misc: bcm_vk: Fix potential deadlock on &vk->ctx_lock

misc: bcm_vk: Fix potential deadlock on &vk->ctx_lock

As &vk->ctx_lock is acquired by timer bcm_vk_hb_poll() under softirq
context, other process context code should disable irq or bottom-half
before acquire the same lock, otherwise deadlock could happen if the
timer preempt the execution while the lock is held in process context
on the same CPU.

Possible deadlock scenario
bcm_vk_open()
    -> bcm_vk_get_ctx()
    -> spin_lock(&vk->ctx_lock)
	<timer iterrupt>
	-> bcm_vk_hb_poll()
	-> bcm_vk_blk_drv_access()
	-> spin_lock_irqsave(&vk->ctx_lock, flags) (deadlock here)

This flaw was found using an experimental static analysis tool we are
developing for irq-related deadlock, which reported the following
warning when analyzing the linux kernel 6.4-rc7 release.

[Deadlock]: &vk->ctx_lock
  [Interrupt]: bcm_vk_hb_poll
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
  [Locking Unit]: bcm_vk_ioctl
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:1181
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512

[Deadlock]: &vk->ctx_lock
  [Interrupt]: bcm_vk_hb_poll
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
  [Locking Unit]: bcm_vk_ioctl
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:1169

[Deadlock]: &vk->ctx_lock
  [Interrupt]: bcm_vk_hb_poll
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
  [Locking Unit]: bcm_vk_open
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:216

[Deadlock]: &vk->ctx_lock
  [Interrupt]: bcm_vk_hb_poll
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
  [Locking Unit]: bcm_vk_release
    -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:306

As suggested by Arnd, the tentative patch fix the potential deadlocks
by replacing the timer with delay workqueue. x86_64 allyesconfig using
GCC shows no new warning. Note that no runtime testing was performed
due to no device on hand.

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Tested-by: Desmond Yan <desmond.branden@broadcom.com>
Tested-by: Desmond Yan <desmond.yan@broadcom.com>
Link: https://lore.kernel.org/r/20230629182941.13045-1-dg573847474@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>