Date: Tue, 12 Sep 2000 11:18:42 +0930 From: Greg Lehey <grog@lemis.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: arch@FreeBSD.ORG Subject: Re: Interruptable mutex aquires. Message-ID: <20000912111842.F88615@wantadilla.lemis.com> In-Reply-To: <20000910193602.B12231@fw.wintelcom.net>; from bright@wintelcom.net on Sun, Sep 10, 2000 at 07:36:02PM -0700 References: <20000910193602.B12231@fw.wintelcom.net>
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On Sunday, 10 September 2000 at 19:36:02 -0700, 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.
>
> 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 suppose there's a case for simpler terminology, but have you looked
at the mutex code? There's a *lot* of code in there, and apart from
the duplication, maintenance would be terrible. I'd suggest the flag
approach, but there's nothing stopping a macro encapsulation with a
name like mtx_enter_interruptable.
> 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"
Ugh. Another use of the word "mutex". I thought we were using mutex
to cover both terms.
Don't forget that BSD/OS 5 hasn't been released yet. Rather
amusingly, ours is available before theirs :-) But it's quite possible
that they will find a use for this kind of 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().
This isn't a question of identifier length, it's a question of
style(9). As long as we have 8 character indentation and only 80
characters text width, we're damned to using short identifiers.
Greg
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