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zvol_state_lock is intended to protect access to the global name->zvol lists (zvol_find_by_name()), but has also been used to control access to OS-side private data, accessed through whatever kernel object is used to represent the volume (gendisk, geom, etc). This appears to have been necessary to some degree because the OS-side object is what's used to get a handle on zvol_state_t, so zv_state_lock and zv_suspend_lock can't be used to manage access, but also, with the private object and the zvol_state_t being shutdown and destroyed at the same time in zvol_os_free(), we must ensure that the private object pointer only ever corresponds to a real zvol_state_t, not one in partial destruction. Taking the global lock seems like a convenient way to ensure this. The problem with this is that zvol_state_lock does not actually protect access to the zvol_state_t internals, so we need to take zv_state_lock and/or zv_suspend_lock. If those are contended, this can then cause OS-side operations (eg zvol_open()) to sleep to wait for them while hold zvol_state_lock. This then blocks out all other OS-side operations which want to get the private data, and any ZFS-side control operations that would take the write half of the lock. It's even worse if ZFS-side operations induce OS-side calls back into the zvol (eg creating a zvol triggers a partition probe inside the kernel, and also a userspace access from udev to set up device links). And it gets even works again if anything decides to defer those ops to a task and wait on them, which zvol_remove_minors_impl() will do under high load. However, since the previous commit, we have a guarantee that the private data pointer will always be NULL'd out in zvol_os_remove_minor() _before_ the zvol_state_t is made invalid, but it won't happen until all users are ejected. So, if we make access to the private object pointer atomic, we remove the need to take a global lockout to access it, and so we can remove all acquisitions of zvol_state_lock from the OS side. While here, I've rewritten much of the locking theory comment at the top of zvol.c. It wasn't wrong, but it hadn't been followed exactly, so I've tried to describe the purpose of each lock in a little more detail, and in particular describe where it should and shouldn't be used. Sponsored-by: Klara, Inc. Sponsored-by: Railway Corporation Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Fedor Uporov <fuporov.vstack@gmail.com> Signed-off-by: Rob Norris <rob.norris@klarasystems.com> Closes #17625 |
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OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.
Official Resources
- Documentation - for using and developing this repo
- ZoL Site - Linux release info & links
- Mailing lists
- OpenZFS site - for conference videos and info on other platforms (illumos, OSX, Windows, etc)
Installation
Full documentation for installing OpenZFS on your favorite operating system can be found at the Getting Started Page.
Contribute & Develop
We have a separate document with contribution guidelines.
We have a Code of Conduct.
Release
OpenZFS is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197
Supported Kernels
- The
METAfile contains the officially recognized supported Linux kernel versions. - Supported FreeBSD versions are any supported branches and releases starting from 13.0-RELEASE.