cvs commit: src/sys/kern kern_rwlock.c src/sys/sys rwlock.h

[Jeff Roberson]

On Wed, 2 Apr 2008, David Schultz wrote:

> On Tue, Apr 01, 2008, Jeff Roberson wrote:
>> On Wed, 2 Apr 2008, Max Laier wrote:
>>
>>> On Tuesday 01 April 2008 22:31:55 Attilio Rao wrote:
>>>> attilio     2008-04-01 20:31:55 UTC
>>>>
>>>>  FreeBSD src repository
>>>>
>>>>  Modified files:
>>>>    sys/kern             kern_rwlock.c
>>>>    sys/sys              rwlock.h
>>>>  Log:
>>>>  Add rw_try_rlock() and rw_try_wlock() to rwlocks.
>>>>  These functions try the specified operation (rlocking and wlocking)
>>>> and true is returned if the operation completes, false otherwise.
>>>
>>> hmmm ... I'm certainly missing something here, but what's a possible
>>> usecase for these?  It seems there is not much you can do if you can't
>>> obtain a rw_lock.  I can understand the need for sx_try_* where you want
>>> to avoid sleeping, but I can't figure out the need for it on a locking
>>> primitive that will only spin or wait (not 100% sure about the
>>> terminology here).  This is especially strange for rw_try_wlock, unless
>>> you plan to sleep manually on fail.  But then again you'd have a good
>>> chance that you have to do it over and over again if timing is
>>> unfortunate.
>>
>> I asked for it.  We have a try operation for mtx already.  I was
>> experimenting with converting some code to use rwlocks from mtx and it
>> required it.  The specific case is in the softdep code where it uses
>> trylock to avoid deadlocking.  With trylock you can violate the lockorder.
>
> One reason that many systems don't provide a rw_try_wlock()
> primitive is that a constant stream of readers can easily starve
> threads attempting to acquire the write lock without waiting.
> There are probably situations where this isn't a problem, though,
> e.g., if readers rarely hold the lock for a long time...
>

The same applies to constant streams of lockers with a normal mutex.  We 
have found a few cases now where trylock was used that basically never 
succeeded and caused pessimal behavior.  I'm not really a fan of it myself 
but it is (ab)used in some scenarios and having it in rw will make 
changing from mtx easier.

I think it would be a good task for someone to attempt to remove uses of 
trylock by restructuring the locking.

Thanks,
Jeff