Re: [PATCH v3 3/3] drm/xe: Route deferred xe_vma/xe_vm teardown off system_dfl_wq

[Matthew Brost <matthew.brost@intel.com>]

On Fri, Sep 25, 2026 at 12:56:43PM -0700, Matthew Brost wrote:
> On Fri, Sep 25, 2026 at 03:33:35PM +0200, Thomas Hellström wrote:
> > xe_vma_destroy() can defer the final teardown of a struct xe_vma to a
> > dma_fence completion callback (vma_destroy_cb()), and xe_vm_free() (the
> > drm_gpuvm_ops.vm_free callback) always defers struct xe_vm teardown to
> > a work item, since destroying a VM needs to sleep. Both used to queue
> > their work on system_dfl_wq, a global, kernel-wide workqueue that xe
> > has no control over and never waits on during module unload.
> > 
> > drm_gpuvm_free() drops its drm_device reference immediately after
> > calling xe_vm_free(), without waiting for the deferred work to run.
> > The same applies one level down: whichever xe_vma or xe_vm reference
> > happens to be the last one can trigger this chain from a dma_fence
> > callback that may fire at an arbitrary time, including after the
> > owning file has already been closed and its own module reference
> > dropped. Since nothing tracks or waits for work queued on
> > system_dfl_wq, `rmmod xe` could succeed and free the module's text
> > while vma_destroy_work_func() or vm_destroy_work_func() is still
> > queued or running on it, jumping into freed code.
> > 
> > Fix this by queueing this work on xe_destroy_wq instead, the existing
> > module-lifetime workqueue already used for GuC exec queue teardown.
> > Unlike a per-device workqueue, this requires no dereference of a
> > struct xe_device that may already be gone by the time a deferred
> > callback fires, and unlike system_dfl_wq it is guaranteed to be
> > drained by xe_destroy_wq_module_exit() before the module is unloaded,
> > following the drm_pagemap_dev_hold()/unhold_work precedent of using a
> > workqueue that is waited on at module unload rather than a bare module
> > reference. The previous commit's reordering of xe_destroy_wq_exit()
> > to run after xe_device_exit() guarantees that xe_destroy_wq is only
> > torn down once the device-count has reached zero, i.e. after any
> > xe_vma or xe_vm whose teardown queues work here has already dropped
> > its drm_device reference and thus already queued that work.
> > 
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Assisted-by: LLM
> > Reviewed-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  drivers/gpu/drm/xe/xe_module.c | 6 ++++--
> >  drivers/gpu/drm/xe/xe_vm.c     | 5 +++--
> >  2 files changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_module.c b/drivers/gpu/drm/xe/xe_module.c
> > index c61bd33546f2..897724cb5cfb 100644
> > --- a/drivers/gpu/drm/xe/xe_module.c
> > +++ b/drivers/gpu/drm/xe/xe_module.c
> > @@ -114,8 +114,10 @@ static void xe_destroy_wq_module_exit(void)
> >   * xe_destroy_wq_queue() - Queue work on the destroy workqueue
> >   * @work: work item to queue
> >   *
> > - * The destroy workqueue has module lifetime and is used for GuC exec queue
> > - * teardown that can outlive a single xe_device. SVM pagemap destroy uses the
> > + * The destroy workqueue has module lifetime, and is guaranteed to outlive
> > + * any xe_device, and to be drained before the module is unloaded. It is used
> > + * for GuC exec queue and xe_vm/xe_vma teardown that can be deferred past the
> > + * lifetime of the xe_device that triggered it. SVM pagemap destroy uses the
> >   * per-device xe->destroy_wq instead.
> >   *
> >   * Return: %true if @work was queued, %false if it was already pending.
> > diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> > index 390da884c727..ee369e6c3b28 100644
> > --- a/drivers/gpu/drm/xe/xe_vm.c
> > +++ b/drivers/gpu/drm/xe/xe_vm.c
> > @@ -29,6 +29,7 @@
> >  #include "xe_exec_queue.h"
> >  #include "xe_gt.h"
> >  #include "xe_migrate.h"
> > +#include "xe_module.h"
> >  #include "xe_pagefault.h"
> >  #include "xe_pat.h"
> >  #include "xe_pm.h"
> > @@ -1249,7 +1250,7 @@ static void vma_destroy_cb(struct dma_fence *fence,
> >  	struct xe_vma *vma = container_of(cb, struct xe_vma, destroy_cb);
> >  
> >  	INIT_WORK(&vma->destroy_work, vma_destroy_work_func);
> > -	queue_work(system_dfl_wq, &vma->destroy_work);
> > +	xe_destroy_wq_queue(&vma->destroy_work);
> >  }
> >  
> >  static void xe_vm_assert_write_mode_or_garbage_collector(struct xe_vm *vm)
> > @@ -2059,7 +2060,7 @@ static void xe_vm_free(struct drm_gpuvm *gpuvm)
> >  	struct xe_vm *vm = container_of(gpuvm, struct xe_vm, gpuvm);
> >  
> >  	/* To destroy the VM we need to be able to sleep */
> > -	queue_work(system_dfl_wq, &vm->destroy_work);
> > +	xe_destroy_wq_queue(&vm->destroy_work);
> 
> Actually this is still unsafe, right?
> 
> destroy_work touches vm->xe which could be gone after gpuvm drops
> potentially the final drm_dev_put, right?
> 

Ignore this - we flush this queue before destorying any device.

Matt

> Matt 
> 
> >  }
> >  
> >  struct xe_vm *xe_vm_lookup(struct xe_file *xef, u32 id)
> > -- 
> > 2.55.0
> >