From: Adrian Huang <ahuang12@xxxxxxxxxx> commit 2fe20210fc5f5e62644678b8f927c49f2c6f42a7 upstream. When booting with amd_iommu=off, the following WARNING message appears: AMD-Vi: AMD IOMMU disabled on kernel command-line ------------[ cut here ]------------ WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2772 flush_workqueue+0x42e/0x450 Modules linked in: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.5.0-rc3-amd-iommu #6 Hardware name: Lenovo ThinkSystem SR655-2S/7D2WRCZ000, BIOS D8E101L-1.00 12/05/2019 RIP: 0010:flush_workqueue+0x42e/0x450 Code: ff 0f 0b e9 7a fd ff ff 4d 89 ef e9 33 fe ff ff 0f 0b e9 7f fd ff ff 0f 0b e9 bc fd ff ff 0f 0b e9 a8 fd ff ff e8 52 2c fe ff <0f> 0b 31 d2 48 c7 c6 e0 88 c5 95 48 c7 c7 d8 ad f0 95 e8 19 f5 04 Call Trace: kmem_cache_destroy+0x69/0x260 iommu_go_to_state+0x40c/0x5ab amd_iommu_prepare+0x16/0x2a irq_remapping_prepare+0x36/0x5f enable_IR_x2apic+0x21/0x172 default_setup_apic_routing+0x12/0x6f apic_intr_mode_init+0x1a1/0x1f1 x86_late_time_init+0x17/0x1c start_kernel+0x480/0x53f secondary_startup_64+0xb6/0xc0 ---[ end trace 30894107c3749449 ]--- x2apic: IRQ remapping doesn't support X2APIC mode x2apic disabled The warning is caused by the calling of 'kmem_cache_destroy()' in free_iommu_resources(). Here is the call path: free_iommu_resources kmem_cache_destroy flush_memcg_workqueue flush_workqueue The root cause is that the IOMMU subsystem runs before the workqueue subsystem, which the variable 'wq_online' is still 'false'. This leads to the statement 'if (WARN_ON(!wq_online))' in flush_workqueue() is 'true'. Since the variable 'memcg_kmem_cache_wq' is not allocated during the time, it is unnecessary to call flush_memcg_workqueue(). This prevents the WARNING message triggered by flush_workqueue(). Link: http://lkml.kernel.org/r/20200103085503.1665-1-ahuang12@xxxxxxxxxx Fixes: 92ee383f6daab ("mm: fix race between kmem_cache destroy, create and deactivate") Signed-off-by: Adrian Huang <ahuang12@xxxxxxxxxx> Reported-by: Xiaochun Lee <lixc17@xxxxxxxxxx> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Joerg Roedel <jroedel@xxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/slab_common.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -850,7 +850,8 @@ static void flush_memcg_workqueue(struct * deactivates the memcg kmem_caches through workqueue. Make sure all * previous workitems on workqueue are processed. */ - flush_workqueue(memcg_kmem_cache_wq); + if (likely(memcg_kmem_cache_wq)) + flush_workqueue(memcg_kmem_cache_wq); } #else static inline int shutdown_memcg_caches(struct kmem_cache *s)