[Intel-gfx] [PATCH 3/6] drm/i915: Always call i915_globals_exit() from i915_exit()

[Jason Ekstrand]

On Tue, Jul 20, 2021 at 3:25 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
>
> On 19/07/2021 19:30, Jason Ekstrand wrote:
> > If the driver was not fully loaded, we may still have globals lying
> > around.  If we don't tear those down in i915_exit(), we'll leak a bunch
> > of memory slabs.  This can happen two ways: use_kms = false and if we've
> > run mock selftests.  In either case, we have an early exit from
> > i915_init which happens after i915_globals_init() and we need to clean
> > up those globals.  While we're here, add an explicit boolean instead of
> > using a random field from i915_pci_device to detect partial loads.
> >
> > The mock selftests case gets especially sticky.  The load isn't entirely
> > a no-op.  We actually do quite a bit inside those selftests including
> > allocating a bunch of mock objects and running tests on them.  Once all
> > those tests are complete, we exit early from i915_init().  Perviously,
> > i915_init() would return a non-zero error code on failure and a zero
> > error code on success.  In the success case, we would get to i915_exit()
> > and check i915_pci_driver.driver.owner to detect if i915_init exited early
> > and do nothing.  In the failure case, we would fail i915_init() but
> > there would be no opportunity to clean up globals.
> >
> > The most annoying part is that you don't actually notice the failure as
> > part of the self-tests since leaking a bit of memory, while bad, doesn't
> > result in anything observable from userspace.  Instead, the next time we
> > load the driver (usually for next IGT test), i915_globals_init() gets
> > invoked again, we go to allocate a bunch of new memory slabs, those
> > implicitly create debugfs entries, and debugfs warns that we're trying
> > to create directories and files that already exist.  Since this all
> > happens as part of the next driver load, it shows up in the dmesg-warn
> > of whatever IGT test ran after the mock selftests.
>
> Story checks out but I totally don't get why it wouldn't be noticed
> until now. Was it perhaps part of the selfetsts contract that a reboot
> is required after failure?

No.  They do unload the driver, though.  They just don't re-load it.

> > While the obvious thing to do here might be to call i915_globals_exit()
> > after selftests, that's not actually safe.  The dma-buf selftests call
> > i915_gem_prime_export which creates a file.  We call dma_buf_put() on
> > the resulting dmabuf which calls fput() on the file.  However, fput()
> > isn't immediate and gets flushed right before syscall returns.  This
> > means that all the fput()s from the selftests don't happen until right
> > before the module load syscall used to fire off the selftests returns
> > which is after i915_init().  If we call i915_globals_exit() in
> > i915_init() after selftests, we end up freeing slabs out from under
> > objects which won't get released until fput() is flushed at the end of
> > the module load.
>
> Nasty. Wasn't visible while globals memory leak was "in place". :I
>
> > The solution here is to let i915_init() return success early and detect
> > the early success in i915_exit() and only tear down globals and nothing
> > else.  This way the module loads successfully, regardless of the success
> > or failure of the tests.  Because we've not enumerated any PCI devices,
> > no device nodes are created and it's entirely useless from userspace.
> > The only thing the module does at that point is hold on to a bit of
> > memory until we unload it and i915_exit() is called.  Importantly, this
> > means that everything from our selftests has the ability to properly
> > flush out between i915_init() and i915_exit() because there are a couple
> > syscall boundaries in between.
>
> When you say "couple of syscall boundaries" you mean exactly two (module
> init/unload) or there is more to it? Like why "couple" is needed and not
> just that the module load syscall has exited? That part sounds
> potentially dodgy. What mechanism is used by the delayed flush?
>
> Have you checked how this change interacts with the test runner and CI?

By the end of the series, a bunch of tests are fixed.  In particular,
https://gitlab.freedesktop.org/drm/intel/-/issues/3746

> >
> > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > Fixes: 32eb6bcfdda9 ("drm/i915: Make request allocation caches global")
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > ---
> >   drivers/gpu/drm/i915/i915_pci.c | 32 +++++++++++++++++++++++++-------
> >   1 file changed, 25 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
> > index 4e627b57d31a2..24e4e54516936 100644
> > --- a/drivers/gpu/drm/i915/i915_pci.c
> > +++ b/drivers/gpu/drm/i915/i915_pci.c
> > @@ -1194,18 +1194,31 @@ static struct pci_driver i915_pci_driver = {
> >       .driver.pm = &i915_pm_ops,
> >   };
> >
> > +static bool i915_fully_loaded = false;
>
> No need to initialize.

Already dropped.

> > +
> >   static int __init i915_init(void)
> >   {
> >       bool use_kms = true;
> >       int err;
> >
> > +     i915_fully_loaded = false;
>
> Ditto.

So, this is something I'm unclear on.  I know that static memory gets
auto-initialized to zero but what happens if you unload and reload a
module?  Is it re-initialized to zero?  If it is then we can drop
this.

> > +
> >       err = i915_globals_init();
> >       if (err)
> >               return err;
> >
> > +     /* i915_mock_selftests() only returns zero if no mock subtests were
>
>
> /*
>   * Please use this multi line comment style in i915.
>   */

Done.

> > +      * run.  If we get any non-zero error code, we return early here.
> > +      * We always return success because selftests may have allocated
> > +      * objects from slabs which will get cleaned up by i915_exit().  We
> > +      * could attempt to clean up immediately and fail module load but,
> > +      * thanks to interactions with other parts of the kernel (struct
> > +      * file, in particular), it's safer to let the module fully load
> > +      * and then clean up on unload.
> > +      */
> >       err = i915_mock_selftests();
> >       if (err)
> > -             return err > 0 ? 0 : err;
> > +             return 0;
> >
> >       /*
> >        * Enable KMS by default, unless explicitly overriden by
> > @@ -1225,6 +1238,12 @@ static int __init i915_init(void)
> >               return 0;
> >       }
> >
> > +     /* After this point, i915_init() must either fully succeed or
> > +      * properly tear everything down and fail.  We don't have separate
> > +      * flags for each set-up bit.
> > +      */
> > +     i915_fully_loaded = true;
> > +
> >       i915_pmu_init();
> >
> >       err = pci_register_driver(&i915_pci_driver);
> > @@ -1240,12 +1259,11 @@ static int __init i915_init(void)
> >
> >   static void __exit i915_exit(void)
> >   {
> > -     if (!i915_pci_driver.driver.owner)
> > -             return;
> > -
> > -     i915_perf_sysctl_unregister();
> > -     pci_unregister_driver(&i915_pci_driver);
> > -     i915_pmu_exit();
> > +     if (i915_fully_loaded) {
> > +             i915_perf_sysctl_unregister();
> > +             pci_unregister_driver(&i915_pci_driver);
> > +             i915_pmu_exit();
> > +     }
> >       i915_globals_exit();
> >   }
> >
> >
>
> Regards,
>
> Tvrtko