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

Cyril Bur [off-list ref] writes:

On Mon, 2017-07-17 at 19:29 +1000, Balbir Singh wrote:

quoted

On Mon, 2017-07-17 at 17:55 +1000, Cyril Bur wrote:

quoted

On Mon, 2017-07-17 at 17:34 +1000, Balbir Singh wrote:

quoted

On Wed, 2017-07-12 at 14:22 +1000, Cyril Bur wrote:

quoted

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 <redacted>
---

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