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Re: (ITS#7813) lmdb multi threaded env creation on Win32 can fail
- To: openldap-its@OpenLDAP.org
- Subject: Re: (ITS#7813) lmdb multi threaded env creation on Win32 can fail
- From: firstname.lastname@example.org
- Date: Thu, 6 Mar 2014 04:24:42 GMT
- Auto-submitted: auto-generated (OpenLDAP-ITS)
> Full_Name: Clayton Stangeland
> Version: gitorious mdb last commit Feb 19, 2014 e2bdd44624a525d4847c22d7ebf1ea4d154ed734
> OS: Windows 7
> URL: ftp://ftp.openldap.org/incoming/
> Submission from: (NULL) (18.104.22.168)
> There is a bug in lightning mdb in mdb.c when running Windows. When creating an
> environment to the same file in multiple threads, an error can occur on the
> OpenMutex call and some of the creations will fail.
> In mdb_env_setup_locks, mdb_env_excl_lock is called.
> If it acquires an exclusive lock CreateMutex is called.
> env->me_rmutex = CreateMutex(&mdb_all_sa, FALSE, env->me_txns->mti_rmname);
> If not OpenMutex is called.
> env->me_rmutex = OpenMutex(SYNCHRONIZE, FALSE, env->me_txns->mti_rmname);
> The OpenMutex call can fail if there is a thread context switch between the time
> the exclusive lock is acquired and the time CreateMutex is called, because the
> mutex to open has not actually been created yet. To be clear OpenMutex in
> another thread gets run before CreateMutex in the first thread.
> The solution is to replace the OpenMutex calls with CreateMutex. It will either
> open the mutex if it exists or create it if it does not. Setting the
> bInitialOwner flag to FALSE as is already done is the recommended way to use
> CreateMutex in this situation.
There is already a note about multiple processes/threads opening the same
environment at the same time. In short, don't do this.
Even if the code is changed as you describe, if such a race condition can
occur, then it also means that the check for mti_magic != MDB_MAGIC can also
fail, so the fix is ineffective.
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/