usb-storage: fix deadlock involving host lock and scsi_done
authorAlan Stern <stern@rowland.harvard.edu>
Wed, 26 Jul 2017 15:49:19 +0000 (11:49 -0400)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Sun, 30 Jul 2017 14:18:27 +0000 (07:18 -0700)
commit8b52291a0743fc4db4a7495c846a6f31ee84d282
tree2e05e2175a5d5d1597ccaf17f1f4344dc680e865
parent89f23d51defcb94a5026d4b5da13faf4e1150a6f
usb-storage: fix deadlock involving host lock and scsi_done

Christoph Hellwig says that since version 4.12, the kernel switched to
using blk-mq by default.  The old code used a softirq for handling
request completions, but blk-mq can handle completions in the caller's
context.  This may cause a problem for usb-storage, because it invokes
the ->scsi_done callback while holding the host lock, and the
completion routine sometimes tries to acquire the same lock (when
running the error handler, for example).

The consequence is that the existing code will sometimes deadlock upon
error completion of a SCSI command (with a lockdep warning).

This is easy enough to fix, since usb-storage doesn't really need to
hold the host lock while the callback runs.  It was simpler to write
it that way, but moving the call outside the locked region is pretty
easy and there's no downside.  That's what this patch does.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Arthur Marsh <arthur.marsh@internode.on.net>
CC: Christoph Hellwig <hch@lst.de>
CC: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/usb/storage/usb.c