cvs commit: src/sys/kern kern_rwlock.c src/sys/sys rwlock.h
jroberson at chesapeake.net
Thu Apr 3 09:29:44 UTC 2008
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
>>>> 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
>> 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.
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