From: Sanjay K Kumar sanjay.k.kumar@intel.com
[ Upstream commit 3cf74230c139f208b7fb313ae0054386eee31a81 ]
If qi_submit_sync() is invoked with 0 invalidation descriptors (for instance, for DMA draining purposes), we can run into a bug where a submitting thread fails to detect the completion of invalidation_wait. Subsequently, this led to a soft lockup. Currently, there is no impact by this bug on the existing users because no callers are submitting invalidations with 0 descriptors. This fix will enable future users (such as DMA drain) calling qi_submit_sync() with 0 count.
Suppose thread T1 invokes qi_submit_sync() with non-zero descriptors, while concurrently, thread T2 calls qi_submit_sync() with zero descriptors. Both threads then enter a while loop, waiting for their respective descriptors to complete. T1 detects its completion (i.e., T1's invalidation_wait status changes to QI_DONE by HW) and proceeds to call reclaim_free_desc() to reclaim all descriptors, potentially including adjacent ones of other threads that are also marked as QI_DONE.
During this time, while T2 is waiting to acquire the qi->q_lock, the IOMMU hardware may complete the invalidation for T2, setting its status to QI_DONE. However, if T1's execution of reclaim_free_desc() frees T2's invalidation_wait descriptor and changes its status to QI_FREE, T2 will not observe the QI_DONE status for its invalidation_wait and will indefinitely remain stuck.
This soft lockup does not occur when only non-zero descriptors are submitted.In such cases, invalidation descriptors are interspersed among wait descriptors with the status QI_IN_USE, acting as barriers. These barriers prevent the reclaim code from mistakenly freeing descriptors belonging to other submitters.
Considered the following example timeline: T1 T2 ======================================== ID1 WD1 while(WD1!=QI_DONE) unlock lock WD1=QI_DONE* WD2 while(WD2!=QI_DONE) unlock lock WD1==QI_DONE? ID1=QI_DONE WD2=DONE* reclaim() ID1=FREE WD1=FREE WD2=FREE unlock soft lockup! T2 never sees QI_DONE in WD2
Where: ID = invalidation descriptor WD = wait descriptor * Written by hardware
The root of the problem is that the descriptor status QI_DONE flag is used for two conflicting purposes: 1. signal a descriptor is ready for reclaim (to be freed) 2. signal by the hardware that a wait descriptor is complete
The solution (in this patch) is state separation by using QI_FREE flag for #1.
Once a thread's invalidation descriptors are complete, their status would be set to QI_FREE. The reclaim_free_desc() function would then only free descriptors marked as QI_FREE instead of those marked as QI_DONE. This change ensures that T2 (from the previous example) will correctly observe the completion of its invalidation_wait (marked as QI_DONE).
Signed-off-by: Sanjay K Kumar sanjay.k.kumar@intel.com Signed-off-by: Jacob Pan jacob.jun.pan@linux.intel.com Reviewed-by: Kevin Tian kevin.tian@intel.com Link: https://lore.kernel.org/r/20240728210059.1964602-1-jacob.jun.pan@linux.intel... Signed-off-by: Lu Baolu baolu.lu@linux.intel.com Signed-off-by: Joerg Roedel jroedel@suse.de Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/iommu/intel/dmar.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c index 84f0459e503cf..7a38e18b18196 100644 --- a/drivers/iommu/intel/dmar.c +++ b/drivers/iommu/intel/dmar.c @@ -1202,9 +1202,7 @@ static void free_iommu(struct intel_iommu *iommu) */ static inline void reclaim_free_desc(struct q_inval *qi) { - while (qi->desc_status[qi->free_tail] == QI_DONE || - qi->desc_status[qi->free_tail] == QI_ABORT) { - qi->desc_status[qi->free_tail] = QI_FREE; + while (qi->desc_status[qi->free_tail] == QI_FREE && qi->free_tail != qi->free_head) { qi->free_tail = (qi->free_tail + 1) % QI_LENGTH; qi->free_cnt++; } @@ -1439,8 +1437,16 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc, raw_spin_lock(&qi->q_lock); }
- for (i = 0; i < count; i++) - qi->desc_status[(index + i) % QI_LENGTH] = QI_DONE; + /* + * The reclaim code can free descriptors from multiple submissions + * starting from the tail of the queue. When count == 0, the + * status of the standalone wait descriptor at the tail of the queue + * must be set to QI_FREE to allow the reclaim code to proceed. + * It is also possible that descriptors from one of the previous + * submissions has to be reclaimed by a subsequent submission. + */ + for (i = 0; i <= count; i++) + qi->desc_status[(index + i) % QI_LENGTH] = QI_FREE;
reclaim_free_desc(qi); raw_spin_unlock_irqrestore(&qi->q_lock, flags);