On 20.01, Thomas Graf wrote:

On 01/20/15 at 02:31pm, Patrick McHardy wrote:

quoted

On 20.01, Thomas Graf wrote:

quoted

Lock out table resizes while dumping Netlink sockets to user space.
This keeps disruptions to a minimum for readers which don't handle
the NLM_F_DUMP_INTR flag.

This doesn't lock them out for the duration of the entire dump of
course, so the benefit seems rather small. Still with this patch,
they will need to handle NLM_F_DUMP_INTR or will get unpredictable
behaviour, in which case I'd think it makes more sense to not even
try this, all it does is hide parts of the brokenness.

If it would lock out the resize for the entire dump I would not have
done patches 1 and 2 ;-)

I does provide better behaviour if the whole dump fits into a single
buffer or if it fits into 2 buffers and we are already dumping into
the 2nd buffer when the resize occurs. Otherwise we will see resizes
and thus tons of duplicates even in those scenarios even if no insert
or removal occurs in parallel.

In the case of Netlink diag that should be typical case. Most systems
will not have 1000s of Netlink sockets in parallel.

I think its preferrable to make the need to handle NETLINK_F_DUMP_INTR
as noticable as possible and not hide it. Silent failure is the worst
kind of failure.

quoted

An alternative would be to set a flag in ht when a dump begins that
indicates to skip resizing operations and on the end of the dump
perform any resizing operations that might be necessary. Herbert
disagrees though and he might be right.

I don't like the flag as it prevents resizes (and possibly rehashes
further down the road) for a long period of time. The hashtable
becomes attackable.

Yeah. The point could be made that this is a regression though. We didn't
require userspace to deal with interruptions before, and the behaviour
was well defined and acceptable for most cases, its not anymore.

So I think it should be handled by the kernel, without changes to
userspace.