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aio: change exit_aio() to load mm->ioctx_table once and avoid rcu_read_lock()

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On 04/30, Benjamin LaHaise wrote:
>
> > -		ctx->mmap_size = 0;
> > -
> > -		kill_ioctx(mm, ctx, NULL);
> > +		if (ctx) {
> > +			ctx->mmap_size = 0;
> > +			kill_ioctx(mm, ctx, NULL);
> > +		}
>
> Rather than indenting and moving the two lines changing mmap_size and the
> kill_ioctx() call, why not just do "if (!ctx) ... continue;"?  That reduces
> the number of lines changed and avoid excessive indentation.

OK. To me the code looks better/simpler with "if (ctx)", but this is subjective
of course, I won't argue.

The patch still removes the empty line between mmap_size = 0 and kill_ioctx(),
we reset mmap_size only for kill_ioctx(). But feel free to remove this change.

-------------------------------------------------------------------------------
Subject: [PATCH v3 1/2] aio: change exit_aio() to load mm->ioctx_table once and avoid rcu_read_lock()

1. We can read ->ioctx_table only once and we do not read rcu_read_lock()
   or even rcu_dereference().

   This mm has no users, nobody else can play with ->ioctx_table. Otherwise
   the code is buggy anyway, if we need rcu_read_lock() in a loop because
   ->ioctx_table can be updated then kfree(table) is obviously wrong.

2. Update the comment. "exit_mmap(mm) is coming" is the good reason to avoid
   munmap(), but another reason is that we simply can't do vm_munmap() unless
   current->mm == mm and this is not true in general, the caller is mmput().

3. We do not really need to nullify mm->ioctx_table before return, probably
   the current code does this to catch the potential problems. But in this
   case RCU_INIT_POINTER(NULL) looks better.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
This commit is contained in:
Oleg Nesterov 2014-04-30 19:02:48 +02:00 committed by Benjamin LaHaise
parent edfbbf388f
commit 4b70ac5fd9
1 changed files with 16 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
fs/aio.c
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@ -791,40 +791,30 @@ EXPORT_SYMBOL(wait_on_sync_kiocb);
*/
void exit_aio(struct mm_struct *mm)
{
struct kioctx_table *table;
struct kioctx *ctx;
unsigned i = 0;
struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table);
int i;
while (1) {
rcu_read_lock();
table = rcu_dereference(mm->ioctx_table);
if (!table)
return;
do {
if (!table || i >= table->nr) {
rcu_read_unlock();
rcu_assign_pointer(mm->ioctx_table, NULL);
if (table)
kfree(table);
return;
}
ctx = table->table[i++];
} while (!ctx);
rcu_read_unlock();
for (i = 0; i < table->nr; ++i) {
struct kioctx *ctx = table->table[i];
if (!ctx)
continue;
/*
* We don't need to bother with munmap() here -
* exit_mmap(mm) is coming and it'll unmap everything.
* Since aio_free_ring() uses non-zero ->mmap_size
* as indicator that it needs to unmap the area,
* just set it to 0; aio_free_ring() is the only
* place that uses ->mmap_size, so it's safe.
* We don't need to bother with munmap() here - exit_mmap(mm)
* is coming and it'll unmap everything. And we simply can't,
* this is not necessarily our ->mm.
* Since kill_ioctx() uses non-zero ->mmap_size as indicator
* that it needs to unmap the area, just set it to 0.
*/
ctx->mmap_size = 0;
kill_ioctx(mm, ctx, NULL);
}
RCU_INIT_POINTER(mm->ioctx_table, NULL);
kfree(table);
}
static void put_reqs_available(struct kioctx *ctx, unsigned nr)