e6135df45e
Yonghong Song says: ==================== Currently, the bpf hashmap iterator takes a bucket_lock, a spin_lock, before visiting each element in the bucket. This will cause a deadlock if a map update/delete operates on an element with the same bucket id of the visited map. To avoid the deadlock, let us just use rcu_read_lock instead of bucket_lock. This may result in visiting stale elements, missing some elements, or repeating some elements, if concurrent map delete/update happens for the same map. I think using rcu_read_lock is a reasonable compromise. For users caring stale/missing/repeating element issues, bpf map batch access syscall interface can be used. Note that another approach is during bpf_iter link stage, we check whether the iter program might be able to do update/delete to the visited map. If it is, reject the link_create. Verifier needs to record whether an update/delete operation happens for each map for this approach. I just feel this checking is too specialized, hence still prefer rcu_read_lock approach. Patch #1 has the kernel implementation and Patch #2 added a selftest which can trigger deadlock without Patch #1. ==================== Signed-off-by: Alexei Starovoitov <ast@kernel.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.
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.