On Tue, Jun 23, 2020 at 02:00:14PM +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 11d6011c2cf29f7c8181ebde6c8bc0c4d83adcd7 Mon Sep 17 00:00:00 2001
From: "Ahmed S. Darwish" <a.darwish@xxxxxxxxxxxxx>
Date: Wed, 3 Jun 2020 16:49:44 +0200
Subject: [PATCH] net: core: device_rename: Use rwsem instead of a seqcount
Sequence counters write paths are critical sections that must never be
preempted, and blocking, even for CONFIG_PREEMPTION=n, is not allowed.
Commit 5dbe7c178d3f ("net: fix kernel deadlock with interface rename and
netdev name retrieval.") handled a deadlock, observed with
CONFIG_PREEMPTION=n, where the devnet_rename seqcount read side was
infinitely spinning: it got scheduled after the seqcount write side
blocked inside its own critical section.
To fix that deadlock, among other issues, the commit added a
cond_resched() inside the read side section. While this will get the
non-preemptible kernel eventually unstuck, the seqcount reader is fully
exhausting its slice just spinning -- until TIF_NEED_RESCHED is set.
The fix is also still broken: if the seqcount reader belongs to a
real-time scheduling policy, it can spin forever and the kernel will
livelock.
Disabling preemption over the seqcount write side critical section will
not work: inside it are a number of GFP_KERNEL allocations and mutex
locking through the drivers/base/ :: device_rename() call chain.
From all the above, replace the seqcount with a rwsem.
Fixes: 5dbe7c178d3f (net: fix kernel deadlock with interface rename and netdev name retrieval.)
Fixes: 30e6c9fa93cf (net: devnet_rename_seq should be a seqcount)
Fixes: c91f6df2db49 (sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface name)
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: kbuild test robot <lkp@xxxxxxxxx> [ v1 missing up_read() on error exit ]
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx> [ v1 missing up_read() on error exit ]
Signed-off-by: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
The conflict was due to a change in comments in commit 2da2b32fd934
("sched/rt, net: Use CONFIG_PREEMPTION.patch"). I've just grabbed
2da2b32fd934 along and queued both of these for 5.4-4.4.
--
Thanks,
Sasha
