Re: rcu_barrier() no longer allowed within mmap_sem?

[Peter Zijlstra]

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@fbcpsr-rgb101010-draw-mmap-gtt.html#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.