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synchronize_rcu() is expensive. The commit phase currently enforces an unconditional synchronize_rcu() after incrementing the generation counter. This is to make sure that a packet always sees a consistent chain, either nft_do_chain is still using old generation (it will skip the newly added rules), or the new one (it will skip old ones that might still be linked into the list). We could just remove the synchronize_rcu(), it would not cause a crash but it could cause us to evaluate a rule that was removed and new rule for the same packet, instead of either-or. To resolve this, add rule pointer array holding two generations, the current one and the future generation. In commit phase, allocate the rule blob and populate it with the rules that will be active in the new generation. Then, make this rule blob public, replacing the old generation pointer. Then the generation counter can be incremented. nft_do_chain() will either continue to use the current generation (in case loop was invoked right before increment), or the new one. Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org> Signed-off-by: Florian Westphal <fw@strlen.de> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
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.
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.