From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753981AbbAVRDE (ORCPT ); Thu, 22 Jan 2015 12:03:04 -0500 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:37510 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753780AbbAVRC6 (ORCPT ); Thu, 22 Jan 2015 12:02:58 -0500 Message-ID: <1421946175.2093.1.camel@HansenPartnership.com> Subject: Re: module: fix module_refcount() return when running in a module exit routine From: James Bottomley To: Christoph Hellwig Cc: Rusty Russell , Masami Hiramatsu , linux-kernel , linux-scsi Date: Thu, 22 Jan 2015 09:02:55 -0800 In-Reply-To: <20150122165018.GA27377@infradead.org> References: <1421600146.2080.8.camel@HansenPartnership.com> <54BC93C3.7010808@hitachi.com> <878ugzco8c.fsf@rustcorp.com.au> <1421683701.2080.25.camel@HansenPartnership.com> <871tmqcmau.fsf@rustcorp.com.au> <1421774615.2080.88.camel@HansenPartnership.com> <20150122165018.GA27377@infradead.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.7 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2015-01-22 at 08:50 -0800, Christoph Hellwig wrote: > On Tue, Jan 20, 2015 at 09:23:35AM -0800, James Bottomley wrote: > > On Tue, 2015-01-20 at 11:15 +1030, Rusty Russell wrote: > > > James Bottomley writes: > > > > On Mon, 2015-01-19 at 16:21 +1030, Rusty Russell wrote: > > > >> Masami Hiramatsu writes: > > > >> > (2015/01/19 1:55), James Bottomley wrote: > > > >> >> From: James Bottomley > > > >> >> > > > >> >> After e513cc1 module: Remove stop_machine from module unloading, > > > >> >> module_refcount() is returning (unsigned long)-1 when called from within > > > >> >> a routine that runs in module_exit. This is confusing the scsi device > > > >> >> put code which is coded to detect a module_refcount() of zero for > > > >> >> running within a module exit routine and not try to do another > > > >> >> module_put. The fix is to restore the original behaviour of > > > >> >> module_refcount() and return zero if we're running inside an exit > > > >> >> routine. > > > >> >> > > > >> >> Fixes: e513cc1c07e2ab93a4514eec9833e031df3e30bb > > > >> >> Reported-by: Bart Van Assche > > > >> >> Signed-off-by: James Bottomley > > > >> >> > > > >> > > > > >> > Yes, this should be fixed as you said, since it must return "unsigned long" value. > > > >> > > > >> But there are only three non-module callers: > > > >> > > > >> drivers/scsi/scsi.c:1012: if (module && module_refcount(module) != 0) > > > >> drivers/staging/lustre/lustre/obdclass/lu_object.c:1359: LINVRNT(module_refcount(key->lct_owner) > 0); > > > >> include/linux/module.h:447:unsigned long module_refcount(struct module *mod); > > > >> kernel/debug/kdb/kdb_main.c:2026: kdb_printf("%4ld ", module_refcount(mod)); > > > >> kernel/module.c:775:unsigned long module_refcount(struct module *mod) > > > >> kernel/module.c:779:EXPORT_SYMBOL(module_refcount); > > > >> kernel/module.c:859: seq_printf(m, " %lu ", module_refcount(mod)); > > > >> kernel/module.c:911: return sprintf(buffer, "%lu\n", module_refcount(mk->mod)); > > > >> > > > >> The first one I think should be eliminated, and the second one is simply > > > >> an assertion before calling module_put() (which should probably be > > > >> eliminated). The others are just printing information. > > > > > > > > If you really want to insist on module_reference() returning -1 when the > > > > module is in it's exit phase, OK, but in that case, I think it should > > > > return a signed value, not an unsigned one. > > > > > > Sure; I just didn't want to paper over the problem here. And I'm not > > > sure we want to lose information, eg. in kgdb we're presumably looking > > > at it because something went wrong... > > > > > > Thanks, > > > Rusty. > > > > > > Subject: module: make module_refcount() a signed integer. > > > > > > James Bottomley points out that it will be -1 during unload. It's > > > only used for diagnostics, so let's not hide that as it could be a > > > clue as to what's gone wrong. > > > > > > Cc: Jason Wessel > > > Signed-off-by: Rusty Russell > > > > > > diff --git a/include/linux/module.h b/include/linux/module.h > > > index ebfb0e153c6a..b653d7c0a05a 100644 > > > --- a/include/linux/module.h > > > +++ b/include/linux/module.h > > > @@ -444,7 +444,7 @@ extern void __module_put_and_exit(struct module *mod, long code) > > > #define module_put_and_exit(code) __module_put_and_exit(THIS_MODULE, code) > > > > > > #ifdef CONFIG_MODULE_UNLOAD > > > -unsigned long module_refcount(struct module *mod); > > > +int module_refcount(struct module *mod); > > > void __symbol_put(const char *symbol); > > > #define symbol_put(x) __symbol_put(VMLINUX_SYMBOL_STR(x)) > > > void symbol_put_addr(void *addr); > > > diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c > > > index f191bddf64b8..7b40c5f07dce 100644 > > > --- a/kernel/debug/kdb/kdb_main.c > > > +++ b/kernel/debug/kdb/kdb_main.c > > > @@ -2023,7 +2023,7 @@ static int kdb_lsmod(int argc, const char **argv) > > > kdb_printf("%-20s%8u 0x%p ", mod->name, > > > mod->core_size, (void *)mod); > > > #ifdef CONFIG_MODULE_UNLOAD > > > - kdb_printf("%4ld ", module_refcount(mod)); > > > + kdb_printf("%4d ", module_refcount(mod)); > > > #endif > > > if (mod->state == MODULE_STATE_GOING) > > > kdb_printf(" (Unloading)"); > > > diff --git a/kernel/module.c b/kernel/module.c > > > index 3965511ae133..2387c98347c1 100644 > > > --- a/kernel/module.c > > > +++ b/kernel/module.c > > > @@ -772,9 +772,9 @@ static int try_stop_module(struct module *mod, int flags, int *forced) > > > return 0; > > > } > > > > > > -unsigned long module_refcount(struct module *mod) > > > +int module_refcount(struct module *mod) > > > { > > > - return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE; > > > + return atomic_read(&mod->refcnt) - MODULE_REF_BASE; > > > } > > > EXPORT_SYMBOL(module_refcount); > > > > > > @@ -856,7 +856,7 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod) > > > struct module_use *use; > > > int printed_something = 0; > > > > > > - seq_printf(m, " %lu ", module_refcount(mod)); > > > + seq_printf(m, " %i ", module_refcount(mod)); > > > > > > /* > > > * Always include a trailing , so userspace can differentiate > > > @@ -908,7 +908,7 @@ EXPORT_SYMBOL_GPL(symbol_put_addr); > > > static ssize_t show_refcnt(struct module_attribute *mattr, > > > struct module_kobject *mk, char *buffer) > > > { > > > - return sprintf(buffer, "%lu\n", module_refcount(mk->mod)); > > > + return sprintf(buffer, "%i\n", module_refcount(mk->mod)); > > > } > > > > Actually, I don't think this is enough. Some Australian once came up > > with a guide to APIs, and lectured on it at length, one of which was > > that the name should be the obvious use and it is unexpected that a > > refcount would go negative. I think we could raise it from -6 on the > > API scale to +3 if we add some documentation like below. > > We'll also still need to change scsi_device_put to deal with > a negative refcount.. I don't believe so ... we never call module_refcount() now, and the actual module ref count never goes negative. That's the point of Rusty's change. In the unload routines, the actual module refcount is zero. __module_get() will increment this and the final module_put() will decrement it without triggering a warning. In the old code, we expected try_module_get() to fail but then used module_refcount() to detect the failure and not do a put. James