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Re: [PATCH] xfs: improve xfs_isilocked

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: improve xfs_isilocked
From: Alex Elder <aelder@xxxxxxx>
Date: Thu, 03 Jun 2010 11:19:27 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20100529095019.GA18859@xxxxxxxxxxxxx>
References: <20100527190533.GB16102@xxxxxxxxxxxxx> <1275075653.2302.38.camel@doink> <20100529095019.GA18859@xxxxxxxxxxxxx>
Reply-to: aelder@xxxxxxx
On Sat, 2010-05-29 at 05:50 -0400, Christoph Hellwig wrote:
> On Fri, May 28, 2010 at 02:40:53PM -0500, Alex Elder wrote:
> > On Thu, 2010-05-27 at 15:05 -0400, Christoph Hellwig wrote:
> > > Use rwsem_is_locked to make the assertations for shared locks work.
> > 
> > So you're changing it so it answers "yes it's locked"
> > even it it's only a read lock now, right?
. . .

> > Now it's "If the exclusive flag is set, but no writer,
> > it's not locked.  Otherwise if the shared flag is
> > set it's locked if rwsem_is_locked() says we are.
> > Otherwise (ASSERT(0) and) it is not locked."
> 
> Not exactly.  Now it's:
> 
>  - if excl is set but shared isn't return true if mr_writer is
>    set, else false
>  - if shared is set either alone or together with excl return
>    if it is locked in any way (rwsem_is_locked).

OK, that makes sense, I get it now.

> Note that xfs_isilocked can be called like:
> 
>       ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL|XFS_ILOCK_SHARED));
> 
> which means that either excl or shared is fine.
> 
>  - if either one or both of excl and shared are set and it's 
> 
> > That last part is wrong I think.  It should be OK to
> > call xfs_isilocked() with neither flag set, in which
> > case the result should be 0.
> 
> We can argue about removing the assert, but we currently don't
> and should't call xfs_isilocked wit ha 0 argument - it's rather
> pointless to do so.

Yes, you're right.  I'd still say the function should
return the right answer even if given an unreasonable
request.  But that's being pedantic.

> 
> > And if the exclusive
> > flag is set, and there *is* a writer, it *is* locked,
> > so it should return 1.
> 
> We do that right now.

Yup.  Thanks for setting me straight.

Reviewed-by: Alex Elder <aelder@xxxxxxx>




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