Message-ID: <94bc32b2-f563-4e56-baea-cf461ed29829@amd.com>
Date: Mon, 22 Sep 2025 15:47:07 +0200
From: Christian König <christian.koenig@....com>
To: Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>,
 Sumit Semwal <sumit.semwal@...aro.org>
Cc: Gustavo Padovan <gustavo@...ovan.org>,
 Eric Engestrom <eric.engestrom@...tec.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-buf/sw-sync: Hide the feature by default

On 22.09.25 15:24, Janusz Krzysztofik wrote:
> When multiple fences of an sw_sync timeline are signaled via
> sw_sync_ioctl_inc(), we now disable interrupts and keep them disabled
> while signaling all requested fences of the timeline in a loop.  Since
> user space may set up an arbitrary long timeline of fences with
> arbitrarily expensive callbacks added to each fence, we may end up running
> with interrupts disabled for too long, longer than NMI watchdog limit.
> That potentially risky scenario has been demonstrated on Intel DRM CI
> trybot[1], on a low end machine fi-pnv-d510, with one of new IGT subtests
> that tried to reimplement wait_* test cases of a dma_fence_chain selftest
> in user space.
> 
> [141.993704] [IGT] syncobj_timeline: starting subtest stress-enable-all-signal-all-forward
> [164.964389] watchdog: CPU3: Watchdog detected hard LOCKUP on cpu 3
> [164.964407] Modules linked in: snd_hda_codec_alc662 snd_hda_codec_realtek_lib snd_hda_codec_generic snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hda_core snd_hwdep snd_pcm snd_timer snd soundcore i915 prime_numbers ttm drm_buddy drm_display_helper cec rc_core i2c_algo_bit video wmi overlay at24 ppdev gpio_ich binfmt_misc nls_iso8859_1 coretemp i2c_i801 i2c_mux i2c_smbus r8169 lpc_ich realtek parport_pc parport nvme_fabrics dm_multipath fuse msr efi_pstore nfnetlink autofs4
> [164.964569] irq event stamp: 1002206
> [164.964575] hardirqs last  enabled at (1002205): [<ffffffff82898ac7>] _raw_spin_unlock_irq+0x27/0x70
> [164.964599] hardirqs last disabled at (1002206): [<ffffffff8287d021>] sysvec_irq_work+0x11/0xc0
> [164.964616] softirqs last  enabled at (1002138): [<ffffffff81341bc5>] fpu_clone+0xb5/0x270
> [164.964631] softirqs last disabled at (1002136): [<ffffffff81341b97>] fpu_clone+0x87/0x270
> [164.964650] CPU: 3 UID: 0 PID: 1515 Comm: syncobj_timelin Tainted: G     U              6.17.0-rc6-Trybot_154715v1-gc1b827f32471+ #1 PREEMPT(voluntary)
> [164.964662] Tainted: [U]=USER
> [164.964665] Hardware name:  /D510MO, BIOS MOPNV10J.86A.0311.2010.0802.2346 08/02/2010
> [164.964669] RIP: 0010:lock_release+0x13d/0x2a0
> [164.964680] Code: c2 01 48 8d 4d c8 44 89 f6 4c 89 ef e8 bc fc ff ff 0b 05 96 ca 42 06 0f 84 fc 00 00 00 b8 ff ff ff ff 65 0f c1 05 0b 71 a9 02 <83> f8 01 0f 85 2f 01 00 00 48 f7 45 c0 00 02 00 00 74 06 fb 0f 1f
> [164.964686] RSP: 0018:ffffc90000170e70 EFLAGS: 00000057
> [164.964693] RAX: 0000000000000001 RBX: ffffffff83595520 RCX: 0000000000000000
> [164.964698] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> [164.964701] RBP: ffffc90000170eb0 R08: 0000000000000000 R09: 0000000000000000
> [164.964706] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff8226a948
> [164.964710] R13: ffff88802423b340 R14: 0000000000000001 R15: ffff88802423c238
> [164.964714] FS:  0000729f4d972940(0000) GS:ffff8880f8e77000(0000) knlGS:0000000000000000
> [164.964720] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [164.964725] CR2: 0000729f4d92e720 CR3: 000000003afe4000 CR4: 00000000000006f0
> [164.964729] Call Trace:
> [164.964734]  <IRQ>
> [164.964750]  dma_fence_chain_get_prev+0x13d/0x240
> [164.964769]  dma_fence_chain_walk+0xbd/0x200
> [164.964784]  dma_fence_chain_enable_signaling+0xb2/0x280
> [164.964803]  dma_fence_chain_irq_work+0x1b/0x80
> [164.964816]  irq_work_single+0x75/0xa0
> [164.964834]  irq_work_run_list+0x33/0x60
> [164.964846]  irq_work_run+0x18/0x40
> [164.964856]  __sysvec_irq_work+0x35/0x170
> [164.964868]  sysvec_irq_work+0x9b/0xc0
> [164.964879]  </IRQ>
> [164.964882]  <TASK>
> [164.964890]  asm_sysvec_irq_work+0x1b/0x20
> [164.964900] RIP: 0010:_raw_spin_unlock_irq+0x2d/0x70
> [164.964907] Code: 00 00 55 48 89 e5 53 48 89 fb 48 83 c7 18 48 8b 75 08 e8 06 63 bf fe 48 89 df e8 be 98 bf fe e8 59 ee d3 fe fb 0f 1f 44 00 00 <65> ff 0d 5c 85 68 01 74 14 48 8b 5d f8 c9 31 c0 31 d2 31 c9 31 f6
> [164.964913] RSP: 0018:ffffc9000070fca0 EFLAGS: 00000246
> [164.964919] RAX: 0000000000000000 RBX: ffff88800c2d8b10 RCX: 0000000000000000
> [164.964923] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> [164.964927] RBP: ffffc9000070fca8 R08: 0000000000000000 R09: 0000000000000000
> [164.964931] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88800c2d8ac0
> [164.964934] R13: ffffc9000070fcc8 R14: ffff88800c2d8ac0 R15: 00000000ffffffff
> [164.964967]  sync_timeline_signal+0x153/0x2c0
> [164.964989]  sw_sync_ioctl+0x98/0x580
> [164.965017]  __x64_sys_ioctl+0xa2/0x100
> [164.965034]  x64_sys_call+0x1226/0x2680
> [164.965046]  do_syscall_64+0x93/0x980
> [164.965057]  ? do_syscall_64+0x1b7/0x980
> [164.965070]  ? lock_release+0xce/0x2a0
> [164.965082]  ? __might_fault+0x53/0xb0
> [164.965096]  ? __might_fault+0x89/0xb0
> [164.965104]  ? __might_fault+0x53/0xb0
> [164.965116]  ? _copy_to_user+0x53/0x70
> [164.965131]  ? __x64_sys_rt_sigprocmask+0x8f/0xe0
> [164.965152]  ? do_syscall_64+0x1b7/0x980
> [164.965169]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> [164.965176] RIP: 0033:0x729f4fb24ded
> [164.965188] Code: 04 25 28 00 00 00 48 89 45 c8 31 c0 48 8d 45 10 c7 45 b0 10 00 00 00 48 89 45 b8 48 8d 45 d0 48 89 45 c0 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1a 48 8b 45 c8 64 48 2b 04 25 28 00 00 00
> [164.965193] RSP: 002b:00007ffdc36220e0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [164.965200] RAX: ffffffffffffffda RBX: 0000000000000007 RCX: 0000729f4fb24ded
> [164.965205] RDX: 00007ffdc3622174 RSI: 0000000040045701 RDI: 0000000000000007
> [164.965209] RBP: 00007ffdc3622130 R08: 0000000000000000 R09: 0000000000000000
> [164.965213] R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffdc3622174
> [164.965217] R13: 0000000040045701 R14: 0000000000000007 R15: 0000000000000003
> [164.965248]  </TASK>
> [166.952984] perf: interrupt took too long (11861 > 6217), lowering kernel.perf_event_max_sample_rate to 16000
> [166.953134] clocksource: Long readout interval, skipping watchdog check: cs_nsec: 13036276804 wd_nsec: 13036274445
> 
> As explained by Christian Köenig[2], "The purpose of the sw-sync is to
> test what happens if drivers exposing dma-fences doesn't behave well.  So
> being able to trigger the NMI watchdog for example is part of why that
> functionality exists in the first place. ... You can actually use the
> functionality to intentionally deadlock drivers and even the core memory
> management."
> 
> Let the feature show up only if EXPERT is selected.
> 
> [1] https://patchwork.freedesktop.org/series/154715/
> [2] https://patchwork.freedesktop.org/patch/675579/#comment_1239269
> 
> Fixes: 35538d7822e86 ("dma-buf/sw_sync: de-stage SW_SYNC")
> Cc: Christian König <christian.koenig@....com>
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>

Good idea, we have previously discussed to taint the kernel if sw_sync is used but that is also a clearly step in the right direction.

Reviewed-by: Christian König <christian.koenig@....com>

Regards,
Christian.

> ---
>  drivers/dma-buf/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index b46eb8a552d7b..e726948b64f67 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -18,7 +18,7 @@ config SYNC_FILE
>  	  Documentation/driver-api/sync_file.rst.
>  
>  config SW_SYNC
> -	bool "Sync File Validation Framework"
> +	bool "Sync File Validation Framework" if EXPERT
>  	default n
>  	depends on SYNC_FILE
>  	depends on DEBUG_FS

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