| Subject | Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of a seqcount | From | Eric Dumazet <> | Date | Tue, 19 May 2020 19:01:38 -0700 |
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On 5/19/20 2:45 PM, Ahmed S. Darwish wrote: > 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@vger.kernel.org> > Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de> > Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > net/core/dev.c | 30 ++++++++++++------------------ > 1 file changed, 12 insertions(+), 18 deletions(-) >
Seems fine to me, assuming rwsem prevent starvation of the writer.
(Presumably this could be a per ndevice rwsem, or per netns, to provide some isolation)
Alternative would be to convert ndev->name from char array to a pointer (rcu protected), but this looks quite invasive change, certainly not for stable branches.
Reviewed-by: Eric Dumazet <edumazet@google.com>
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