From: Alexei Starovoitov <ast@kernel.org>
Date: Fri, 4 Sep 2020 00:36:41 +0000 (-0700)
Subject: Merge branch 'hashmap_iter_bucket_lock_fix'
X-Git-Tag: v5.15~2870^2~30^2~7
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=e6135df45e21f1815a5948f452593124b1544a3e;p=platform%2Fkernel%2Flinux-starfive.git

Merge branch 'hashmap_iter_bucket_lock_fix'

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>
---

e6135df45e21f1815a5948f452593124b1544a3e