> -----Original Message----- > From: [email protected] [mailto:owner-freebsd- > [email protected]] On Behalf Of Dmitry Krivenok > Sent: Thursday, February 24, 2011 7:47 AM > To: [email protected] > Subject: mtx_init/lock_init and uninitialized struct mtx > > Hello Hackers, > > Is it allowed to call mtx_init on a mutex defined as an auto variable > and not initialized explicitly, i.e.: > > static int foo() > { > struct mtx m; // Uninitialized auto variable, so it's value is > undefined. > mtx_init(&m, "my_mutex", NULL, MTX_DEF); > ... > // Do something > ... > mtx_destroy(&m); > return 0; > } > > I encountered a problem with such code on a kernel compiled with > INVARIANTS option. > The problem is that mtx_init calls lock_init(&m->lock_object) and > lock_init, in turn, calls: > > 79 /* Check for double-init and zero object. */ > 80 KASSERT(!lock_initalized(lock), ("lock \"%s\" %p already > initialized", > 81 name, lock)); > > lock_initialized() just checks that a bit is set in lo_flags field of > struct lock_object: > > 178 #define lock_initalized(lo) ((lo)->lo_flags & LO_INITIALIZED) > > However, the structure containing this field is never initialized > (neither in mtx_init nor in lock_init). > So, assuming that the mutex was defined as auto variable, the content > of lock_object field of struct mtx > is also undefined: > > 37 struct mtx { > 38 struct lock_object lock_object; /* Common lock > properties. */ > 39 volatile uintptr_t mtx_lock; /* Owner and flags. */ > 40 }; > > In some cases, the initial value of lo_flags _may_ have the > "initialized" bit set and KASSERT will call panic. > > Is it user's responsibility to properly (how exactly?) initialize > struct mtx, e.g. > memset(&m, '\0', sizeof(struct mtx)); > > Or should mtx_init() explicitly initialize all fields of struct mtx? > > Thanks in advance! >
When dealing with stack variables, I always initialize them to a known state, (mostly doing as you do above, with memset() though in the kernel I then to use bzero()) Given that if it is global then it is either in .bss or in .data if it has been initialized. If it is part of a structure then most often you zero out the structure right after allocation. So there is an implicit assumption that the structure will be zero'd before calling mtx_init(). Patrick ---------------------------------------------------- Patrick Mahan Lead Technical Kernel Engineer Adara Networks Disclaimer: The opinions expressed here are solely the responsibility of the author and are not to be construed as an official opinion of Adara Networks. > -- > Sincerely yours, Dmitry V. Krivenok > e-mail: [email protected] > skype: krivenok_dmitry > jabber: [email protected] > icq: 242-526-443 > _______________________________________________ > [email protected] mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "[email protected]" _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[email protected]"
