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[212.114.229.10]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43eb5fd1194sm29516765e9.11.2025.04.02.12.13.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Apr 2025 12:13:05 -0700 (PDT) Message-ID: <3296aaff-a591-4fcd-b421-9cfcc6291d2b@suse.com> Date: Wed, 2 Apr 2025 21:13:04 +0200 Precedence: bulk X-Mailing-List: linux-usb@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Handling incoming ZLP in cdc-wdm To: Robert Hodaszi , Oliver Neukum , "linux-usb@vger.kernel.org" , =?UTF-8?Q?Bj=C3=B8rn_Mork?= References: <28a5c207-6c2e-42e1-9568-2f949554026e@suse.com> <8de930f3-2e87-443d-9e8f-9d6f16f7a399@digi.com> Content-Language: en-US From: Oliver Neukum In-Reply-To: <8de930f3-2e87-443d-9e8f-9d6f16f7a399@digi.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 02.04.25 17:01, Robert Hodaszi wrote: > > But we cannot return with -EAGAIN. If we do that, we're back to sqrt(1), and get stuck again. Then we have a problem. If we are servicing a read() syscall, we have to either return data, or, if we cannot do that we either sleep or return -EAGAIN depending on O_NONBLOCK. There is nothing we can do about that. This makes me think that the issue here is poll() rather than in wdm_read() > So what about modifying the service_interrupt_work to no simply set WDM_READ if resp_count is 0, but instead to check if there's any real message in the buffer, to not confuse consumers. Something like this: That specific proposal will not work because the issue is in service_interrupt_work() which can already be scheduled. We cannot prevent that. >    diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c >    index 37873acd18f4..9037379f3603 100644 >    --- a/drivers/usb/class/cdc-wdm.c >    +++ b/drivers/usb/class/cdc-wdm.c >    @@ -1010,7 +1010,7 @@ static void service_interrupt_work(struct >    work_struct *work) > >             spin_lock_irq(&desc->iuspin); >             service_outstanding_interrupt(desc); >    -       if (!desc->resp_count) { >    +       if (!desc->resp_count && (desc->length || desc->rerr)) { >                     set_bit(WDM_READ, &desc->flags); >                     wake_up(&desc->wait); And what happens if wdm_read() wakes up because a signal is delivered? Regards Oliver