Messages in this thread | | | Date | Fri, 19 Sep 2003 23:17:32 +0400 | From | Andrew Zabolotny <> | Subject | __make_request() bug and a fix variant |
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Hello!
While developing a driver I found something that I think is a bug in kernel (I'm using 2.4.20, I just hope it is already fixed in 2.6.x series). It manifests itself by bread() randomly crashing (in different places) if called for, say, reading 1024 bytes from block 0 of device 0300 from a driver's module_init() (at least in my very stripped debug environment, this could differ from system to system).
Here's a somewhat long description of the problem roots:
bread() in the first place calls getblk(), which first tries to find the requested buffer in hash tables, and if it is not there, it calls grow_buffers(), the later calls grow_dev_page() and finally that calls create_buffers(). The later gets a set of free buffer_head's from the pool, and puts them in a chain attached to a page. Many fields are left in a indefinite state since they are initialized before usage. The b_reqnext field is left in a indefinite state as well, and it happens to be filled with garbage in my case (actually it's a leftover from previous usage of the buffer_head).
Now when bread() gets this buffer, it is passed to ll_rw_block() which is passing it to generic_make_request(), and, in turn (for many block devices including IDE) to __make_request.
And finally, if elevator returns ELEVATOR_NO_MERGE, the b_reqnext field of the buffer_head structure is left uninitialized! So when b_end_io (and in turn end_that_request_first) is called, it looks at b_reqnext and sees there's another bufhead waiting for processing. What happens next is limited just by your imagination :-)
Also I observed the ELEVATOR_BACK_MERGE case also has the same problem (bh->b_reqnext is left in a indefinite state). So maybe __make_request always assumes that b_reqnext is initially NULL? In this case the bug is in create_buffers which should NULLify this field. In any case, I'm leaving the final solution up to kernel wizards.
-- Greetings, Andrew
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