On Wed, May 24, 2017 at 11:24:21AM +1000, Neil Brown wrote:
> On Mon, May 22 2017, Nix wrote:
>
> > On 22 May 2017, NeilBrown told this:
> >
> >> Probably something like this:
> >>
> >> diff --git a/drivers/md/md.c b/drivers/md/md.c
> >> index f6ae1d67bcd0..dbca31be22a1 100644
> >> --- a/drivers/md/md.c
> >> +++ b/drivers/md/md.c
> >> @@ -8364,8 +8364,6 @@ static void md_start_sync(struct work_struct *ws)
> >> */
> >> void md_check_recovery(struct mddev *mddev)
> >> {
> >> - if (mddev->suspended)
> >> - return;
> >>
> >> if (mddev->bitmap)
> >> bitmap_daemon_work(mddev);
> >> @@ -8484,6 +8482,7 @@ void md_check_recovery(struct mddev *mddev)
> >> clear_bit(MD_RECOVERY_DONE, &mddev->recovery);
> >>
> >> if (!test_and_clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery) ||
> >> + mddev->suspended ||
> >> test_bit(MD_RECOVERY_FROZEN, &mddev->recovery))
> >> goto not_running;
> >> /* no recovery is running.
> >>
> >> though it's late so don't trust anything I write.
> >>
> >> If you try again it will almost certainly succeed. I suspect this is a
> >> hard race to hit - well done!!!
> >
> > Definitely not a hard race to hit :( I just hit it again with this
> > patch.
> >
> > Absolutely identical hang:
> >
> > [ 495.833520] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [ 495.840618] mdadm D 0 2700 2537 0x00000000
> > [ 495.847762] Call Trace:
> > [ 495.854825] __schedule+0x290/0x810
> > [ 495.861905] schedule+0x36/0x80
> > [ 495.868934] mddev_suspend+0xb3/0xe0
> > [ 495.875926] ? wake_atomic_t_function+0x60/0x60
> > [ 495.882976] level_store+0x1a7/0x6c0
> > [ 495.889953] ? md_ioctl+0xb7/0x1c10
> > [ 495.896901] ? putname+0x53/0x60
> > [ 495.903807] md_attr_store+0x83/0xc0
> > [ 495.910684] sysfs_kf_write+0x37/0x40
> > [ 495.917547] kernfs_fop_write+0x110/0x1a0
> > [ 495.924429] __vfs_write+0x28/0x120
> > [ 495.931270] ? kernfs_iop_get_link+0x172/0x1e0
> > [ 495.938126] ? __alloc_fd+0x3f/0x170
> > [ 495.944906] vfs_write+0xb6/0x1d0
> > [ 495.951646] SyS_write+0x46/0xb0
> > [ 495.958338] entry_SYSCALL_64_fastpath+0x13/0x94
> >
> > Everything else hangs the same way, too. This was surprising enough that
> > I double-checked to be sure the patch was applied: it was. I suspect the
> > deadlock is somewhat different than you supposed... (and quite possibly
> > not a race at all, or I wouldn't be hitting it so consistently, every
> > time. I mean, I only need to miss it *once* and I'll have reshaped... :) )
> >
> > It seems I can reproduce this on demand, so if you want to throw a patch
> > with piles of extra printks my way, feel free.
>
> Did you have md_write_start being called by syslog-ng again?
> I wonder what syslog is logging - presumably something about the reshape
> starting.
> If you kill syslog-ng, can you start the reshape?
>
> Alternately, this might do it.
> I think the root problem is that it isn't safe to call mddev_suspend()
> while holding the reconfig_mutex.
> For complete safety I probably need to move the request_module() call
> earlier, as that could block if a device was suspended (no memory
> allocation allowed while device is suspended).
>
> Thanks,
> NeilBrown
>
>
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 10367ffe92e3..a7b9c0576479 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -324,8 +324,12 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
> void mddev_suspend(struct mddev *mddev)
> {
> WARN_ON_ONCE(mddev->thread && current == mddev->thread->tsk);
> +
> if (mddev->suspended++)
> return;
> +#ifdef CONFIG_LOCKDEP
> + WARN_ON_ONCE(debug_locks && lockdep_is_held(&mddev->reconfig_mutex));
> +#endif
> synchronize_rcu();
> wait_event(mddev->sb_wait, atomic_read(&mddev->active_io) == 0);
> mddev->pers->quiesce(mddev, 1);
> @@ -3594,9 +3598,12 @@ level_store(struct mddev *mddev, const char *buf, size_t len)
> if (slen == 0 || slen >= sizeof(clevel))
> return -EINVAL;
>
> + mddev_suspend(mddev);
> rv = mddev_lock(mddev);
> - if (rv)
> + if (rv) {
> + mddev_resume(mddev);
> return rv;
> + }
>
> if (mddev->pers == NULL) {
> strncpy(mddev->clevel, buf, slen);
> @@ -3687,7 +3694,6 @@ level_store(struct mddev *mddev, const char *buf, size_t len)
> }
>
> /* Looks like we have a winner */
> - mddev_suspend(mddev);
> mddev_detach(mddev);
>
> spin_lock(&mddev->lock);
> @@ -3771,13 +3777,13 @@ level_store(struct mddev *mddev, const char *buf, size_t len)
> blk_set_stacking_limits(&mddev->queue->limits);
> pers->run(mddev);
> set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
> - mddev_resume(mddev);
> if (!mddev->thread)
> md_update_sb(mddev, 1);
> sysfs_notify(&mddev->kobj, NULL, "level");
> md_new_event(mddev);
> rv = len;
> out_unlock:
> + mddev_resume(mddev);
> mddev_unlock(mddev);
> return rv;
> }
> @@ -4490,6 +4496,7 @@ action_store(struct mddev *mddev, const char *page, size_t len)
> int err;
> if (mddev->pers->start_reshape == NULL)
> return -EINVAL;
> + mddev_suspend(mddev);
> err = mddev_lock(mddev);
> if (!err) {
> if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))
> @@ -4500,6 +4507,7 @@ action_store(struct mddev *mddev, const char *page, size_t len)
> }
> mddev_unlock(mddev);
> }
> + mddev_resume(mddev);
> if (err)
> return err;
> sysfs_notify(&mddev->kobj, NULL, "degraded");
The analysis makes a lot of sense, thanks! The patch looks not solving the
problem though, because check_recovery will not write super if suspended isn't
0.
I'm thinking of alternative wayt to fix this issue. A request which stalls in
md_write_start reallys isn't an active IO. We could add another counter in
md_write_start to record stalled request there. Then in mddev_suspend, wait
event will wait for atomic_read(&mddev->active_io) == the_new_counter.
Thanks,
Shaohua
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