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

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Jul 20, 2021 at 09:55:22AM -0500, Jason Ekstrand wrote:
> On Tue, Jul 20, 2021 at 9:18 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Mon, Jul 19, 2021 at 01:30:44PM -0500, 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.
> > >
> > > 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.
> > >
> > > 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.
> > >
> > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > > Fixes: 32eb6bcfdda9 ("drm/i915: Make request allocation caches global")
> > > Cc: Daniel Vetter <daniel@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;
> > > +
> > >  static int __init i915_init(void)
> > >  {
> > >       bool use_kms = true;
> > >       int err;
> > >
> > > +     i915_fully_loaded = false;
> > > +
> > >       err = i915_globals_init();
> > >       if (err)
> > >               return err;
> > >
> > > +     /* i915_mock_selftests() only returns zero if no mock subtests were
> > > +      * 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;
> >
> > At least the module options still claim that you can run selftests and
> > still load the driver. Which makes sense for perf/hw selftests, since
> > those need the driver, but would result in the same old bug resurfacing
> > that you're trying to fix there.
> >
> > Is that description just confused and needs some fixing, or do we have a
> > gap here?
> 
> I don't think there's real need for a fully loaded driver after mock
> selftests.  They exist entirely to run against a mock driver, not the
> real one.

Can you pls update the module option help then for the next round?
-Daniel

> 
> > Patch itself looks reasonable, with the nits from Tvrtko addressed:
> 
> Done
> 
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> Thanks
> 
> --Jason
> 
> > >
> > >       /*
> > >        * 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();
> > >  }
> > >
> > > --
> > > 2.31.1
> > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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