tree 46cb7832d38a3bd7538ac7208210f25117a87075
parent c79f01497208e3cea20d4d563dee158e0e6f4674
author Willy Tarreau <w@1wt.eu> 1634819618 +0200
committer Willy Tarreau <w@1wt.eu> 1634822922 +0200
encoding latin1

OPTIM: dns: use an atomic check for the list membership

The crash that was fixed by commit 7045590d8 ("BUG/MAJOR: dns: attempt
to lock globaly for msg waiter list instead of use barrier") was now
completely analysed and confirmed to be partially a result of the
debugging code added to LIST_INLIST(), which was looking at both
pointers and their reciprocals, and that, if used in a concurrent
context, could perfectly return false if a neighbor was being added or
removed while the current one didn't change, allowing the LIST_APPEND
to fail.

As the LIST API was not designed to be used in a concurrent context,
we should not rely on LIST_INLIST() but on the newly introduced
LIST_INLIST_ATOMIC().

This patch simply reverts the commit above to switch to the new test,
saving a lock during potentially long operations. It was verified that
the check doesn't fail anymore.

It is unsure what the performance impact of the fix above could be in
some contexts. If any performance regression is observed, it could make
sense to backport this patch, along with the previous commit introducing
the LIST_INLIST_ATOMIC() macro.