Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965018AbaD2Sjm (ORCPT ); Tue, 29 Apr 2014 14:39:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:10681 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757530AbaD2Sjl (ORCPT ); Tue, 29 Apr 2014 14:39:41 -0400 Date: Tue, 29 Apr 2014 20:39:30 +0200 From: Oleg Nesterov To: Andrew Morton , Benjamin LaHaise , Kent Overstreet Cc: Al Viro , linux-aio@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/1] aio: change exit_aio() to load mm->ioctx_table once and avoid rcu_read_lock() Message-ID: <20140429183930.GA32521@redhat.com> References: <20140429183915.GA32513@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140429183915.GA32513@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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(). Signed-off-by: Oleg Nesterov --- fs/aio.c | 47 ++++++++++++++++++----------------------------- 1 files changed, 18 insertions(+), 29 deletions(-) diff --git a/fs/aio.c b/fs/aio.c index 12a3de0..5fd1fe7 100644 --- a/fs/aio.c +++ b/fs/aio.c @@ -777,40 +777,29 @@ 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; - } - - ctx = table->table[i++]; - } while (!ctx); + struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table); + int i; - rcu_read_unlock(); + if (!table) + return; + for (i = 0; i < table->nr; ++i) { + struct kioctx *ctx = table->table[i]; /* - * 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); + if (ctx) { + ctx->mmap_size = 0; + kill_ioctx(mm, ctx); + } } + + rcu_assign_pointer(mm->ioctx_table, NULL); + kfree(table); } static void put_reqs_available(struct kioctx *ctx, unsigned nr) -- 1.5.5.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/