Messages in this thread |  | | Subject | Re: [PATCH v2] dmaengine: check device and channel list for empty | From | Jiri Slaby <> | Date | Thu, 9 Jul 2020 07:35:36 +0200 |
| |
On 07. 07. 20, 17:42, Dave Jiang wrote: > On 7/6/2020 11:05 PM, Jiri Slaby wrote: >> On 26. 06. 20, 20:09, Dave Jiang wrote: >>> Check dma device list and channel list for empty before iterate as the >>> iteration function assume the list to be not empty. With devices and >>> channels now being hot pluggable this is a condition that needs to be >>> checked. Otherwise it can cause the iterator to spin forever. >> >> Could you be a little bit more specific how this can spin forever? I.e. >> can you attach a stacktrace of such a behaviour? > > I can't seem to find the original splat that lead me to the conclusion > of it's spinning forever. As I recall, the issue seems to produce > different splats and not always consistent in being reproduced. Here's a > partial splat that was tracked by the internal bug database. Since with > the dma device and channel list being are hot added and removed, the > device and channel lists can be empty. The list_entry() and friends > expect the list to not be empty (according to header comment), I added > the check to ensure that isn't the case before using them in dmaengine.
Yes, the comment states that as it is true: you receive a wild/non-checkable pointer if you do list_entry on an empty list. BUT have you actually read what I wrote:
>> As in the empty case, "&pos->member" is "head" (look into >> list_for_each_entry) and the for loop should loop exactly zero times.
HERE ^^^^
> With the fix, we can no longer produce any of the splats. So maybe the > above was a bad description of the issue.
No, not only the description, worse, the patch proper looks wrong.
> [ 4216.048375] ? dma_channel_rebalance+0x7b/0x250 > [ 4216.056360] dma_async_device_register+0x349/0x3a0 > [ 4216.064604] idxd_register_dma_device+0x90/0xc0 [idxd] > [ 4216.073175] idxd_config_bus_probe.cold+0x7d/0x1fc [idxd]
So, the good part in the patch is the fixed locking in dma_async_device_register. Otherwise it adds nonsense checks. So you fixed the issue only by a chance, by a side effect as Peter pointed out. Leaving aside that you broke dma_request_chan -- that could happen to anybody.
Vinod, please drop/revert this patch. Then start over only with dma_async_device_register fixed locking.
thanks, -- js
|  |