linux-kernel - [PATCH v3] sg: O_EXCL and other lock handling

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Date:	Tue, 12 Nov 2013 19:58:28 -0500
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	SCSI development list <linux-scsi@...r.kernel.org>,
	James Bottomley <james.bottomley@...senpartnership.com>
CC:	vaughan <vaughan.cao@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Hannes Reinecke <hare@...e.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH v3] sg: O_EXCL and other lock handling

After feedback on version 2 and a new report of a failure
in the vicinity of sg_remove() [remove device] during
a shutdown on a large machine, the locking has been
revised again.

ChangeLog v3:
   - change Sg_device::exclude and detached (renamed to
     detaching) to atomic_t
   - introduce atomic_t Sg_device::open_cnt and use for
     open(O_EXCL) logic. Hence stop using list_empty(sfds)
     which decouples the open/release logic from
     sg_remove_device() and other post-release cleanup
     functions
   - use a mutex to stop races between sg_open() and
     sg_release() on the same device
   - reduce the use of driver wide sg_index_lock so now
     it only protects sg_index_idr (the device array)
   - expand cleanups requested by checkpatch.pl to the
     remaining code in the driver

ChangeLog v2:
   - favour non O_EXCL open()s over open(dev, O_EXCL)s
   - wake all open(dev)s if dev is removed (detached)
   - wake all read(dev_fd)s that are waiting for a response
     if dev is removed (detached)
   - other cleanups requested by checkpatch.pl

ChangeLog v1:
   - introduce a finer grain (per device) lock to protect
     access and changes to the file descriptor objects
   - introduce a semaphore for mutual exclusion of co-incident
     open and release calls to the same device
   - improve the O_EXCL handling of sg_open() when multiple
     callers are waiting for an O_EXCL condition to clear
   - change some seq_printf()s to seq_puts()s as requested
     by checkpatch.pl
   - update copyright notice, version number and date


The patch is against lk 3.12.0 (and should work on lk 3.10
and lk 3.11 as the sg driver hasn't changed).

Testing is ongoing (see the v2 post) with focus on host
removal and shutdown. The driver survives bombarding 4 LUs
with queued requests spread across 6000 scsi_debug LUs.
Some log noise is generated, but it is not from the sg
driver:
   scsi 9:0:33:3: rejecting I/O to offline device
   scsi 9:0:33:3: [sg1000] killing request
<multiple times>

This is not seen when there are only 600 LUs.


Signed-off-by: Douglas Gilbert <dgilbert@...erlog.com>

View attachment "sg.c_race_excl_dg2.patch" of type "text/x-patch" (38759 bytes)