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    SubjectRe: [PATCH v9 08/20] gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL
    On Thu, Sep 24, 2020 at 5:39 AM Kent Gibson <warthog618@gmail.com> wrote:
    > On Wed, Sep 23, 2020 at 06:41:45PM +0300, Andy Shevchenko wrote:
    > > On Tue, Sep 22, 2020 at 5:35 AM Kent Gibson <warthog618@gmail.com> wrote:

    ...

    > > > + memcpy(info_v1->consumer, info_v2->consumer,
    > > > + sizeof(info_v1->consumer));
    > >
    > > One line?
    > >
    >
    > It can be now the line length limit has been raised - it just breaks the
    > old 80 character limit.

    I really wouldn't care about this if it's only for a couple of characters.

    ...

    > > > +static int lineinfo_ensure_abi_version(struct gpio_chardev_data *cdata,
    > > > + unsigned int version)
    > > > +{
    > >
    > > > + int abiv = atomic_read(&cdata->watch_abi_version);
    > > > +
    > > > + if (abiv == 0) {
    > >
    > > > + atomic_cmpxchg(&cdata->watch_abi_version, 0, version);
    > > > + abiv = atomic_read(&cdata->watch_abi_version);
    > >
    > > atomic_cmpxchng() returns a value.
    >
    > Yep, it returns the old value - which we don't care about - see below.

    Then what's the point to read back?..

    > > Also there are no barriers here...
    > >
    >
    > No barriers required - the atomic_cmpxchg() is sufficient.
    >
    > > > + }
    > > > + if (abiv != version)
    > > > + return -EPERM;
    > >
    > > I'm not sure I understand why this is atomic.
    > >
    >
    > The algorithm requires some level of protection and atomic is
    > sufficient.
    >
    > > Also this seems to be racy if cdata changed in background.
    > >
    >
    > Can you provide a case?

    CPU0: CPU1:
    xchg() ...
    ... xchg()
    ... read() -> OK
    read() ->NOK

    > The atomic_cmpxchg() ensures cdata->watch_abi_version is only set
    > once - first in wins. The atomic_read() is so we can check that
    > the set version matches what the caller wants.
    > Note that multiple callers may request the same version - and all
    > should succeed.

    So, that's basically what you need when using _old_ value.

    0 means you were first, right?
    Anything else you simply compare and bail out if it's not the same as
    what has been asked.

    >
    > > Shouldn't be rather
    > >
    > > if (atomic_cmpxchg() == 0) {
    > > if (atomic_read() != version)
    > > return ...;
    > > }
    > >
    >
    > My algorithm allows for multiple callers requesting the same version
    > to all succeed. Yours would fail the first conditional for all but
    > the first, and you haven't provided an else for that case...
    >
    > ... but it would probably look the same so the conditional is pointless,
    > or it would reject the request - which would be wrong.
    >
    > > But here is still the question: why do you expect the version to be
    > > changed on background? And what about barriers?
    > >
    >
    > While it is unlikely that userspace will be attempting to use both ABI
    > versions simultaneously on the same chip request, it is a possiblity and
    > so needs to be protected against. And better to have the watch request
    > fail than the read fail or worse - return the wrong struct version.
    >
    > The atomic_cmpxchg() is sufficient for this algorithm - no barriers
    > required. It could also be written with a spinlock but I was trying to
    > avoid locks unless they were absolutely necessary. A spinlock version
    > may arguably be more readable, but it would certainly be more verbose,
    > larger and slower.
    >
    > I'm happy to add some documentation to the function if that would help.

    Yes, I guess documentation is what is eagerly needed here.

    > > > + return 0;
    > > > +}
    > > > +#endif
    > > > +
    > > > +static int lineinfo_get(struct gpio_chardev_data *cdev, void __user *ip,
    > > > + bool watch)
    > > > +{
    > > > + struct gpio_desc *desc;
    > > > + struct gpio_v2_line_info lineinfo;
    > > > +
    > > > + if (copy_from_user(&lineinfo, ip, sizeof(lineinfo)))
    > > > + return -EFAULT;
    > > > +
    > > > + if (memchr_inv(lineinfo.padding, 0, sizeof(lineinfo.padding)))
    > > > + return -EINVAL;
    > > > +
    > > > + desc = gpiochip_get_desc(cdev->gdev->chip, lineinfo.offset);
    > > > + if (IS_ERR(desc))
    > > > + return PTR_ERR(desc);
    > > > +
    > > > + if (watch) {
    > > > +#ifdef CONFIG_GPIO_CDEV_V1
    > >
    > > > + if (lineinfo_ensure_abi_version(cdev, 2))
    > > > + return -EPERM;
    > >
    > > Can't you propagate error code from the function?
    > >
    >
    > You mean:
    > + ret = lineinfo_ensure_abi_version(cdev, 2)
    > + if (ret)
    > + return ret;
    >
    > That seems more verbose and less clear. And I'd need to conditionally
    > declare a ret - as this test is compiled out if CDEV_V1 is not defined.
    >
    > I did flip-flop on what lineinfo_ensure_abi_version() should return -
    > either a bool or an error code.
    >
    > If a bool then the code would include the dreaded negative conditional
    > ;-(:
    >
    > + if (!lineinfo_is_abi_version(cdev, 2))
    > + return -EPERM;
    >
    > so I eventually settled for the error code. But I'm on the fence on
    > this one and happy to change it if you think the bool form is clearer.
    >
    > Cheers,
    > Kent.



    --
    With Best Regards,
    Andy Shevchenko

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