https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207848
Bug ID: 207848
Summary: knote list lock may be held forever when
knlist_remove_kq() is called while another function is
trying to acquire the lock
Product: Base System
Version: 10.2-STABLE
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: kern
Assignee: freebsd-bugs@FreeBSD.org
Reporter: j...@freebsd.org
As reported in review D4893, it is possible for a knote list lock to be held
forever.
The sequence of events is:
Thread 1 obtains the knlist lock.
Thread 2 hits KN_LIST_LOCK(). It finds that kn->kn_knlist is non-NULL, so it
waits to acquire the knlist lock.
Thread 1 calls knlist_remove_kq(). This sets kn->kn_knlist to NULL. It then
releases the knlist lock.
Thread 2 obtains the knlist lock.
Thread 2 hits KN_LIST_UNLOCK(). It finds that kn->kn_knlist is NULL, so it does
not release the knlist lock.
The fix seems to be to recheck kn->kn_knlist after acquiring the lock, and then
immediately release the lock. The submitter of review D4893 is going to work on
a patch.
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