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Re: mutexes, locks and so on...

On 11/17/10 11:38, Johnny Billquist wrote:
On 11/17/10 05:15, Matt Thomas wrote:

On Nov 16, 2010, at 8:04 PM, Johnny Billquist wrote:

On 2010-11-17 04:52, Matt Thomas wrote:

Not true. Spinlocks must enter through mutex_spin_enter and adaptive
enter through mutex_enter. The corresponding is true for exiting as
The only reason mutex_vector_{enter,exit} is called to cause a panic.

That may be true in theory, but I happen to know (now) that it's not
true in reality. I've found atleast one place in the kernel where
this is done.

Then that is a bug and should be fixed. File a PR.

Ouch. Should I also file bugs for mutexes that are released, but already
appear to be free? Because that also happens.

I actually wonder if it is a bug, or just a chosen design. I'd rather
say we should get rid of the mutex_spin_{} functions, since the kernel
is so confused about the whole thing anyway.

I'm adding a bunch of counters now, and will come back with how often
these things are abused in a minute...

Oh, and mutex_vector_exit will not cause a panic, just because it was
called from mmutex_spin_exit with a adaptive mutex. mutex_vector_exit
will simply just handle it as an adaptive mutex, and release it.

That should probably be fixed.

How? mutex_vector_{enter,exit} have been written to handle both spin and
adaptive mutexes. And it don't know that it might have been called from
mutex_spin_{} with an adaptive mutex. It might in fact be the only
function available for both kinds... You don't have to write your own
mutex_[]_{enter,exit} functions, and if not, the mutex_vector ones will
be used directly.

Some results from having put in a bunch of counters. This on a freshly booted system:

mutex exit adapt with unlocked                    0        0 misc
mutex exit spin with unlocked                  1004        2 misc
mutex enter spin with lock                        0        0 misc
mutex exit adapt with spin                    41864       92 misc
mutex exit spin with adapt                        0        0 misc
mutex enter adapt with spin                   40935       89 misc
mutex enter spin with adapt                       0        0 misc

There are obviously a lot of calls to mutex_enter and mutex_exit with spin mutexes... Also a fair number of calls to mutex_spin_exit with the spin lock not appearing to be held (I actually set and clear a bit to indicate if the mutex is held, in addition to bumping the IPL).


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