https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207085
Bug ID: 207085
Summary: pmc assertion failure: pmc %p non-NULL
Product: Base System
Version: 10.2-RELEASE
Hardware: amd64
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
CC: [email protected]
I see these panics with INVARIANTS enabled on 10.2-R on an SMP machine running
many processes using hwpmc in parallel.
[core,2376] PHW pmc non-NULL
cpuid = 1
pmc_process_exit() does the following:
sx_xlock();
critical_enter();
free_attached_pmcs...
critical_exit();
free_owner_pmcs...
sx_unlock();
and the various drivers do *_release_pmc():
... KASSERT(phw->phw_pmc == NULL) ...
What I'm seeing is on an SMP machine, from time to time a process which
owns PMCs races with a process that has attached PMCs. free_owner_pmcs()
calls *_release_pmc() which checks to make sure the per-cpu counter
isn't in use. A process with attached pmcs will have freed them with
both the pmc_lock and scheduler lock held. But, a process being scheduled
onto the pmc can sneak in between the critical_exit() and the sx_unlock(),
but before the free_owner_pmcs() resulting in an assertion failure.
free_owner_pmcs can't be protected by the scheduler lock because it
may need to sleep waiting on sampling counters to come off the cpu. I'm
not sure how to approach a fix other than disabling the KASSERT, and I'm not
sure if that's safe.
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