Messages in this thread | | | Date | Sat, 20 Sep 2003 13:37:37 +0200 | From | Jens Axboe <> | Subject | Re: __make_request() bug and a fix variant |
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On Fri, Sep 19 2003, Andrew Zabolotny wrote: > 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.
I dunno if you were the one posting this issue here some months ago?
Show me a regular kernel path that passes invalid b_reqnext to __make_request? That would be a bug, indeed, but I've never heard of such a bug. Most likely it's a bug in your driver, not initialising b_reqnext. You can see the initialisor for buffer_heads is init_buffer_head, which memsets the entire buffer_head. When a buffer_head is detached from the request list, b_reqnext is cleared too.
-- Jens Axboe
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