geom->access problem and workaround

[Andriy Gapon]

On 23/03/2018 15:31, Warner Losh wrote:
> 
> 
> On Fri, Mar 23, 2018 at 4:38 AM, Andriy Gapon <avg at freebsd.org
> <mailto:avg at freebsd.org>> wrote:
> 
>     On 12/03/2018 20:07, Poul-Henning Kamp wrote:
>     > If we want to have an architectural sound way to do slow operations
>     > before any "user-I/O" is initiated, the right way to do so is to
>     > define new BIO_OPEN and BIO_CLOSE  operation, and insist via asserts
>     > than all BIO_{READ|WRITE|DELETE} are wrapped in these.
> 
> 
>     In support of this proposal I want to add another example.
>     The problem is not only with doing I/O in access, but also with doing blocking
>     I/O in the normal I/O path.
>     The following happened when a userland program tried to read from a CD-ROM while
>     its tray was open:
> 
>     panic: sleepq_add: td 0xfffff80008e1c000 to sleep on wchan 0xfffff801e58b8048
>     with sleeping prohibited
> 
> 
> cdstrategy shouldn't be sleeping. It can start I/O, but it can't wait for it to
> finish. strategy has been a no-sleep-zone since at least v6. The fact it's
> waiting for prevent is the bug here.

I guess that what you suggest is that cdstrategy should place the incoming
request on a queue and then start a state machine for issuing management
commands and handling their completions.  After the state machine goes back to
the normal state only then the driver would start processing queued I/O requests.
Something like that?


-- 
Andriy Gapon