tree bb498b980a73cb8e648a89208343d7659574e329
parent 5c3e686d11b3232514006877f63333cdd9b08ad0
author Willy Tarreau <w@1wt.eu> 1643296248 +0100
committer Willy Tarreau <w@1wt.eu> 1645077237 +0100
encoding latin1

BUG/MEDIUM: fd: always align fdtab[] to 64 bytes

There's a risk that fdtab is not 64-byte aligned. The first effect is that
it may cause false sharing between cache lines resulting in contention
when adjacent FDs are used by different threads. The second is related
to what is explained in commit "BUG/MAJOR: compiler: relax alignment
constraints on certain structures", i.e. that modern compilers might
make use of aligned vector operations to zero some entries, and would
crash. We do not use any memset() or so on fdtab, so the risk is almost
inexistent, but that's not a reason for violating some valid assumptions.

This patch addresses this by allocating 64 extra bytes and aligning the
structure manually (this is an extremely cheap solution for this specific
case). The original address is stored in a new variable "fdtab_addr" and
is the one that gets freed. This remains extremely simple and should be
easily backportable. A dedicated aligned allocator later would help, of
course.

This needs to be backported as far as 2.2. No issue related to this was
reported yet, but it could very well happen as compilers evolve. In
addition this should preserve high performance across restarts (i.e.
no more dependency on allocator's alignment).

(cherry picked from commit 97ea9c49f1d95c7e91e544e8ad9ba09bffbcc023)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
(cherry picked from commit 6cebd0fd75e3ef511d5227df476c6d8c6c2d3378)
Signed-off-by: Willy Tarreau <w@1wt.eu>