options WITNESS and locks
yonyossef.lists at gmail.com
Tue Jul 15 19:48:13 UTC 2008
Problem solved :-)
Thank you very much for your patience Kris..
On Tue, Jul 15, 2008 at 5:14 PM, Kris Kennaway <kris at freebsd.org> wrote:
> Yony Yossef wrote:
>> Something is still unclear. All my locks are MTX_DEF type, which
>> sleepable, including the one specified in the crash report
>> (MTNIC state
>> This crash happens when I try to call bus_resource_alloc_any for
>> a SYS_REQ
>> which is trying to obtain the second lock (ACPI root bus). How
>> come my
>> MTX_DEF lock is being treated as as non-sleepable?
>> MTX_DEF locks *are* non-sleepable :-)
>> Good news :-)
>> Yet the documentation is confusing me:
>> man pages say:
>> MTX_DEF Default mutexes will always allow the current thread to be
>> suspended to avoid deadlock conditions against
>> threads. The implementation of this lock type may spin
>> for a while before suspending the current thread.
>> and in mutex.h:
>> * Mutex types and options passed to mtx_init(). MTX_QUIET and MTX_DUPOK
>> * can also be passed in.
>> #define MTX_DEF 0x00000000 /* DEFAULT (sleep) lock */
>> #define MTX_SPIN 0x00000001 /* Spin lock (disables interrupts) */
>> #define MTX_RECURSE 0x00000004 /* Option: lock allowed to recurse */
>> #define MTX_NOWITNESS 0x00000008 /* Don't do any witness checking. */
>> #define MTX_NOPROFILE 0x00000020 /* Don't profile this lock */
>> What does the "DEFAULT (sleep) lock" comment near MTX_DEF means?
>> And which ones are sleepable?.. I guess I'm missing something here.
> "sleep mutex" means that the mutex holder may be put to sleep by the kernel
> while the mutex is held by another process (if the lock holder is running
> then it will spin, though). It is in contrast to "spin" mutexes that always
> spin forever while the lock is held, even when it might be inefficient for
> them to do so because the lock holder is no longer running after the CPU
> context switched to another process before it released the lock.
> This describes the behaviour of the mutex while it is waiting to acquire a
> "sleepable" vs "non-sleepable" locks means what it is legal for your code
> to do while holding a mutex and doesn't relate to the internal
> implementation of the lock.
> This describes what it is legal for your code to do once it has acquired
> the lock.
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