Re: [PATCH 8/8] scsi: ufs: Fix deadlock between power management and error handler

Adrian Hunter
2022-09-27T17:06:55Z

On 23/09/22 23:11, Bart Van Assche wrote:
> The following deadlock has been observed on multiple test setups:
> * ufshcd_wl_suspend() is waiting for blk_execute_rq() to complete while it
> holds host_sem.
> * ufshcd_eh_host_reset_handler() invokes ufshcd_err_handler() and the
> latter function tries to obtain host_sem.
> This is a deadlock because blk_execute_rq() can't execute SCSI commands
> while the host is in the SHOST_RECOVERY state and because the error
> handler cannot make progress either.
>
> Fix this deadlock by calling the UFS error handler directly during system
> suspend instead of relying on the error handling mechanism in the SCSI
> core.
>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
> drivers/ufs/core/ufshcd.c | 43 ++++++++++++++++++++++++++++++++++++---
> 1 file changed, 40 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index abeb120b12eb..996641fc1176 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -6405,9 +6405,19 @@ static void ufshcd_err_handler(struct work_struct *work)
> {
> struct ufs_hba *hba = container_of(work, struct ufs_hba, eh_work);
>
> - down(&hba->host_sem);
> - ufshcd_recover_link(hba);
> - up(&hba->host_sem);
> + /*
> + * Serialize suspend/resume and error handling because a deadlock
> + * occurs if the error handler runs concurrently with
> + * ufshcd_set_dev_pwr_mode().
> + */
> + if (mutex_trylock(&system_transition_mutex)) {

This is effectively disabling the UFS driver's error handler work.
It would be better to add the ability to do that explicitly within
the UFS driver and avoid using system_transition_mutex.

> + down(&hba->host_sem);
> + ufshcd_recover_link(hba);
> + up(&hba->host_sem);
> + mutex_unlock(&system_transition_mutex);
> + } else {
> + pr_info("%s: suspend or resume is ongoing\n", __func__);
> + }
> }
>
> /**
> @@ -8298,6 +8308,25 @@ static void ufshcd_async_scan(void *data, async_cookie_t cookie)
> }
> }
>
> +static enum scsi_timeout_action ufshcd_eh_timed_out(struct scsi_cmnd *scmd)
> +{
> + struct ufs_hba *hba = shost_priv(scmd->device->host);
> +
> + if (!hba->system_suspending) {

Is a PM notifier needed - couldn't hba->system_suspending
have been set in ufshcd_wl_suspend() ?

Doesn't resume have the same issue ?

> + /* Apply the SCSI core error handling strategy. */
> + return SCSI_EH_NOT_HANDLED;
> + }
> +
> + /*
> + * Fail START STOP UNIT commands without activating the SCSI error
> + * handler to prevent a deadlock between ufshcd_set_dev_pwr_mode() and
> + * ufshcd_err_handler().
> + */
> + set_host_byte(scmd, DID_TIME_OUT);
> + scsi_done(scmd);
> + return SCSI_EH_DONE;
> +}
> +
> static const struct attribute_group *ufshcd_driver_groups[] = {
> &ufs_sysfs_unit_descriptor_group,
> &ufs_sysfs_lun_attributes_group,
> @@ -8332,6 +8361,7 @@ static struct scsi_host_template ufshcd_driver_template = {
> .eh_abort_handler = ufshcd_abort,
> .eh_device_reset_handler = ufshcd_eh_device_reset_handler,
> .eh_host_reset_handler = ufshcd_eh_host_reset_handler,
> + .eh_timed_out = ufshcd_eh_timed_out,
> .this_id = -1,
> .sg_tablesize = SG_ALL,
> .cmd_per_lun = UFSHCD_CMD_PER_LUN,
> @@ -8791,6 +8821,13 @@ static int ufshcd_set_dev_pwr_mode(struct ufs_hba *hba,
> break;
> if (host_byte(ret) == 0 && scsi_status_is_good(ret))
> break;
> + /*
> + * Calling the error handler directly when suspending or
> + * resuming the system since the callers of this function hold
> + * hba->host_sem in that case.

Runtime PM doesn't hold host_sem

> + */
> + if (host_byte(ret) != 0 && hba->system_suspending)
> + ufshcd_recover_link(hba);
> }
> if (ret) {
> sdev_printk(KERN_WARNING, sdp,