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

[Daniel Vetter]

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 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;
> +
>  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?

Patch itself looks reasonable, with the nits from Tvrtko addressed:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

>  
>  	/*
>  	 * 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