一个同事forward过来一个, 公司某产品线遇到的一个低概率, 但长时间出现了几次的Core的bt信息, 找我帮忙分析下原因.
bt栈如下(路径信息以*代替):
| 以下是代码片段： #0 0x00000000004a75e5 in _zend_mm_alloc_int (heap=0xd61260, size=79) at /*/php-5.2.6/Zend/zend_alloc.c:1879 #1 0x000000000048d3cd in vspprintf (pbuf=0x7fbffe9cd8, max_len=1024, format=Variable "format" is not available. ) at /*/php-5.2.6/main/spprintf.c:224 #2 0x0000000000489747 in php_error_cb (type=1, error_filename=0x2a9a787ee8 "/*/application/helpers/util.php", error_lineno=1149, format=Variable "format" is not available. ) at /*/php-5.2.6/main/main.c:799 #3 0x000000000061db35 in soap_error_handler (error_num=1, error_filename=0x2a9a787ee8 "/*/application/helpers/util.php", error_lineno=1149, format=0x7b9cb8 "Maximum execution time of %d second%s exceeded", args=0x7fbffea3b0) at /*/php-5.2.6/ext/soap/soap.c:2178 #4 0x00000000004c2576 in zend_error (type=1, format=0x7b9cb8 "Maximum execution time of %d second%s exceeded") at /*/php-5.2.6/Zend/zend.c:976 #5 <signal handler called> #6 0x00000000004a720f in _zend_mm_free_int (heap=0xd61260, p=Variable "p" is not available. ) at /*/php-5.2.6/Zend/zend_alloc.c:844 ...以下省略 |
| 以下是代码片段： HANDLE_BLOCK_INTERRUPTIONS(); .... HANDLE_UNBLOCK_INTERRUPTIONS(); |
好吧, 不管他了, 那么这个core到底是怎么产生的呢?
首先要介绍下PHP内存管理中的核心结构zend_mm_heap, 如下图:
Zend MM heap
对于free_buckets来说, 它存储了所有小块内存的指针, 对应的通过一个free_bitmap来指明在free_buckets中, 那些索引是可用的(有实际内存),
然后, 在zend_mm_free_int的函数调用中, 在回收内存的时候, 如果发现内存相邻的内存是空闲的, 则会进行合并, 具体的逻辑:
| 以下是代码片段： HANDLE_BLOCK_INTERRUPTIONS(); heap->size -= size; next_block = ZEND_MM_BLOCK_AT(mm_block, size); if (ZEND_MM_IS_FREE_BLOCK(next_block)) { zend_mm_remove_from_free_list(heap, (zend_mm_free_block *) next_block); size += ZEND_MM_FREE_BLOCK_SIZE(next_block); } //以下省略 |
| 以下是代码片段： //有省略 prev->next_free_block = next; next->prev_free_block = prev; if (EXPECTED(ZEND_MM_SMALL_SIZE(ZEND_MM_FREE_BLOCK_SIZE(mm_block)))) { if (EXPECTED(prev == next)) { size_t index = ZEND_MM_BUCKET_INDEX(ZEND_MM_FREE_BLOCK_SIZE(mm_block)); if (EXPECTED(heap->free_buckets[index*2] == heap->free_buckets[index*2+1])) { //注意这一行 heap->free_bitmap &= ~(ZEND_MM_LONG_CONST(1) << index); } } } else if (UNEXPECTED(mm_block->parent != NULL)) { goto subst_block; } //有省略 |
问题就出在这里
当代码执行到这句的时候, 业务逻辑超时信号被触发, 导致这行代码没有被执行, 就转入了信号处理流程.
| 以下是代码片段： #4 0x00000000004c2576 in zend_error (type=1, format=0x7b9cb8 "Maximum execution time of %d second%s exceeded") at /*/php-5.2.6/Zend/zend.c:976 #5 <signal handler called> #6 0x00000000004a720f in _zend_mm_free_int (heap=0xd61260, p=Variable "p" is not available. ) at /*/php-5.2.6/Zend/zend_alloc.c:844 |
| 以下是代码片段： #0 0x00000000004a75e5 in _zend_mm_alloc_int (heap=0xd61260, size=79) at /*/php-5.2.6/Zend/zend_alloc.c:1879 #1 0x000000000048d3cd in vspprintf (pbuf=0x7fbffe9cd8, max_len=1024, format=Variable "format" is not available. ) |
从而导致在alloc_init中的逻辑, 段错误退出:
| 以下是代码片段： # define ZEND_MM_CHECK_BLOCK_LINKAGE(block) \ if (UNEXPECTED((block)->info._size != ZEND_MM_BLOCK_AT(block, ZEND_MM_FREE_BLOCK_SIZE(block))->info._prev) || \ UNEXPECTED(!UNEXPECTED(ZEND_MM_IS_FIRST_BLOCK(block)) && \ UNEXPECTED(ZEND_MM_PREV_BLOCK(block)->info._size != (block)->info._prev))) { \ zend_mm_panic("zend_mm_heap corrupted"); \ } |
其实, 在出core的时候, 业务逻辑已经超时出错了, 而这个core是PHP本身的一些特点造就的, 也不能算是PHP的bug(起码它有一个看似屏蔽信号的操作), 只能说, 低概率的事件吧…