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[ 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.com 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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.