cvs commit: src/sys/kern subr_witness.c
jhb at FreeBSD.org
Mon Aug 29 17:32:20 GMT 2005
On Monday 29 August 2005 01:10 pm, Don Lewis wrote:
> On 29 Aug, John Baldwin wrote:
> > On Wednesday 24 August 2005 11:47 pm, Don Lewis wrote:
> >> truckman 2005-08-25 03:47:37 UTC
> >> FreeBSD src repository
> >> Modified files:
> >> sys/kern subr_witness.c
> >> Log:
> >> Track all lock relationships instead of pruning direct relationships
> >> if an indirect relationship exists (keep both A->B->C and A->C).
> >> This allows witness_checkorder() to use isitmychild() instead of
> >> the much more expensive isitmydescendant() to check for valid lock
> >> ordering.
> >> Don't do an expensive tree walk to update the w_level values when
> >> the tree is updated. Only update the w_level values when using the
> >> debugger to display the tree.
> >> Nuke the experimental "witness_watch > 1" mode that only compared
> >> w_level for the two locks. This information is no longer maintained
> >> at run time, and the use of isitmychild() in witness_checkorder
> >> should bring performance close enough to the acceptable level that
> >> this hack is not needed.
> >> Report witness data structure allocation statistics under the
> >> debug.witness sysctl.
> >> Reviewed by: jhb
> >> MFC after: 30 days
> >> Revision Changes Path
> >> 1.198 +31 -71 src/sys/kern/subr_witness.c
> > I didn't think of this until now, but I think this breaks indirect lock
> > order relationships that aren't explicit. That is, suppose at some point
> > the following lock order pairs are recorded:
> > A -> B
> > C -> D
> Ok ...
That should have been B -> C rather than C -> D.
> > That will give you a tree structure of something like:
> > A --> B --> C
> You lost me here. How did this transformation happen?
> > If you then do C -> A, since C isn't a direct descendant of A, witness
> > won't catch it anymore. So, you might need to back this out until a
> > solution to this problem is solved.
> No, C -> A should still be caught. We first check for known direct
> relationships by calling isitmychild(C, A), which will return 0, and
> then start checking for reversals. In the loop, we will call
> isitmydescendent(A, C), which will find the A -> B -> C relationship,
> return 1, and cause witness_checkorder() to fall through to the "lock
> order reversal" message.
Ah, I thought you only checked the direct children since the log message said:
This allows witness_checkorder() to use isitmychild() instead of
the much more expensive isitmydescendant() to check for valid lock
> The entire tree is still checked for reversals. The optimization is to
> only check for direct relationships when validating that the lock is
> being grabbed in the direct order, so if anything, witness_checkorder()
> should fall through into the lock order reversal checking code more
Yeah, I see that now, sorry for the noise.
John Baldwin <jhb at FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
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