linux-kernel - Re: [PATCH 8/18] 2.6.17.9 perfmon2 patch for review: event sets and multiplexing support

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Message-Id: <20060906192135.83fc4231.akpm@osdl.org>
Date:	Wed, 6 Sep 2006 19:21:35 -0700
From:	Andrew Morton <akpm@...l.org>
To:	eranian@....hp.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/18] 2.6.17.9 perfmon2 patch for review: event sets and
 multiplexing support

On Wed, 6 Sep 2006 07:50:31 -0700
Stephane Eranian <eranian@....hp.com> wrote:

> > > +
> > > +	cachep = ctx->flags.mapset ? pfm_set_cachep : pfm_lg_set_cachep;
> > > +
> > > +	new_set = kmem_cache_alloc(cachep, SLAB_ATOMIC);
> > 
> > SLAB_ATOMIC is unreliable.  Is it possible to use SLAB_KERNEL here?  If
> > coms ecallers can sleep and others cannot then passing in the gfp_flags
> > would permit improvement here.
> > 
> 
> I made some changes and now I know I execute this part of the function
> with interrupts disabled, holding only the perfmon context lock. I assume
> SLAB_KERNEL means, we can sleep. I think I can make this change safely.
> 
> 
> > 
> > > +		if (ctx->flags.mapset) {
> > > +			view_size = PAGE_ALIGN(sizeof(struct pfm_set_view));
> > > +			view      = vmalloc(view_size);
> > 
> > vmalloc() sleeps, so this _could_ have used SLAB_ATOMIC.
> > 
> 
> I am not sure I follow you here. Are you talking about eh kmem_cache_alloc()
> above?
> 

My logic was as follows:

a) vmalloc() can sleep

b) Stephane at some time tested this conde with
   CONFIG_DEBUG_SPINLOCK_SLEEP and didn't get sleep-while-atomic warnings out of
   that vmalloc().

c) Hence this code is never called under spinlock, or with local
   interrupts disabled.

d) Hence it is safe to convert the earlier SLAB_ATOMIC into SLAB_KERNEL.


If b) is false then it's the vmalloc() call which is incorrect, not the
SLAB_ATOMIC.

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