irq_work: Fix IRQ_WORK_BUSY bit clearing

While attempting to clear the busy bit at the end of a work execution,
atomic_cmpxchg() expects the value of the flags with the pending bit
cleared as the old value. However by mistake the value of the flags is
passed without clearing the pending bit first.

As a result, clearing the busy bit fails and irq_work_sync() may stall:

 watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [blktrace:4948]
 CPU: 0 PID: 4948 Comm: blktrace Not tainted 5.4.0-rc7-00003-gfeb4a51323bab #1
 RIP: 0010:irq_work_sync+0x4/0x10
 Call Trace:
  relay_close_buf+0x19/0x50
  relay_close+0x64/0x100
  blk_trace_free+0x1f/0x50
  __blk_trace_remove+0x1e/0x30
  blk_trace_ioctl+0x11b/0x140
  blkdev_ioctl+0x6c1/0xa40
  block_ioctl+0x39/0x40
  do_vfs_ioctl+0xa5/0x700
  ksys_ioctl+0x70/0x80
  __x64_sys_ioctl+0x16/0x20
  do_syscall_64+0x5b/0x1d0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

So clear the appropriate bit before passing the old flags to cmpxchg().

Fixes: feb4a51323 ("irq_work: Slightly simplify IRQ_WORK_PENDING clearing")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Link: https://lkml.kernel.org/r/20191113171201.14032-1-frederic@kernel.org

kernel/irq_work.c

@@ -158,6 +158,7 @@ static void irq_work_run_list(struct llist_head *list)
 		 * Clear the BUSY bit and return to the free state if
 		 * no-one else claimed it meanwhile.
 		 */
+		flags &= ~IRQ_WORK_PENDING;
 		(void)atomic_cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
 	}
 }