[BUG] fpga: dfl: possible ABBA deadlock in dfl_fpga_cdev_assign_port() and fme_pr()

From: Jia-Ju Bai
Date: Tue Nov 23 2021 - 02:55:24 EST


Hello,

My static analysis tool reports a possible ABBA deadlock in the dfl driver in Linux 5.10:

dfl_fpga_cdev_assign_port()
  mutex_lock(&cdev->lock); --> Line 1067 (Lock A)
  mutex_lock(&pdata->lock); --> Line 1624 (Lock B)

fme_pr()
  mutex_lock(&pdata->lock); --> Line 126 (Lock B)
  fpga_region_program_fpga()
    fpga_bridges_enable()
      fpga_bridge_enable()
        fme_bridge_enable_set() --> function pointer via "bridge->br_ops->enable_set()"
          dfl_fpga_cdev_find_port()
            mutex_lock(&cdev->lock); --> Line 499 (LockA)

When dfl_fpga_cdev_assign_port() and fme_pr() are concurrently executed, the deadlock can occur.

I am not quite sure whether this possible deadlock is real and how to fix it if it is real.
Any feedback would be appreciated, thanks

Reported-by: TOTE Robot <oslab@xxxxxxxxxxxxxxx>


Best wishes,
Jia-Ju Bai