Re: [PATCH v2] drm/amdkfd: change kfd process kref count at creation

[Felix Kuehling]

On 2024-10-11 10:41, Xiaogang.Chen wrote:
From: Xiaogang Chen <xiaogang.c...@amd.com>

kfd process kref count(process->ref) is initialized to 1 by kref_init. After
it is created not need to increaes its kref. Instad add kfd process kref at kfd
process mmu notifier allocation since we decrease the ref at free_notifier of
mmu_notifier_ops, so pair them.

When user process opens kfd node multiple times the kfd process kref is
increased each time to balance kfd node close operation.

Signed-off-by: Xiaogang Chen <xiaogang.c...@amd.com>

Thanks, this is an elegant solution, IMO. The reference returned by 
kfd_create_process comes either from find_process or create_process. And 
the extra reference that gets released by the free_notifier gets 
allocated by the alloc_notifier. I think there is a race condition, 
though. See inline.


---
  drivers/gpu/drm/amd/amdkfd/kfd_process.c | 15 ++++++++++-----
  1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c 
b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
index d07acf1b2f93..78bf918abf92 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
@@ -850,8 +850,10 @@ struct kfd_process *kfd_create_process(struct task_struct 
*thread)
                goto out;
        }
  
-	/* A prior open of /dev/kfd could have already created the process. */
-       process = find_process(thread, false);
+       /* A prior open of /dev/kfd could have already created the process.
+        * find_process will increase process kref in this case
+        */
+       process = find_process(thread, true);
        if (process) {
                pr_debug("Process already found\n");
        } else {
@@ -899,8 +901,6 @@ struct kfd_process *kfd_create_process(struct task_struct 
*thread)
                init_waitqueue_head(&process->wait_irq_drain);
        }
  out:
-       if (!IS_ERR(process))
-               kref_get(&process->ref);
        mutex_unlock(&kfd_processes_mutex);
        mmput(thread->mm);
  
@@ -1191,7 +1191,12 @@ static struct mmu_notifier *kfd_process_alloc_notifier(struct mm_struct *mm)
  
  	srcu_read_unlock(&kfd_processes_srcu, idx);
  
-	return p ? &p->mmu_notifier : ERR_PTR(-ESRCH);
+       if (p) {
+               kref_get(&p->ref);

This should be inside the srcu. I think you could use 
kfd_lookup_process_by_mm instead of open-coding the SRCU locking and 
find_process_by_mm. This does the lookup and reference counting safely 
already.

Regards,
  Felix

+               return &p->mmu_notifier;
+       }
+
+       return ERR_PTR(-ESRCH);
  }
  
  static void kfd_process_free_notifier(struct mmu_notifier *mn)