Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1609753

Re: deadlock in synchronize_srcu() in debugfs?

Show all headers | View raw

Hi,

> > Before I go hunting - has anyone seen a deadlock in
> > synchronize_srcu() in debugfs_remove() before?
> 
> Not yet. How reproducible is this?

So ... this turned out to be a livelock of sorts.

We have a debugfs file (not upstream (yet?), it seems) that basically
blocks reading data.

At the point of system hanging, there was a process reading from that
file, with no data being generated.

A second process was trying to remove a completely unrelated debugfs
file (*), with the RTNL held.

A third and many other processes were waiting to acquire the RTNL.


Obviously, in light of things like nfp_net_debugfs_tx_q_read(),
wil_write_file_reset(), lowpan_short_addr_get() and quite a few more,
nobody in the whole system can now remove debugfs files while holding
the RTNL. Not sure how many people that affects, but it's IMHO a pretty
major new restriction, and one that isn't even flagged at all.


Similarly, nobody should be blocking in debugfs files, like we did in
ours, but also smsdvb_stats_read(), crtc_crc_open() look like they
could block for quite a while. Again, there's no warning here that
blocking in debugfs files can now indefinitely defer completely
unrelated debugfs_remove() calls in the entire system.

Overall, while I can solve this problem for our driver, possibly by
making the debugfs file return some dummy data periodically if no real
data exists, which may not easily be possible for all such files, I'm
not convinced that all of this really is the right thing to actually
impose. Perhaps if it was per directory, or per some kind of subsystem?

johannes

(*) before removing first first we'd obviously wake up and thereby more
or less terminate the readers first

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:00 +0100
  Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:50 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-27 14:00 +0200
      Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-30 09:40 +0200
        Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-30 10:00 +0200
          Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-30 12:30 +0200
            Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-30 13:20 +0200
              Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-31 11:10 +0200
                Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-31 11:50 +0200
  Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:50 +0100
  Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 10:00 +0100
      Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 10:30 +0100
        Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 18:50 +0100
          Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 20:00 +0100
            Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 20:40 +0100
              Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 21:30 +0100
                Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-27 13:20 +0200

csiph-web