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pick_link() needs to push onto stack; we start with using two-element array embedded into struct nameidata and the first time we need more than that we switch to separately allocated array. Allocation can fail, of course, and handling of that would be simple enough - we need to drop 'link' and bugger off. However, the things get more complicated in RCU mode. There we must do GFP_ATOMIC allocation. If that fails, we try to switch to non-RCU mode and repeat the allocation. To switch to non-RCU mode we need to grab references to 'link' and to everything in nameidata. The latter done by unlazy_walk(); the former - legitimize_path(). 'link' must go first - after unlazy_walk() we are out of RCU-critical period and it's too late to call legitimize_path() since the references in link->mnt and link->dentry might be pointing to freed and reused memory. So we do legitimize_path(), then unlazy_walk(). And that's where it gets too subtle: what to do if the former fails? We MUST do path_put(link) to avoid leaks. And we can't do that under rcu_read_lock(). Solution in mainline was to empty then nameidata manually, drop out of RCU mode and then do put_path(). In effect, we open-code the things eventual terminate_walk() would've done on error in RCU mode. That looks badly out of place and confusing. We could add a comment along the lines of the explanation above, but... there's a simpler solution. Call unlazy_walk() even if legitimaze_path() fails. It will take us out of RCU mode, so we'll be able to do path_put(link). Yes, it will do unnecessary work - attempt to grab references on the stuff in nameidata, only to have them dropped as soon as we return the error to upper layer and get terminate_walk() called there. So what? We are thoroughly off the fast path by that point - we had GFP_ATOMIC allocation fail, we had ->d_seq or mount_lock mismatch and we are about to try walking the same path from scratch in non-RCU mode. Which will need to do the same allocation, this time with GFP_KERNEL, so it will be able to apply memory pressure for blocking stuff. Compared to that the cost of several lockref_get_not_dead() is noise. And the logics become much easier to understand that way. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
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| net | ||
| samples | ||
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| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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.