Re: (ITS#7969) LMDB: Globally shared fields of meta-data are not 'volatile'.

[hyc@symas.com]

leo@yuriev.ru wrote:
> The attached files is derived from OpenLDAP Software. All of the modifications
> to OpenLDAP Software represented in the following patch(es) were developed by
> Peter-Service LLC, Moscow, Russia. Peter-Service LLC has not assigned rights
> and/or interest in this work to any party. I, Leonid Yuriev am authorized by
> Peter-Service LLC, my employer, to release this work under the following terms.
>
> Peter-Service LLC hereby places the following modifications to OpenLDAP Software
> (and only these modifications) into the public domain. Hence, these
> modifications may be freely used and/or redistributed for any purpose with or
> without attribution and/or other notice.

Committed to mdb.master. The whitespace is wrong in all of the patches you 
submitted, had to apply them manually.

>
> commit 5e83ea8a76f81dac8d5a9dcd74d5b10a01330102
> Author: Leo Yuriev <leo@yuriev.ru>
> Date:   2014-09-12 01:32:13 +0400
>
>      ITS#7969 for LMDB: volatile & __synchronize().
>
> diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
> index 6cc3433..3286ffb 100644
> --- a/libraries/liblmdb/mdb.c
> +++ b/libraries/liblmdb/mdb.c
> @@ -580,11 +580,11 @@ typedef struct MDB_rxbody {
>    * started from so we can avoid overwriting any data used in that
>    * particular version.
>    */
> - txnid_t mrb_txnid;
> + volatile txnid_t mrb_txnid;
>    /** The process ID of the process owning this reader txn. */
> - MDB_PID_T mrb_pid;
> + volatile MDB_PID_T mrb_pid;
>    /** The thread ID of the thread owning this txn. */
> - MDB_THR_T mrb_tid;
> + volatile MDB_THR_T mrb_tid;
>   } MDB_rxbody;
>
>    /** The actual reader record, with cacheline padding. */
> @@ -632,12 +632,12 @@ typedef struct MDB_txbody {
>    * This is recorded here only for convenience; the value can always
>    * be determined by reading the main database meta pages.
>    */
> - txnid_t mtb_txnid;
> + volatile txnid_t mtb_txnid;
>    /** The number of slots that have been used in the reader table.
>    * This always records the maximum count, it is not decremented
>    * when readers release their slots.
>    */
> - unsigned mtb_numreaders;
> + volatile unsigned mtb_numreaders;
>   } MDB_txbody;
>
>    /** The actual reader table definition. */
> @@ -908,7 +908,7 @@ typedef struct MDB_meta {
>    /** Any persistent environment flags. @ref mdb_env */
>   #define mm_flags mm_dbs[0].md_flags
>    pgno_t mm_last_pg; /**< last used page in file */
> - txnid_t mm_txnid; /**< txnid that committed this page */
> + volatile txnid_t mm_txnid; /**< txnid that committed this page */
>   } MDB_meta;
>
>    /** Buffer for a stack-allocated meta page.
> @@ -3559,6 +3559,10 @@ mdb_env_write_meta(MDB_txn *txn)
>    mp->mm_dbs[0] = txn->mt_dbs[0];
>    mp->mm_dbs[1] = txn->mt_dbs[1];
>    mp->mm_last_pg = txn->mt_next_pgno - 1;
> +#if !(defined(_MSC_VER) || defined(__i386__) || defined(__x86_64__))
> + /* LY: issue a memory barrier, if not x86. */
> + __sync_synchronize();
> +#endif
>    mp->mm_txnid = txn->mt_txnid;
>    if (!(env->me_flags & (MDB_NOMETASYNC|MDB_NOSYNC))) {
>    unsigned meta_size = env->me_psize;
>
>
>
>


-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/