[PATCH 1/1] mm: memcg/slab: Call flush_memcg_workqueue() only if memcg workqueue is valid

[Adrian Huang <adrianhuang0701@gmail.com>]

From: Adrian Huang <ahuang12@lenovo.com>

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
  RSP: 0000:ffffffff96203d80 EFLAGS: 00010246
  RAX: ffffffff96203dc8 RBX: 0000000000000000 RCX: 0000000000000000
  RDX: ffffffff96a63120 RSI: ffffffff95efcba2 RDI: ffffffff96203dc0
  RBP: ffffffff96203e08 R08: 0000000000000000 R09: ffffffff962a1828
  R10: 00000000f0000080 R11: dead000000000100 R12: ffff8d8a87c0a770
  R13: dead000000000100 R14: 0000000000000456 R15: ffffffff96203da0
  FS:  0000000000000000(0000) GS:ffff8d8dbd000000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffff8d91cfbff000 CR3: 000000078920a000 CR4: 00000000000406b0
  Call Trace:
   ? wait_for_completion+0x51/0x180
   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().

Cc: Shakeel Butt <shakeelb@google.com>
Cc: Joerg Roedel <jroedel@suse.de>
Reported-by: Xiaochun Lee <lixc17@lenovo.com> 
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
---
 mm/slab_common.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index f0ab6d4ceb4c..0d95ddea13b0 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -903,7 +903,8 @@ static void flush_memcg_workqueue(struct kmem_cache *s)
 	 * 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);
 
 	/*
 	 * If we're racing with children kmem_cache deactivation, it might
-- 
2.17.1