From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH 3/3] drm/amdgpu: Switch to interrupted wait to recover from ring hang. Date: Wed, 25 Apr 2018 15:55:24 -0500 Message-ID: <87h8nzt39f.fsf@xmission.com> References: <1524583836-12130-1-git-send-email-andrey.grodzovsky@amd.com> <1524583836-12130-4-git-send-email-andrey.grodzovsky@amd.com> <87bme8bm9g.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: (Andrey Grodzovsky's message of "Wed, 25 Apr 2018 13:17:40 -0400") Sender: linux-kernel-owner@vger.kernel.org To: Andrey Grodzovsky Cc: "Panariti, David" , "linux-kernel@vger.kernel.org" , "amd-gfx@lists.freedesktop.org" , "Deucher, Alexander" , "Koenig, Christian" , "oleg@redhat.com" , "akpm@linux-foundation.org" List-Id: amd-gfx.lists.freedesktop.org Andrey Grodzovsky writes: > On 04/24/2018 12:30 PM, Eric W. Biederman wrote: >> "Panariti, David" writes: >> >>> Andrey Grodzovsky writes: >>>> Kind of dma_fence_wait_killable, except that we don't have such API >>>> (maybe worth adding ?) >>> Depends on how many places it would be called, or think it might be called. Can always factor on the 2nd time it's needed. >>> Factoring, IMO, rarely hurts. The factored function can easily be visited using `M-.' ;-> >>> >>> Also, if the wait could be very long, would a log message, something like "xxx has run for Y seconds." help? >>> I personally hate hanging w/no info. >> Ugh. This loop appears susceptible to loosing wake ups. There are >> races between when a wake-up happens, when we clear the sleeping state, >> and when we test the stat to see if we should stat awake. So yes >> implementing a dma_fence_wait_killable that handles of all that >> correctly sounds like an very good idea. > > I am not clear here - could you be more specific about what races will happen > here, more bellow >> >> Eric >> >> >>>> If the ring is hanging for some reason allow to recover the waiting by sending fatal signal. >>>> >>>> Originally-by: David Panariti >>>> Signed-off-by: Andrey Grodzovsky >>>> --- >>>> drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 14 ++++++++++---- >>>> 1 file changed, 10 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c >>>> index eb80edf..37a36af 100644 >>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c >>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c >>>> @@ -421,10 +421,16 @@ int amdgpu_ctx_wait_prev_fence(struct amdgpu_ctx *ctx, unsigned ring_id) >>>> >>>> if (other) { >>>> signed long r; >>>> - r = dma_fence_wait_timeout(other, false, MAX_SCHEDULE_TIMEOUT); >>>> - if (r < 0) { >>>> - DRM_ERROR("Error (%ld) waiting for fence!\n", r); >>>> - return r; >>>> + >>>> + while (true) { >>>> + if ((r = dma_fence_wait_timeout(other, true, >>>> + MAX_SCHEDULE_TIMEOUT)) >= 0) >>>> + return 0; >>>> + > > Do you mean that by the time I reach here some other thread from my group > already might dequeued SIGKILL since it's a shared signal and hence > fatal_signal_pending will return false ? Or are you talking about the > dma_fence_wait_timeout implementation in dma_fence_default_wait with > schedule_timeout ? Given Oleg's earlier comment about the scheduler having special cases for signals I might be wrong. But in general there is a pattern: for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (loop_is_done()) break; schedule(); } set_current_state(TASK_RUNNING); If you violate that pattern by testing for a condition without having first set your task as TASK_UNINTERRUPTIBLE (or whatever your sleep state is). Then it is possible to miss a wake-up that tests the condidtion. Thus I am quite concerned that there is a subtle corner case where you can miss a wakeup and not retest fatal_signal_pending(). Given that there is is a timeout the worst case might have you sleep MAX_SCHEDULE_TIMEOUT instead of indefinitely. Without a comment why this is safe, or having fatal_signal_pending check integrated into dma_fence_wait_timeout I am not comfortable with this loop. Eric >>>> + if (fatal_signal_pending(current)) { >>>> + DRM_ERROR("Error (%ld) waiting for fence!\n", r); >>>> + return r; >>>> + } >>>> } >>>> } >>>> >>>> -- >>>> 2.7.4 >>>> >> Eric