[PATCH v3 02/10] mtd: powernv_flash: Lock around concurrent access to OPAL

[Michael Ellerman]

Cyril Bur <cyrilbur at gmail.com> writes:

> On Mon, 2017-07-17 at 19:29 +1000, Balbir Singh wrote:
>> On Mon, 2017-07-17 at 17:55 +1000, Cyril Bur wrote:
>> > On Mon, 2017-07-17 at 17:34 +1000, Balbir Singh wrote:
>> > > On Wed, 2017-07-12 at 14:22 +1000, Cyril Bur wrote:
>> > > > OPAL can only manage one flash access at a time and will return an
>> > > > OPAL_BUSY error for each concurrent access to the flash. The simplest
>> > > > way to prevent this from happening is with a mutex.
>> > > > 
>> > > > Signed-off-by: Cyril Bur <cyrilbur at gmail.com>
>> > > > ---
>> > > 
>> > > Should the mutex_lock() be mutex_lock_interruptible()? Are we OK waiting on
>> > > the mutex while other operations with the lock are busy?
>> > > 
>> > 
>> > This is a good question. My best interpretation is that
>> > _interruptible() should be used when you'll only be coming from a user
>> > context. Which is mostly true for this driver, however, MTD does
>> > provide kernel interfaces, so I was hesitant, there isn't a great deal
>> > of use of _interruptible() in drivers/mtd. 
>> > 
>> > Thoughts?
>> 
>> What are the kernel interfaces (I have not read through mtd in detail)?
>> I would still like to see us not blocked in mutex_lock() across threads
>> for parallel calls, one option is to use mutex_trylock() and return if
>> someone already holds the mutex with -EBUSY, but you'll need to evaluate
>> what that means for every call.
>
> Yeah maybe mutex_trylock() is the way to go, thinking quickly, I don't
> see how it could be a problem for userspace using powernv_flash. I'm
> honestly not too sure about the depths of the mtd kernel interfaces but
> I've seen a tonne of cool stuff you could do, hence my reluctance to go
> with _interruptible()

If you use trylock that means all your callers now need to handle EBUSY,
which I doubt they do. Which means it goes up to userspace, which most
users will just treat as a hard error.

So that sounds like a bad plan to me.

cheers