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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): [] _raw_spin_unlock_irq+0x27/0x70 > [164.964599] hardirqs last disabled at (1002206): [] sysvec_irq_work+0x11/0xc0 > [164.964616] softirqs last enabled at (1002138): [] fpu_clone+0xb5/0x270 > [164.964631] softirqs last disabled at (1002136): [] 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] > [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] > [164.964882] > [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] > [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 > Signed-off-by: Janusz Krzysztofik 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 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