kernel_mutex--mysql5.5.18

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在mysql innodb中，随处可见的kernel_mutex。全局锁，事务开始的一段代码段 /trx/trx0trx.c 中 trx_start（）函数

/****************************************************************//**
Starts a new transaction.
@return   TRUE */
UNIV_INTERN
ibool
trx_start(
/*======*/
   trx_t*   trx,   /*!< in: transaction */
   ulint   rseg_id)/*!< in: rollback segment id; if ULINT_UNDEFINED
           is passed, the system chooses the rollback segment
           automatically in a round-robin fashion */
{
   ibool   ret;

   /* Update the info whether we should skip XA steps that eat CPU time
   For the duration of the transaction trx->support_xa is not reread
   from thd so any changes in the value take effect in the next
   transaction. This is to avoid a scenario where some undo
   generated by a transaction, has XA stuff, and other undo,
   generated by the same transaction, doesn't. */
   trx->support_xa = thd_supports_xa(trx->mysql_thd);

   mutex_enter(&kernel_mutex);

   ret = trx_start_low(trx, rseg_id);

   mutex_exit(&kernel_mutex);

   return(ret);
}

http://blog.csdn.net/zhaiwx1987/article/details/7108610

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