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md devices allocate a bio_set and use it for two distinct purposes. mddev->bio_set is used to clone bios as part of sending upper level requests down to lower level devices, and it is also use for synchronous IO such as superblock and bitmap updates, and for correcting read errors. This multiple usage can lead to deadlocks. It is likely that cloned bios might be queued for write and to be waiting for a metadata update before the write can be permitted. If the cloning exhausted mddev->bio_set, the metadata update may not be able to proceed. This scenario has been seen during heavy testing, with lots of IO and lots of memory pressure. Address this by adding a new bio_set specifically for synchronous IO. All synchronous IO goes directly to the underlying device and is not queued at the md level, so request using entries from the new mddev->sync_set will complete in a timely fashion. Requests that use mddev->bio_set will sometimes need to wait for synchronous IO, but will no longer risk deadlocking that iO. Also: small simplification in mddev_put(): there is no need to wait until the spinlock is released before calling bioset_free(). Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Shaohua Li <shli@fb.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.