drain_workqueue() cannot be called safely in a spinlocked context due to
possible task rescheduling. In the multi-task scenario, calling
queue_work() while drain_workqueue() will lead to a Call Trace as
pushing a work on a draining workqueue is not permitted in spinlocked
context.
    Call Trace:
    <TASK>
    ? __warn+0x7d/0x140
    ? __queue_work+0x2b2/0x440
    ? report_bug+0x1f8/0x200
    ? handle_bug+0x3c/0x70
    ? exc_invalid_op+0x18/0x70
    ? asm_exc_invalid_op+0x1a/0x20
    ? __queue_work+0x2b2/0x440
    queue_work_on+0x28/0x30
    idxd_misc_thread+0x303/0x5a0 [idxd]
    ? __schedule+0x369/0xb40
    ? __pfx_irq_thread_fn+0x10/0x10
    ? irq_thread+0xbc/0x1b0
    irq_thread_fn+0x21/0x70
    irq_thread+0x102/0x1b0
    ? preempt_count_add+0x74/0xa0
    ? __pfx_irq_thread_dtor+0x10/0x10
    ? __pfx_irq_thread+0x10/0x10
    kthread+0x103/0x140
    ? __pfx_kthread+0x10/0x10
    ret_from_fork+0x31/0x50
    ? __pfx_kthread+0x10/0x10
    ret_from_fork_asm+0x1b/0x30
    </TASK>

The current implementation uses a spinlock to protect event log workqueue
and will lead to the Call Trace due to potential task rescheduling.

To address the locking issue, convert the spinlock to mutex, allowing
the drain_workqueue() to be called in a safe mutex-locked context.

This change ensures proper synchronization when accessing the event log
workqueue, preventing potential Call Trace and improving the overall
robustness of the code.

Fixes: c40bd7d9 ("dmaengine: idxd: process user page faults for completion record")
Signed-off-by: Rex Zhang <rex.zhang@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Lijun Pan <lijun.pan@intel.com>
Link: https://lore.kernel.org/r/20240404223949.2885604-1-fenghua.yu@intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>