock_free_func()". 

This bug fix also addresses Bug #72809.

Thank you for the bug report.

$ bzr log -r 6163
------------------------------------------------------------
revno: 6163
committer: Yasufumi Kinoshita <yasufumi.kinoshita@oracle.com>
branch nick: mysql-5.6
timestamp: Tue 2014-09-16 16:46:24 +0900
message:
  Some memory barrier were added for non-intel architecture (POWER, ARM, etc..)
  
  Bug#18870970 : INNODB LINUX NATIVE AIO SETUP MISSING BARRIER AFTER SETUP (Bug#72809)
  For explicit cache coherency with the parent thread, write barrier is added to the head of os_thread_create_func()
  
  Bug#13364876 : RW_LOCK_FREE MIGHT CAUSE ASSERTION FAILURE BY RACE CONDITION (Bug#62692)
  For explicit cache coherency with the other thread, read barrier is added to before the assertion codes at rw_lock_free_func()
  
  Approved by Vasil in rb#6255