The TCP timewait timer is proving to be problematic for setups where scheduler
CPU isolation is achieved at runtime via cpusets (as opposed to statically via
isolcpus=domains).

What happens there is a CPU goes through tcp_time_wait(), arming the time_wait
timer, then gets isolated. TCP_TIMEWAIT_LEN later, the timer fires, causing
interference for the now-isolated CPU. This is conceptually similar to the issue
described in
  e02b93124855 ("workqueue: Unbind kworkers before sending them to exit()")

Keep softirqs disabled, but make the timer un-pinned and arm it *after* the
hashdance.

This introduces the following (non-fatal) race:

  CPU0                        CPU1
    allocates a tw
    insert it in hash table
                                finds the TW and removes it
                                (timer cancel does nothing)
    arms a TW timer, lasting

This partially reverts
  ed2e92394589 ("tcp/dccp: fix timewait races in timer handling")
and
  ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")

This also reinstores a comment from
  ec94c2696f0b ("tcp/dccp: avoid one atomic operation for timewait hashdance")
as inet_twsk_hashdance() had a "Step 1" and "Step 3" comment, but the "Step
2" had gone missing.

Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/ (local)
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 net/dccp/minisocks.c          | 16 +++++++---------
 net/ipv4/inet_timewait_sock.c | 20 +++++++++++++++-----
 net/ipv4/tcp_minisocks.c      | 16 +++++++---------
 3 files changed, 29 insertions(+), 23 deletions(-)

diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
index 64d805b27adde..2f0fad4255e36 100644
--- a/net/dccp/minisocks.c
+++ b/net/dccp/minisocks.c

@@ -53,16 +53,14 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
                if (state == DCCP_TIME_WAIT)
                        timeo = DCCP_TIMEWAIT_LEN;

-               /* tw_timer is pinned, so we need to make sure BH are disabled
-                * in following section, otherwise timer handler could run before
-                * we complete the initialization.
-                */
-               local_bh_disable();
-               inet_twsk_schedule(tw, timeo);
-               /* Linkage updates.
-                * Note that access to tw after this point is illegal.
-                */
+              local_bh_disable();
+
+               // Linkage updates
                inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
+               inet_twsk_schedule(tw, timeo);