Re: dangling pointers and/or reentrancy in scmd_eh_abort_handler?

[Bart Van Assche <bvanassche@acm.org>]

On 05/19/14 18:43, Bart Van Assche wrote:
> On 05/19/14 18:09, Paolo Bonzini wrote:
>> Il 19/05/2014 17:08, Bart Van Assche ha scritto:
>>> On 05/19/14 16:08, Paolo Bonzini wrote:
>>>> 2) reentrancy: the softirq handler and scmd_eh_abort_handler can run
>>>> concurrently, and call scsi_finish_command without any lock protecting
>>>> the calls.  You can then get memory corruption.
>>>
>>> I'm not sure what the recommended approach is to address this race. But
>>> it is possible to address this in the LLD. See e.g. the srp_claim_req()
>>> function in the SRP LLD and how it is invoked from the reply handler,
>>> the abort handler and the reset handlers in that LLD.
>>
>> That's not enough, unless I'm missing something.  Say the request
>> handler claims the request and the abort handler doesn't:
>>
>> - the request handler calls scsi_done and ends up in scsi_finish_command.
>>
>> - the abort handler will return SUCCESS, and scmd_eh_abort_handler then
>> calls scsi_finish_command.
> 
> It depends on how the SCSI abort handler gets invoked. If the SCSI abort
> handler gets invoked because a SCSI command timed out that means that
> the block layer has already detected a timeout and also that the
> REQ_ATOM_COMPLETE bit has already been set. In this scenario if a SCSI
> LLD invokes scsi_done() that causes blk_complete_request() to return
> without invoking __blk_complete_request() and hence without invoking
> scsi_softirq_done().

(replying to my own e-mail)

Please note that scsi_eh_abort_cmds() neither checks nor sets the
REQ_ATOM_COMPLETE bit before it invokes hostt->eh_abort_handler(). Would
it make sense to modify that function such that it invokes
blk_abort_request() instead ? That last function atomically
test-and-sets the REQ_ATOM_COMPLETE bit before invoking the timeout handler.

Bart.