linux-kernel - Re: [PATCH 14/25] aio: Make aio_read

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Message-ID: <20121129193154.GI15094@google.com>
Date:	Thu, 29 Nov 2012 11:31:54 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Zach Brown <zab@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, bcrl@...ck.org, jmoyer@...hat.com,
	axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 14/25] aio: Make aio_read_evt() more efficient

On Wed, Nov 28, 2012 at 04:38:16PM -0800, Zach Brown wrote:
> > We can't use cmpxchg() on the ring buffer's head pointer directly, since
> > it's modded to nr_events and would be susceptible to ABA. So instead we
> > maintain a shadow head that uses the full 32 bits, and cmpxchg() that
> > and then updated the real head pointer.
> 
> Time to update this comment to reflect the mutex instead of the shadow
> head? :)

Yeah :P

> > +		ev = kmap(page);
> > +		copy_ret = copy_to_user(event + ret, ev + pos, sizeof(*ev) * i);
> > +		kunmap(page);
> 
> For lack of a better time: do we want to bring up the missing
> flush_dcache_page() calls around the kernel mappings of ring pages that
> are also mapped to user addresses?

Well, it's mainly aio_complete() that needs to be doing
flush_dcache_page()... technically io_getevents should be too, but only
because it's updating the head pointer. Should probably do it now and
not forget about it again, but I think it should be its own patch.

> >  		prepare_to_wait_exclusive(&ctx->wait, &wait,
> >  					  TASK_INTERRUPTIBLE);
> >  
> > +		ret = aio_read_events(ctx, event + i, nr - i);
> > +		if (ret < 0)
> >  			break;
> 
> As mentioned offlist: we don't want to be blocking under
> TASK_INTERRUPTIBLE.  Is the plan to do a non-blocking check and pop
> outside the wait loop to do a blocking copy?

Argh. This thing is getting irritating. Gonna mull this over on irc
more.
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