From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?iso-8859-1?Q?N=E9lio?= Laranjeiro Subject: Re: [PATCH] net/mlx5: fix error number handling Date: Thu, 7 Jun 2018 09:39:44 +0200 Message-ID: <20180607073944.zewdysx6ddrdygoz@laranjeiro-vm.dev.6wind.com> References: <20180604173731.29125-1-yskoh@mellanox.com> <20180605065246.mw7xnk24cfwxy4an@laranjeiro-vm.dev.6wind.com> <84C956B6-28EE-4F82-97AE-5E9C371DD115@mellanox.com> <20180606065501.hhrfrti47nr5xigo@laranjeiro-vm.dev.6wind.com> <20180606183926.GA1446@yongseok-MBP.local> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Cc: Adrien Mazarguil , "dev@dpdk.org" , Shahaf Shuler , dpdk stable , "Xueming(Steven) Li" To: Yongseok Koh Return-path: Received: from mail-wr0-f194.google.com (mail-wr0-f194.google.com [209.85.128.194]) by dpdk.org (Postfix) with ESMTP id DAE8D1AFEC for ; Thu, 7 Jun 2018 09:39:14 +0200 (CEST) Received: by mail-wr0-f194.google.com with SMTP id d2-v6so8889226wrm.10 for ; Thu, 07 Jun 2018 00:39:14 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20180606183926.GA1446@yongseok-MBP.local> List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Wed, Jun 06, 2018 at 11:39:27AM -0700, Yongseok Koh wrote: > On Wed, Jun 06, 2018 at 08:55:01AM +0200, Nélio Laranjeiro wrote: > > On Tue, Jun 05, 2018 at 09:36:32PM +0000, Yongseok Koh wrote: > > > > On Jun 4, 2018, at 11:52 PM, Nélio Laranjeiro wrote: > > > > > > > > On Mon, Jun 04, 2018 at 10:37:31AM -0700, Yongseok Koh wrote: > > > >> rte_errno should be saved only if error has occurred because rte_errno > > > >> could have garbage value. > > > >> > > > >> Fixes: a6d83b6a9209 ("net/mlx5: standardize on negative errno values") > > > >> Cc: stable@dpdk.org > > > >> > > > >> Signed-off-by: Yongseok Koh > > > >> --- > > > >> drivers/net/mlx5/mlx5_flow.c | 3 ++- > > > >> 1 file changed, 2 insertions(+), 1 deletion(-) > > > >> > > > >> diff --git a/drivers/net/mlx5/mlx5_flow.c b/drivers/net/mlx5/mlx5_flow.c > > > >> index 994be05be..eaffe7495 100644 > > > >> --- a/drivers/net/mlx5/mlx5_flow.c > > > >> +++ b/drivers/net/mlx5/mlx5_flow.c > > > >> @@ -3561,7 +3561,8 @@ mlx5_fdir_filter_delete(struct rte_eth_dev *dev, > > > >> /* The flow does not match. */ > > > >> continue; > > > >> } > > > >> - ret = rte_errno; /* Save rte_errno before cleanup. */ > > > >> + if (ret) > > > >> + ret = rte_errno; /* Save rte_errno before cleanup. */ > > > >> if (flow) > > > >> mlx5_flow_list_destroy(dev, &priv->flows, flow); > > > >> exit: > > > >> -- > > > >> 2.11.0 > > > > > > > > This patch is not enough, the returned value being -rte_errno if no > > > > error is detected by the function it cannot set rte_errno nor return it. > > > > > > We may need to refactor this kind of code (saving and restoring rte_errno). I > > > still don't understand why we should preserve rte_errno like this. > > > > > > Even if this function returns success, there's no obligation to preserve > > > rte_errno in the function. Once it is called, the ownership of rte_errno belongs > > > to this function. > > > > > > I can't find how we define this per-lcore variable but, from > > > the man page of errno, > > > > > > The header file defines the integer variable errno, which > > > is set by system calls and some library functions in the event of an > > > error to indicate what went wrong. Its value is significant only when > > > the return value of the call indicated an error (i.e., -1 from most > > > system calls; -1 or NULL from most library functions); > > > a function that succeeds is allowed to change errno. > > > > > > So, I still think an API can change rte_errno even if it succeeds, no need to > > > preserve it. If needed, the caller has to save it. > > > > Functions in this PMD are defined as is: > > > > * @return > > * 0 on success, a negative errno value otherwise and rte_errno is set. > > > > Which means rte_errno is only modified in case of error. > > > > This fix does not respect the documentation of the function or any other > > function of the PMD which can return errors. > > That's logically a wrong interpretation. According to the description, if > returning error, rte_errno is set but the opposite isn't always true. Even if > rte_errno is set, it doesn't mean there's an error. So the description coincides > with that of errno. If you want to enforce preserving rte_errno in case of > success, you should amend the documentation. > > > rte_errno is only set if an error is encountered and contains only the error > > code of the first error sub-sequent ones are considered consequences of the > > first one and thus not preserved. > > > > Not preserving the rte_errno in roll backs is equivalent to not setting > > it at all as a function called by the rollback may also set it, example: > > > > { > > void * a; > > > > foo_do(); > > a = malloc(10); > > if (!a) { > > rte_errno = ENOMEM; > > foo_undo(); > > This example is weird. You can simply set rte_errno after foo_undo() in this > case. > > > return -rte_errno; > > } > > } > > > > If foo_undo() also encounter an error it will modify the rte_errno which > > may have a value different from ENOMEM, for the callee won't be informed > > the error is due to a memory issue and thus cannot make counter parts. > > In such situation the rte_errno must be preserved to keep the ENOMEM > > error code. > > I knew it. That's why rte_errno is saved before calling another API which may > change the rte_errno inside. But, we are talking about a case where an API > returns success. If caller is supposed to save rte_errno (when it's needed), why > does callee have to put some effort to preserve it even in case of success? If > rte_errno must be preserved even in case of success, we have to make a big > change to preserve rte_errno for cases where a void function is called (or cases > where we don't check its return value of non-void function). > > > This is also the main reason almost all system function only update > > errno when no error is encountered. > > 'Almost' doesn't mean 'all", does it? It is true that such functions must update > errno when it returns error but it doesn't care about the value when it returns > success. Like the man page I attached above, the errno is significant only when > it returns an error. And "a function that succeeds is allowed to change errno." It is "almost" because a system function touching the errno when the function succeed it not common. But as the man page says it is not impossible. > So, the decision point is whether we want to preserve rte_errno in case of > success? My opinion is no. I did not understood it was only a concern about the success of the function, even it is better to avoid as most as possible a useless store, in this specific case, as errno (rte_errno) has a garbage value, I fully agree with you. Regards, -- Nélio Laranjeiro 6WIND