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dcssblk_remove_store() holds the dcssblk_devices_sem semaphore while
calling del_gendisk(dev_info->gd), which in turn tries to acquire
disk->open_mutex. Then there is dcssblk_release(), which is called
with disk->open_mutex held, and tries to acquire dcssblk_devices_sem.
Lockdep reports this as possible circular locking dependency (CPU0 =
dcssblk_remove_store, CPU1 = dcssblk_release):
[ 44.948865] Possible unsafe locking scenario:
[ 44.948866] CPU0 CPU1
[ 44.948867] ---- ----
[ 44.948868] lock(&dcssblk_devices_sem);
[ 44.948870] lock(&disk->open_mutex);
[ 44.948872] lock(&dcssblk_devices_sem);
[ 44.948874] lock(&disk->open_mutex);
[ 44.948876]
*** DEADLOCK ***
In practice, this deadlock should not happen, since dcssblk_remove_store()
checks for dev_info->use_count != 0 after acquiring dcssblk_devices_sem,
and breaks out before calling del_gendisk(). dev_info->use_count will be
decremented in dcssblk_release(), protected by dcssblk_devices_sem.
Still there is no need for dcssblk_remove_store() to hold the
dcssblk_devices_sem until after calling del_gendisk(), as this only
protects dcssblk internal data. So fix the lockdep warning by releasing
dcssblk_devices_sem earlier. Also move the segment_unload() loop up,
similar to dcssblk_shared_store() error path, no need to do that after
calling del_gendisk().
Also change dcssblk_shared_store() error path, where dcssblk_devices_sem
was also released only after calling del_gendisk(), and a similar lockdep
warning could be triggered (but also deadlock prevented by check for
dev_info->use_count).
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.