Re: [PATCH v3 4/8] scsi: call scsi_stop_queue() without state_mutex held

Bart Van Assche
2023-06-12T13:41:26Z

On 6/12/23 04:15, Martin Wilck wrote:
> I guess the race that Bart was hinting at is hard to trigger.

Are you sure about this? I think this scenario can be triggered by
writing into the sysfs attribute that changes the SCSI device state
while a scsi_target_block() call is in progress. See also
store_state_field().

> I would like to remark that the fact that we need to hold the SCSI
> state_mutex while calling blk_mq_quiesce_queue_nowait() looks like a
> layering issue to me. Not sure if, and how, this could be avoided,
> though.

I do not agree that this is a layering issue. Is holding a mutex around
a call of a function in a lower layer ever a layering issue?

What matters is to be very careful with locks while invoking callback
functions. See also slide 7 in Ousterhout's presentation "Why Threads
Are A Bad Idea (for most purposes)" from 1996
(https://web.stanford.edu/~ouster/cgi-bin/papers/threads.pdf).

Bart.