From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?ISO-8859-1?Q?Christian_K=F6nig?= Subject: Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation for fences Date: Thu, 15 May 2014 11:21:06 +0200 Message-ID: <53748702.6070606@vodafone.de> References: <20140514145134.21163.32350.stgit@patser> <20140514145809.21163.64947.stgit@patser> <53738BCC.2070809@vodafone.de> <5374131D.4010906@canonical.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1"; Format="flowed" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <5374131D.4010906-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: nouveau-bounces-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org Sender: "Nouveau" To: Maarten Lankhorst , airlied-cv59FeDIM0c@public.gmane.org Cc: nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org List-Id: dri-devel@lists.freedesktop.org Am 15.05.2014 03:06, schrieb Maarten Lankhorst: > op 14-05-14 17:29, Christian K=F6nig schreef: >>> + /* did fence get signaled after we enabled the sw irq? */ >>> + if = >>> (atomic64_read(&fence->rdev->fence_drv[fence->ring].last_seq) >=3D = >>> fence->seq) { >>> + radeon_irq_kms_sw_irq_put(fence->rdev, fence->ring); >>> + return false; >>> + } >>> + >>> + fence->fence_wake.flags =3D 0; >>> + fence->fence_wake.private =3D NULL; >>> + fence->fence_wake.func =3D radeon_fence_check_signaled; >>> + __add_wait_queue(&fence->rdev->fence_queue, &fence->fence_wake); >>> + fence_get(f); >> That looks like a race condition to me. The fence needs to be added = >> to the wait queue before the check, not after. >> >> Apart from that the whole approach looks like a really bad idea to = >> me. How for example is lockup detection supposed to happen with this? = > It's not a race condition because fence_queue.lock is held when this = > function is called. Ah, I see. That's also the reason why you moved the wake_up_all out of = the processing function. > > Lockup's a bit of a weird problem, the changes wouldn't allow core ttm = > code to handle the lockup any more, > but any driver specific wait code would still handle this. I did this = > by design, because in future patches the wait > function may be called from outside of the radeon driver. The official = > wait function takes a timeout parameter, > so lockups wouldn't be fatal if the timeout is set to something like = > 30*HZ for example, it would still return > and report that the function timed out. Timeouts help with the detection of the lockup, but not at all with the = handling of them. What we essentially need is a wait callback into the driver that is = called in non atomic context without any locks held. This way we can block for the fence to become signaled with a timeout = and can then also initiate the reset handling if necessary. The way you designed the interface now means that the driver never gets = a chance to wait for the hardware to become idle and so never has the = opportunity to the reset the whole thing. Christian. > > ~Maarten