From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8AEFD2C2374; Mon, 23 Mar 2026 23:52:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.167.242.64 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774309970; cv=none; b=LxFkXiMaJpqiOa32MCcV+1DmR/azOSy0qnGwS7OABgnXLBIRJmTi7ghMx3f/Q3OgrNSix5ReWcx2MmuBo/ZNO9gSIrMfZ6pKy7lx08ZJM7uGbI7ZjCetM841td8d0hEs4JIHRPCfFixkRxkbENxbheB2M2Jy7CixO8itV7/ZnFw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774309970; c=relaxed/simple; bh=8NOTqTm6GBIgnbyNEwFerZ3BM38VpLsHMWVyEESeMCI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kP/kScQY8+G1hGG4F2R9VSseUqO+YYz83onscBysvB1hUKzbBut22RPKmMF9IQxAgsXTFN4dyiN5RwG1ubvFBI0+gWo2EEvdhhc4zn0WhS8U4r+KEm6xzRmAJ7KykgK5ca6miqvO76hHYdf8tquz09L3nkyYBJAqNdTyaEJf2iY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com; spf=pass smtp.mailfrom=ideasonboard.com; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b=J5A2fJRz; arc=none smtp.client-ip=213.167.242.64 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="J5A2fJRz" Received: from killaraus.ideasonboard.com (2001-14ba-703d-e500--2a1.rev.dnainternet.fi [IPv6:2001:14ba:703d:e500::2a1]) by perceval.ideasonboard.com (Postfix) with UTF8SMTPSA id BF436741; Tue, 24 Mar 2026 00:51:29 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1774309889; bh=8NOTqTm6GBIgnbyNEwFerZ3BM38VpLsHMWVyEESeMCI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=J5A2fJRzbzJmX9kWnxnS7mWlTBHodzO5TC4GaB77vQJTPW9J8vtD9XsjrmObjE+IO ll35RCSljDWVVh9iL8zZdDB1qhmbLrxt0CkHxx4SR0CU+28pES1mLdgMjUpjRkeTPT bQi9HdUs1Y8Yb6srFearv8IEJGBwV0rQqZRwhgTs= Date: Tue, 24 Mar 2026 01:52:45 +0200 From: Laurent Pinchart To: Sean Anderson Cc: Hans de Goede , Ricardo Ribalda , linux-media@vger.kernel.org, Mauro Carvalho Chehab , linux-kernel@vger.kernel.org, Hans Verkuil Subject: Re: [PATCH v2] media: uvcvideo: Fix deadlock if uvc_status_stop is called from async_ctrl.work Message-ID: <20260323235245.GB2334070@killaraus.ideasonboard.com> References: <20260316155823.1855434-1-sean.anderson@linux.dev> Precedence: bulk X-Mailing-List: linux-media@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260316155823.1855434-1-sean.anderson@linux.dev> Hi Sean, Thank you for the patch. On Mon, Mar 16, 2026 at 11:58:22AM -0400, Sean Anderson wrote: > If a UVC camera has an asynchronous control, uvc_status_stop may be > called from async_ctrl.work: > > uvc_ctrl_status_event_work() > uvc_ctrl_status_event() > uvc_ctrl_clear_handle() > uvc_pm_put() > uvc_status_put() > uvc_status_stop() > cancel_work_sync() > > This will cause a deadlock, since cancel_work_sync will wait for > uvc_ctrl_status_event_work to complete before returning. > > Fix this by returning early from uvc_status_stop if we are currently in > the work function. flush_status now remains false until uvc_status_start > is called again, ensuring that uvc_ctrl_status_event_work won't resubmit > the URB. > > Fixes: a32d9c41bdb8 ("media: uvcvideo: Make power management granular") > Closes: https://lore.kernel.org/all/6733bdfb-3e88-479f-8956-ab09c04c433e@linux.dev/ > Signed-off-by: Sean Anderson > --- > > Changes in v2: > - Update comments with review feedback > - Use flush_work instead of cancel_work_sync since the work should never > be rescheduled. > > drivers/media/usb/uvc/uvc_status.c | 27 ++++++++++++++++++--------- > 1 file changed, 18 insertions(+), 9 deletions(-) > > diff --git a/drivers/media/usb/uvc/uvc_status.c b/drivers/media/usb/uvc/uvc_status.c > index 231cfee8e7c2c..ea45b11642e59 100644 > --- a/drivers/media/usb/uvc/uvc_status.c > +++ b/drivers/media/usb/uvc/uvc_status.c > @@ -316,6 +316,15 @@ static int uvc_status_start(struct uvc_device *dev, gfp_t flags) > if (!dev->int_urb) > return 0; > > + /* > + * If the work called uvc_status_stop it may still be running. Wait for > + * it to finish before we submit the urb. > + */ > + flush_work(&dev->async_ctrl.work); I assume the only way this can happen is for uvc_status_start() to run after the uvc_status_put() call in uvc_ctrl_status_event_work() uvc_ctrl_status_event() uvc_ctrl_clear_handle() uvc_pm_put() uvc_status_put() returns. Is that right ? If so I'd like to capture that better in the comment, as I have a feeling we'll need to revisit this code at some point. /* * If the previous uvc_status_stop() call was from the async work, the * work may still be running. Wait for it to finish before we submit the * urb. */ And if that's not correct, there's something I don't get and a better comment is even more important :-) If the above change is fine, I can update the comment when applying, and Reviewed-by: Laurent Pinchart This being said, I wonder if we could have a simpler implementation if we always stopped the work and URB when the last userspace user goes away, instead of keeping the device active until we get the asynchronous control notification. > + > + /* Clear the flush status if we were previously stopped. */ > + smp_store_release(&dev->flush_status, false); > + > return usb_submit_urb(dev->int_urb, flags); > } > > @@ -336,6 +345,15 @@ static void uvc_status_stop(struct uvc_device *dev) > */ > smp_store_release(&dev->flush_status, true); > > + /* > + * If we are called from the event work function, the URB is guaranteed > + * to not be in flight as it has completed and has not been resubmitted. > + * There's no need to cancel the work (which would deadlock), or to kill > + * the URB. > + */ > + if (current_work() == &w->work) > + return; > + > /* > * Cancel any pending asynchronous work. If any status event was queued, > * process it synchronously. > @@ -354,15 +372,6 @@ static void uvc_status_stop(struct uvc_device *dev) > */ > if (cancel_work_sync(&w->work)) > uvc_ctrl_status_event(w->chain, w->ctrl, w->data); > - > - /* > - * From this point, there are no events on the queue and the status URB > - * is dead. No events will be queued until uvc_status_start() is called. > - * The barrier is needed to make sure that flush_status is visible to > - * uvc_ctrl_status_event_work() when uvc_status_start() will be called > - * again. > - */ > - smp_store_release(&dev->flush_status, false); > } > > int uvc_status_resume(struct uvc_device *dev) -- Regards, Laurent Pinchart