On Tue, 26 Dec 2017, Ryota Ozaki wrote:
On Tue, Dec 26, 2017 at 11:35 AM, Paul Goyette <p...@whooppee.com> wrote:
To generate a diff of this commit:
# cvs rdiff -u -r1.139 -r1.140 src/sys/netinet6/nd6_nbr.c
@@ -1097,7 +1097,11 @@ nd6_dad_stoptimer(struct dadq *dp)
#ifdef NET_MPSAFE
callout_halt(&dp->dad_timer_ch, NULL);
#else
- callout_halt(&dp->dad_timer_ch, softnet_lock);
+ /* XXX still need the trick for softnet_lock */
+ if (mutex_owned(softnet_lock))
+ callout_halt(&dp->dad_timer_ch, softnet_lock);
+ else
+ callout_halt(&dp->dad_timer_ch, NULL);
#endif
}
This goes against the restriction noted in the mutex(9) man page:
[mutex_owned()] should not be used to make locking decisions at run
time. ...
Please find a different way to make this run-time decision.
I know the restriction well, but for softnet_lock, following the restriction
is sometimes hard. I don't have an option to statically decide if
softnet_lock is held or not without messing up many codes.
An option we can have here is to give up using callout_halt and use
callout_stop that may be unsafe.
Which is better for us?
Well, since the lock _might_ be released (and subsequently reacquired)
by callout_halt(), it might be easiest to modify all the callers to
just unlock it before calling nd6_dad_stoptimer(), and reacquire the
mutex after it returns (as well as re-establishing any values that
might have changed while the mutex was released).
The callers needs to be prepared for the release/reacquire situation
anyway, so the change should not be significant. As noted in the
callout_halt(9) man page
...to avoid race conditions the caller should always assume that
[the] interlock has been released and reacquired, and act
accordingly.
Alternatively, you could modify all the callers to always acquire the
mutex before calling nd6_dad_stoptimer().
But making a run-time decision with mutex_owned() is not a good idea.
+------------------+--------------------------+----------------------------+
| Paul Goyette | PGP Key fingerprint: | E-mail addresses: |
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| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd dot org |
+------------------+--------------------------+----------------------------+