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

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>

1 files changed, 16 insertions, 26 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 955947ef3e02..b6696462e345 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -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;
-
-        while (1) {
-                rcu_read_lock();
-                table = rcu_dereference(mm->ioctx_table);
-
-                do {
-                        if (!table || i >= table->nr) {
-                                rcu_read_unlock();
-                                rcu_assign_pointer(mm->ioctx_table, NULL);
-                                if (table)
-                                        kfree(table);
-                                return;
-                        }
+        struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table);
+        int i;
 
-                        ctx = table->table[i++];
-                } while (!ctx);
+        if (!table)
+                return;
 
-                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)