[PATCH 2/2] drm/amdkfd: Downgrade mmap lock in SVM restore work

[Philip Yang <Philip.Yang@amd.com>]

svm_range_restore_work holds the mmap write lock across the entire loop
validating and mapping every invalid range, which blocks all mmap,
munmap and page fault activity of the process, not only the KFD paths
waiting for the same locks.

Only the deferred work flush handshake in
svm_range_list_lock_and_flush_work needs the write lock.
svm_range_validate_and_map runs under the read lock, as
svm_range_set_attr already does after its mmap_write_downgrade. The
flush stays valid because svm_range_deferred_list_work takes the write
lock, so it cannot run while the read lock is held.

Downgrade the mmap lock before walking the range list.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
---
 drivers/gpu/drm/amd/amdkfd/kfd_svm.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
index 28b687133cf8..f63bd8cf4174 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
@@ -1987,6 +1987,10 @@ static void svm_range_restore_work(struct work_struct *work)
 
 	mutex_lock(&process_info->lock);
 	svm_range_list_lock_and_flush_work(svms, mm);
+	/* Validate and map needs the read lock only, and deferred list work
+	 * takes the write lock, so the flush above stays valid.
+	 */
+	mmap_write_downgrade(mm);
 	mutex_lock(&svms->lock);
 
 	evicted_ranges = atomic_read(&svms->evicted_ranges);
@@ -2045,7 +2049,7 @@ static void svm_range_restore_work(struct work_struct *work)
 					       AMDGPU_SVM_RANGE_RESTORE_DELAY_MAX_MS);
 
 	mutex_unlock(&svms->lock);
-	mmap_write_unlock(mm);
+	mmap_read_unlock(mm);
 	mutex_unlock(&process_info->lock);
 
 	/* If validation failed, reschedule another attempt */
-- 
2.50.1