On Mon, Mar 30, 2020 at 03:00:35PM +0200, Daniel Vetter wrote: > Hi all, for all = rcu, cpuhotplug and perf maintainers > > We've hit an interesting new lockdep splat in our drm/i915 CI: > > https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_17096/shard-tglb7/igt@kms_frontbuffer_tracking@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx#dmesg-warnings861 > > Summarizing away the driver parts we have > > < gpu locks which are held within mm->mmap_sem in various gpu fault handlers > > > -> #4 (&mm->mmap_sem#2){++++}: > <4> [604.892615] __might_fault+0x63/0x90 > <4> [604.892617] _copy_to_user+0x1e/0x80 > <4> [604.892619] perf_read+0x200/0x2b0 > <4> [604.892621] vfs_read+0x96/0x160 > <4> [604.892622] ksys_read+0x9f/0xe0 > <4> [604.892623] do_syscall_64+0x4f/0x220 > <4> [604.892624] entry_SYSCALL_64_after_hwframe+0x49/0xbe > <4> [604.892625] > -> #3 (&cpuctx_mutex){+.+.}: > <4> [604.892626] __mutex_lock+0x9a/0x9c0 > <4> [604.892627] perf_event_init_cpu+0xa4/0x140 > <4> [604.892629] perf_event_init+0x19d/0x1cd > <4> [604.892630] start_kernel+0x362/0x4e4 > <4> [604.892631] secondary_startup_64+0xa4/0xb0 > <4> [604.892631] > -> #2 (pmus_lock){+.+.}: > <4> [604.892633] __mutex_lock+0x9a/0x9c0 > <4> [604.892633] perf_event_init_cpu+0x6b/0x140 > <4> [604.892635] cpuhp_invoke_callback+0x9b/0x9d0 > <4> [604.892636] _cpu_up+0xa2/0x140 > <4> [604.892637] do_cpu_up+0x61/0xa0 > <4> [604.892639] smp_init+0x57/0x96 > <4> [604.892639] kernel_init_freeable+0x87/0x1dc > <4> [604.892640] kernel_init+0x5/0x100 > <4> [604.892642] ret_from_fork+0x24/0x50 > <4> [604.892642] > -> #1 (cpu_hotplug_lock.rw_sem){++++}: > <4> [604.892643] cpus_read_lock+0x34/0xd0 > <4> [604.892644] rcu_barrier+0xaa/0x190 > <4> [604.892645] kernel_init+0x21/0x100 > <4> [604.892647] ret_from_fork+0x24/0x50 > <4> [604.892647] > -> #0 (rcu_state.barrier_mutex){+.+.}: > The last backtrace boils down to i915 driver code which holds the same > locks we are holding within mm->mmap_sem, and then ends up calling > rcu_barrier(). From what I can see i915 is just the messenger here, > any driver with this pattern of a lock held within mmap_sem which also > has a path of calling rcu_barrier while holding that lock should be > hitting this splat. > > Two questions: > - This suggests that calling rcu_barrier() isn't ok anymore while > holding mmap_sem, or anything that has a dependency upon mmap_sem. I > guess that's not the idea, please confirm. > - Assuming this depedency is indeed not intended, where should the > loop be broken? It goes through perf, cpuhotplug and rcu subsystems, > and I don't have a clue about any of those. I wonder what is new here; the 1-4 chain there has been true for a long time, see also the comment at perf_event_ctx_lock_nested(). That said; it _might_ be possible to break 3->4, that is, all the copy_{to,from}_user() usage in perf can be lifted out from under the various locks by re-arranging code, but I have a nagging feeling there was more to it than that. Of course, while I did document the locking rules, I seem to have forgotten to comment on exactly why these rules are as they are.. oh well. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
