On Wed, 12 Jul 2017, Christoph Hellwig wrote:
> On Wed, Jul 12, 2017 at 12:10:02PM -0400, Alan Stern wrote:
> > This is pretty conclusive. The problem comes about because
> > usb_stor_control_thread() calls scsi_mq_done() while holding
> > shost->host_lock, and then scsi_eh_scmd_add() tries to acquire that
> > same lock.
> >
> > I don't know why this didn't show up in earlier kernels. I guess some
> > element of the call chain listed above must be new in 4.12.
> >
> > Christoph, what's the best way to fix this? Should usb-storage release
> > the host lock before issuing the ->scsi_done callback? If so, does
> > that change need to be applied to any kernels before 4.12?
>
> 4.12 switched to blk-mq by default, and while the old code used
> a softirq for completions, which is always a difference context
> the blk-mq code might execute in the same context it's called in.
>
> So yes, for that we'd need to drop host_lock. But I wonder how
> many more of these are lingering somewhere and if we can find
> another workaround.
All right. In the meantime, changing usb-storage won't hurt.
Arthur, can you test the patch below?
Alan Stern
Index: usb-4.x/drivers/usb/storage/usb.c
===================================================================
--- usb-4.x.orig/drivers/usb/storage/usb.c
+++ usb-4.x/drivers/usb/storage/usb.c
@@ -315,6 +315,7 @@ static int usb_stor_control_thread(void
{
struct us_data *us = (struct us_data *)__us;
struct Scsi_Host *host = us_to_host(us);
+ struct scsi_cmnd *srb;
for (;;) {
usb_stor_dbg(us, "*** thread sleeping\n");
@@ -330,6 +331,7 @@ static int usb_stor_control_thread(void
scsi_lock(host);
/* When we are called with no command pending, we're done */
+ srb = us->srb;
if (us->srb == NULL) {
scsi_unlock(host);
mutex_unlock(&us->dev_mutex);
@@ -398,14 +400,11 @@ static int usb_stor_control_thread(void
/* lock access to the state */
scsi_lock(host);
- /* indicate that the command is done */
- if (us->srb->result != DID_ABORT << 16) {
- usb_stor_dbg(us, "scsi cmd done, result=0x%x\n",
- us->srb->result);
- us->srb->scsi_done(us->srb);
- } else {
+ /* was the command aborted? */
+ if (us->srb->result == DID_ABORT << 16) {
SkipForAbort:
usb_stor_dbg(us, "scsi command aborted\n");
+ srb = NULL; /* Don't call srb->scsi_done() */
}
/*
@@ -429,6 +428,13 @@ SkipForAbort:
/* unlock the device pointers */
mutex_unlock(&us->dev_mutex);
+
+ /* now that the locks are released, notify the SCSI core */
+ if (srb) {
+ usb_stor_dbg(us, "scsi cmd done, result=0x%x\n",
+ srb->result);
+ srb->scsi_done(srb);
+ }
} /* for (;;) */
/* Wait until we are told to stop */
--
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