Re: [PATCH] Rewrite proc seq operations via seq_list_xxx ones

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Pavel Emelyanov (xemul@openvz.org) wrote:
> Mathieu Desnoyers wrote:
> > * Pavel Emelyanov (xemul@openvz.org) wrote:
> >> Some time ago I posted a set of patches that consolidated
> >> the seq files, walking the list_head-s. This set was merged
> >> in the mm tree. /proc/diskstats and /proc/partitions files 
> >> were included in this set, but a bit later this part was 
> >> dropped because of conflicts with some other changes.
> >>
> >> Here's the fixed version against the latest git block repo.
> >>
> > 
> > Hi Pavel,
> > 
> > You might want to try implementing this patch on top of the sorted seq
> > list patch I proposed there. It deals with a race between proc file read
> > and list modification between iterations.
> 
> AFAIS there's no race - everything is protected with the mutex.
> 
> OTOH, If you mean the problem that the list can change while the 
> seq file reallocates its buffers, then this problem exists for all
> the kernel and I don't think it's a good idea to stick to the block
> proc files only.
> 

The problem is that the list can change between two consecutive reads.
And yes, all proc files using lists are affected, but changing every
proc file users is a quite large task.

A problematic sequence would be:

1st procfs read()
*_start()
- mutex down
- seq list start

*_next()
- seq list next

*_stop()
- mutex up
end of 1st procfs read()

mutex down
-> Removing a list entry
mutex up

2nd procfs read()
*_start()
- mutex down
... we now skip a list entry depending on the relative position of our
iterator and the deletion.

The mutex is taken/released at each and every read, therefore not
protecting against modifications across reads.

Mathieu


> > http://lkml.org/lkml/2007/9/6/175
> > 
> > See this patch for a user implementation example and test code
> > (/proc/modules) :
> > 
> > http://lkml.org/lkml/2007/9/6/176
> > 
> > Mathieu
> > 
> >> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> >>
> >> ---
> >>
> >> diff --git a/block/genhd.c b/block/genhd.c
> >> index 3af1e7a..27f7061 100644
> >> --- a/block/genhd.c
> >> +++ b/block/genhd.c
> >> @@ -264,39 +264,34 @@ void __init printk_all_partitions(void)
> >>  
> >>  #ifdef CONFIG_PROC_FS
> >>  /* iterator */
> >> -static void *part_start(struct seq_file *part, loff_t *pos)
> >> +static void *disk_start(struct seq_file *part, loff_t *pos)
> >>  {
> >> -	struct list_head *p;
> >> -	loff_t l = *pos;
> >> -
> >>  	mutex_lock(&block_subsys_lock);
> >> -	list_for_each(p, &block_subsys.list)
> >> -		if (!l--)
> >> -			return list_entry(p, struct gendisk, kobj.entry);
> >> -	return NULL;
> >> +	return seq_list_start_head(&block_subsys.list, *pos);
> >>  }
> >>  
> >> -static void *part_next(struct seq_file *part, void *v, loff_t *pos)
> >> +static void *disk_next(struct seq_file *part, void *v, loff_t *pos)
> >>  {
> >> -	struct list_head *p = ((struct gendisk *)v)->kobj.entry.next;
> >> -	++*pos;
> >> -	return p==&block_subsys.list ? NULL :
> >> -		list_entry(p, struct gendisk, kobj.entry);
> >> +	return seq_list_next(v, &block_subsys.list, pos);
> >>  }
> >>  
> >> -static void part_stop(struct seq_file *part, void *v)
> >> +static void disk_stop(struct seq_file *part, void *v)
> >>  {
> >>  	mutex_unlock(&block_subsys_lock);
> >>  }
> >>  
> >>  static int show_partition(struct seq_file *part, void *v)
> >>  {
> >> -	struct gendisk *sgp = v;
> >> +	struct gendisk *sgp;
> >>  	int n;
> >>  	char buf[BDEVNAME_SIZE];
> >>  
> >> -	if (&sgp->kobj.entry == block_subsys.list.next)
> >> +	if (v == &block_subsys.list) {
> >>  		seq_puts(part, "major minor  #blocks  name\n\n");
> >> +		return 0;
> >> +	}
> >> +
> >> +	sgp = list_entry(v, struct gendisk, kobj.entry);
> >>  
> >>  	/* Don't show non-partitionable removeable devices or empty devices */
> >>  	if (!get_capacity(sgp) ||
> >> @@ -325,9 +320,9 @@ static int show_partition(struct seq_fil
> >>  }
> >>  
> >>  struct seq_operations partitions_op = {
> >> -	.start =part_start,
> >> -	.next =	part_next,
> >> -	.stop =	part_stop,
> >> +	.start =disk_start,
> >> +	.next =	disk_next,
> >> +	.stop =	disk_stop,
> >>  	.show =	show_partition
> >>  };
> >>  #endif
> >> @@ -616,44 +611,22 @@ decl_subsys(block, &ktype_block, &block_
> >>   */
> >>  
> >>  /* iterator */
> >> -static void *diskstats_start(struct seq_file *part, loff_t *pos)
> >> -{
> >> -	loff_t k = *pos;
> >> -	struct list_head *p;
> >> -
> >> -	mutex_lock(&block_subsys_lock);
> >> -	list_for_each(p, &block_subsys.list)
> >> -		if (!k--)
> >> -			return list_entry(p, struct gendisk, kobj.entry);
> >> -	return NULL;
> >> -}
> >> -
> >> -static void *diskstats_next(struct seq_file *part, void *v, loff_t *pos)
> >> -{
> >> -	struct list_head *p = ((struct gendisk *)v)->kobj.entry.next;
> >> -	++*pos;
> >> -	return p==&block_subsys.list ? NULL :
> >> -		list_entry(p, struct gendisk, kobj.entry);
> >> -}
> >> -
> >> -static void diskstats_stop(struct seq_file *part, void *v)
> >> -{
> >> -	mutex_unlock(&block_subsys_lock);
> >> -}
> >> -
> >>  static int diskstats_show(struct seq_file *s, void *v)
> >>  {
> >> -	struct gendisk *gp = v;
> >> +	struct gendisk *gp;
> >>  	char buf[BDEVNAME_SIZE];
> >>  	int n = 0;
> >>  
> >> +	if (v == &block_subsys.list)
> >>  	/*
> >> -	if (&sgp->kobj.entry == block_subsys.kset.list.next)
> >>  		seq_puts(s,	"major minor name"
> >>  				"     rio rmerge rsect ruse wio wmerge "
> >>  				"wsect wuse running use aveq"
> >>  				"\n\n");
> >>  	*/
> >> +		return 0;
> >> +
> >> +	gp = list_entry(v, struct gendisk, kobj.entry);
> >>   
> >>  	preempt_disable();
> >>  	disk_round_stats(gp);
> >> @@ -686,9 +659,9 @@ static int diskstats_show(struct seq_fil
> >>  }
> >>  
> >>  struct seq_operations diskstats_op = {
> >> -	.start	= diskstats_start,
> >> -	.next	= diskstats_next,
> >> -	.stop	= diskstats_stop,
> >> +	.start	= disk_start,
> >> +	.next	= disk_next,
> >> +	.stop	= disk_stop,
> >>  	.show	= diskstats_show
> >>  };
> >>  
> >> -
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> >> Please read the FAQ at  http://www.tux.org/lkml/
> >>
> > 
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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