Date: Sun, 10 Sep 2000 23:36:11 -0400 (EDT) From: Daniel Eischen <eischen@vigrid.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: arch@FreeBSD.ORG Subject: Re: Interruptable mutex aquires. Message-ID: <Pine.SUN.3.91.1000910231601.12837A-100000@pcnet1.pcnet.com> In-Reply-To: <20000910193602.B12231@fw.wintelcom.net>
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On Sun, 10 Sep 2000, Alfred Perlstein wrote:
>
> There's a lot of code in the kernel that does this:
>
> while(lock != mine) {
> error = tsleep(&lock, PRIBIO, "foo", PCATCH);
> if (error)
> return(error);
> }
> lock = mine;
>
> Ok, well the equivelant way to do this with mutexes would be
> to add a new function 'mtx_enter_interruptable()' [*]
> that attempts to aquire a mutex but if interrupted returns
> an error suitable for aborting the operation.
I don't think we want to equate tsleep() with mtx_enter().
I'd rather see us use msleep_sig() until condition variables
are added and then use cv_wait_sig() (or something like that).
Mutexes should be taken for very short times, and we shouldn't
worry about them being interrupted unless they surround a
tsleep()/msleep().
>
> Nice, easy, simple and fits well as a replacement tool for
> the current while(lock != mine) mechnism.
>
> Some people have discussed making a new flag 'MTX_CATCH', personally
> that sounds terrible, I'm already not really thrilled with the
> 'do-all' functions that have been put into place as our primatives,
> as they are not very intuative, it's hard to tell at a glance
> exactly what is a spinlock and what is a task switching mutex,
> let's _please_ try to keep this readable and stick away from the
> flags.
I sort of agree but can see why the flags were used instead of
relying on mtx_init() to set the type of mutex. I wonder if
we are too concerned about trying to squeeze an extra .5%
performance for recursive mutexes, than with trying to create
a simple and understandable interface.
>
> I also think that we don't really need the overhead of a
> return value if at all possible.
>
> At the BSDi/FreeBSD SMP meeting I remeber asking Chuck why he
> implemented it this way, his respnonse was something along the
> lines of "it seemed like a good idea to have an object that one
> could spin or sleep on depending on the locatation or state of the
> object encapsulating the mutex, but in practice we've never used
> nor needed to use a mutex as a spinlock or a spinlock as a mutex"
>
> [*] yes, today's C compilers actually support identifiers longer
> than 6 characters! if mtx_enter_interruptable is too offensive
> I could live with mtx_enter_intr().
I prefer *_sig()
--
Dan Eischen
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