workqueue: Fix pwq->nr_in_flight corruption in try_to_grab_pending()

dd6c3c544126 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work
item handling") relocated pwq_dec_nr_in_flight() after
set_work_pool_and_keep_pending(). However, the latter destroys information
contained in work->data that's needed by pwq_dec_nr_in_flight() including
the flush color. With flush color destroyed, flush_workqueue() can stall
easily when mixed with cancel_work*() usages.

This is easily triggered by running xfstests generic/001 test on xfs:

     INFO: task umount:6305 blocked for more than 122 seconds.
     ...
     task:umount          state:D stack:13008 pid:6305  tgid:6305  ppid:6301   flags:0x00004000
     Call Trace:
      <TASK>
      __schedule+0x2f6/0xa20
      schedule+0x36/0xb0
      schedule_timeout+0x20b/0x280
      wait_for_completion+0x8a/0x140
      __flush_workqueue+0x11a/0x3b0
      xfs_inodegc_flush+0x24/0xf0
      xfs_unmountfs+0x14/0x180
      xfs_fs_put_super+0x3d/0x90
      generic_shutdown_super+0x7c/0x160
      kill_block_super+0x1b/0x40
      xfs_kill_sb+0x12/0x30
      deactivate_locked_super+0x35/0x90
      deactivate_super+0x42/0x50
      cleanup_mnt+0x109/0x170
      __cleanup_mnt+0x12/0x20
      task_work_run+0x60/0x90
      syscall_exit_to_user_mode+0x146/0x150
      do_syscall_64+0x5d/0x110
      entry_SYSCALL_64_after_hwframe+0x6c/0x74

Fix it by stashing work_data before calling set_work_pool_and_keep_pending()
and using the stashed value for pwq_dec_nr_in_flight().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Chandan Babu R <chandanbabu@kernel.org>
Link: http://lkml.kernel.org/r/87o7cxeehy.fsf@debian-BULLSEYE-live-builder-AMD64
Fixes: dd6c3c544126 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work item handling")

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ffb625db97717c2a867d8e363035057588a26e25..55c9816506b06be3245faceacaf2bf804b002e74 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1999,6 +1999,8 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
         */
        pwq = get_work_pwq(work);
        if (pwq && pwq->pool == pool) {
+               unsigned long work_data;
+
                debug_work_deactivate(work);
 
                /*
@@ -2016,11 +2018,15 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
 
                list_del_init(&work->entry);
 
-               /* work->data points to pwq iff queued, point to pool */
+               /*
+                * work->data points to pwq iff queued. Let's point to pool. As
+                * this destroys work->data needed by the next step, stash it.
+                */
+               work_data = *work_data_bits(work);
                set_work_pool_and_keep_pending(work, pool->id);
 
                /* must be the last step, see the function comment */
-               pwq_dec_nr_in_flight(pwq, *work_data_bits(work));
+               pwq_dec_nr_in_flight(pwq, work_data);
 
                raw_spin_unlock(&pool->lock);
                rcu_read_unlock();