Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3 7/9] drm: update global mutex lock in the ioctl handler | From | Desmond Cheong Zhi Xi <> | Date | Thu, 19 Aug 2021 18:52:58 +0800 |
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On 18/8/21 7:02 pm, Daniel Vetter wrote: > On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote: >> In a future patch, a read lock on drm_device.master_rwsem is >> held in the ioctl handler before the check for ioctl >> permissions. However, this produces the following lockdep splat: >> >> ====================================================== >> WARNING: possible circular locking dependency detected >> 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U >> ------------------------------------------------------ >> kms_lease/1752 is trying to acquire lock: >> ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 >> >> but task is already holding lock: >> ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: >> drm_ioctl_kernel+0xfb/0x1a0 >> >> which lock already depends on the new lock. >> >> the existing dependency chain (in reverse order) is: >> >> -> #2 (&dev->master_rwsem){++++}-{3:3}: >> lock_acquire+0xd3/0x310 >> down_read+0x3b/0x140 >> drm_master_internal_acquire+0x1d/0x60 >> drm_client_modeset_commit+0x10/0x40 >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 >> drm_fb_helper_set_par+0x34/0x40 >> intel_fbdev_set_par+0x11/0x40 [i915] >> fbcon_init+0x270/0x4f0 >> visual_init+0xc6/0x130 >> do_bind_con_driver+0x1de/0x2c0 >> do_take_over_console+0x10e/0x180 >> do_fbcon_takeover+0x53/0xb0 >> register_framebuffer+0x22d/0x310 >> __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 >> intel_fbdev_initial_config+0xf/0x20 [i915] >> async_run_entry_fn+0x28/0x130 >> process_one_work+0x26d/0x5c0 >> worker_thread+0x37/0x390 >> kthread+0x13b/0x170 >> ret_from_fork+0x1f/0x30 >> >> -> #1 (&helper->lock){+.+.}-{3:3}: >> lock_acquire+0xd3/0x310 >> __mutex_lock+0xa8/0x930 >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 >> intel_fbdev_restore_mode+0x2b/0x50 [i915] >> drm_lastclose+0x27/0x50 >> drm_release_noglobal+0x42/0x60 >> __fput+0x9e/0x250 >> task_work_run+0x6b/0xb0 >> exit_to_user_mode_prepare+0x1c5/0x1d0 >> syscall_exit_to_user_mode+0x19/0x50 >> do_syscall_64+0x46/0xb0 >> entry_SYSCALL_64_after_hwframe+0x44/0xae >> >> -> #0 (drm_global_mutex){+.+.}-{3:3}: >> validate_chain+0xb39/0x1e70 >> __lock_acquire+0x5a1/0xb70 >> lock_acquire+0xd3/0x310 >> __mutex_lock+0xa8/0x930 >> drm_open+0x64/0x280 >> drm_stub_open+0x9f/0x100 >> chrdev_open+0x9f/0x1d0 >> do_dentry_open+0x14a/0x3a0 >> dentry_open+0x53/0x70 >> drm_mode_create_lease_ioctl+0x3cb/0x970 >> drm_ioctl_kernel+0xc9/0x1a0 >> drm_ioctl+0x201/0x3d0 >> __x64_sys_ioctl+0x6a/0xa0 >> do_syscall_64+0x37/0xb0 >> entry_SYSCALL_64_after_hwframe+0x44/0xae >> >> other info that might help us debug this: >> Chain exists of: >> drm_global_mutex --> &helper->lock --> &dev->master_rwsem >> Possible unsafe locking scenario: >> CPU0 CPU1 >> ---- ---- >> lock(&dev->master_rwsem); >> lock(&helper->lock); >> lock(&dev->master_rwsem); >> lock(drm_global_mutex); >> >> *** DEADLOCK *** >> >> The lock hierarchy inversion happens because we grab the >> drm_global_mutex while already holding on to master_rwsem. To avoid >> this, we do some prep work to grab the drm_global_mutex before >> checking for ioctl permissions. >> >> At the same time, we update the check for the global mutex to use the >> drm_dev_needs_global_mutex helper function. > > This is intentional, essentially we force all non-legacy drivers to have > unlocked ioctl (otherwise everyone forgets to set that flag). > > For non-legacy drivers the global lock only ensures ordering between > drm_open and lastclose (I think at least), and between > drm_dev_register/unregister and the backwards ->load/unload callbacks > (which are called in the wrong place, but we cannot fix that for legacy > drivers). > > ->load/unload should be completely unused (maybe radeon still uses it), > and ->lastclose is also on the decline. >
Ah ok got it, I'll change the check back to drm_core_check_feature(dev, DRIVER_LEGACY) then.
> Maybe we should update the comment of drm_global_mutex to explain what it > protects and why. >
The comments in drm_dev_needs_global_mutex make sense I think, I just didn't read the code closely enough.
> I'm also confused how this patch connects to the splat, since for i915 we
Right, my bad, this is a separate instance of circular locking. I was too hasty when I saw that for legacy drivers we might grab master_rwsem then drm_global_mutex in the ioctl handler.
> shouldn't be taking the drm_global_lock here at all. The problem seems to > be the drm_open_helper when we create a new lease, which is an entirely > different can of worms. > > I'm honestly not sure how to best do that, but we should be able to create > a file and then call drm_open_helper directly, or well a version of that > which never takes the drm_global_mutex. Because that is not needed for > nested drm_file opening: > - legacy drivers never go down this path because leases are only supported > with modesetting, and modesetting is only supported for non-legacy > drivers > - the races against dev->open_count due to last_close or ->load callbacks > don't matter, because for the entire ioctl we already have an open > drm_file and that wont disappear. > > So this should work, but I'm not entirely sure how to make it work. > -Daniel >
One idea that comes to mind is to change the outcome of drm_dev_needs_global_mutex while we're in the ioctl, but that requires more locking which sounds like a bad idea.
Another idea, which is quite messy, but just for thoughts, uses the idea of pushing the master_rwsem read lock down:
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 7f523e1c5650..5d05e744b728 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = { DRM_RENDER_ALLOW), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0), - DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER), + DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER), diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c index 983701198ffd..a25bc69522b4 100644 --- a/drivers/gpu/drm/drm_lease.c +++ b/drivers/gpu/drm/drm_lease.c @@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, return -EINVAL; } + /* Clone the lessor file to create a new file for us */ + DRM_DEBUG_LEASE("Allocating lease file\n"); + lessee_file = file_clone_open(lessor_file); + if (IS_ERR(lessee_file)) + return PTR_ERR(lessee_file); + + down_read(&dev->master_rwsem); + + if (!drm_is_current_master(lessor_priv)) { + ret = -EACCES; + goto out_file; + } + lessor = drm_file_get_master(lessor_priv); /* Do not allow sub-leases */ if (lessor->lessor) { @@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, goto out_leases; } - /* Clone the lessor file to create a new file for us */ - DRM_DEBUG_LEASE("Allocating lease file\n"); - lessee_file = file_clone_open(lessor_file); - if (IS_ERR(lessee_file)) { - ret = PTR_ERR(lessee_file); - goto out_lessee; - } - lessee_priv = lessee_file->private_data; /* Change the file to a master one */ drm_master_put(&lessee_priv->master); @@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, fd_install(fd, lessee_file); drm_master_put(&lessor); + up_read(&dev->master_rwsem); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); return 0; -out_lessee: - drm_master_put(&lessee); - out_leases: put_unused_fd(fd); out_lessor: drm_master_put(&lessor); + +out_file: + up_read(&dev->master_rwsem); + fput(lessee_file); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret); return ret; }
Something like this would also address the other deadlock we'd hit in drm_mode_create_lease_ioctl():
drm_ioctl_kernel(): down_read(&master_rwsem); <--- down_read() drm_mode_create_lease_ioctl(): drm_lease_create(): file_clone_open(): ... drm_open(): drm_open_helper(): drm_master_open(): down_write(&master_rwsem); <--- down_write()
Overall, I think the suggestion to push master_rwsem write locks down into ioctls would solve the nesting problem for those ioctls.
Although I'm still a little concerned that, just like here, there might be deeply embedded nested locking, so locking becomes prone to breaking. It does smell a bit to me.
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> >> --- >> drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- >> 1 file changed, 9 insertions(+), 9 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c >> index 880fc565d599..2cb57378a787 100644 >> --- a/drivers/gpu/drm/drm_ioctl.c >> +++ b/drivers/gpu/drm/drm_ioctl.c >> @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, >> if (drm_dev_is_unplugged(dev)) >> return -ENODEV; >> >> + /* Enforce sane locking for modern driver ioctls. */ >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) >> + mutex_lock(&drm_global_mutex); >> + >> retcode = drm_ioctl_permit(flags, file_priv); >> if (unlikely(retcode)) >> - return retcode; >> + goto out; >> >> - /* Enforce sane locking for modern driver ioctls. */ >> - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || >> - (flags & DRM_UNLOCKED)) >> - retcode = func(dev, kdata, file_priv); >> - else { >> - mutex_lock(&drm_global_mutex); >> - retcode = func(dev, kdata, file_priv); >> + retcode = func(dev, kdata, file_priv); >> + >> +out: >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) >> mutex_unlock(&drm_global_mutex); >> - } >> return retcode; >> } >> EXPORT_SYMBOL(drm_ioctl_kernel); >> -- >> 2.25.1 >> >
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