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SubjectRe: [PATCH v3 6/9] trace_uprobe: Support SDT markers having reference count (semaphore)
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Thanks Oleg for the review,

On 05/24/2018 09:56 PM, Oleg Nesterov wrote:
> On 04/17, Ravi Bangoria wrote:
>>
>> @@ -941,6 +1091,9 @@ typedef bool (*filter_func_t)(struct uprobe_consumer *self,
>> if (ret)
>> goto err_buffer;
>>
>> + if (tu->ref_ctr_offset)
>> + sdt_increment_ref_ctr(tu);
>> +
>
> iiuc, this is probe_event_enable()...
>
> Looks racy, but afaics the race with uprobe_mmap() will be closed by the next
> change. However, it seems that probe_event_disable() can race with trace_uprobe_mmap()
> too and the next 7/9 patch won't help,
>
>> + if (tu->ref_ctr_offset)
>> + sdt_decrement_ref_ctr(tu);
>> +
>> uprobe_unregister(tu->inode, tu->offset, &tu->consumer);
>> tu->tp.flags &= file ? ~TP_FLAG_TRACE : ~TP_FLAG_PROFILE;
>
> so what if trace_uprobe_mmap() comes right after uprobe_unregister() ?
> Note that trace_probe_is_enabled() is T until we update tp.flags.

Sure, I'll look at your comments.

Apart from these, I've also found a deadlock between uprobe_lock and
mm->mmap_sem. trace_uprobe_mmap() takes these locks in

mm->mmap_sem
uprobe_lock

order but some other code path is taking these locks in reverse order. I've
mentioned sample lockdep warning at the end. The issue is, mm->mmap_sem is
not in control of trace_uprobe_mmap() and we have to take uprobe_lock to
loop over all trace_uprobes.

Any idea how this can be resolved?


Sample lockdep warning:

[ 499.258006] ======================================================
[ 499.258205] WARNING: possible circular locking dependency detected
[ 499.258409] 4.17.0-rc3+ #76 Not tainted
[ 499.258528] ------------------------------------------------------
[ 499.258731] perf/6744 is trying to acquire lock:
[ 499.258895] 00000000e4895f49 (uprobe_lock){+.+.}, at: trace_uprobe_mmap+0x78/0x130
[ 499.259147]
[ 499.259147] but task is already holding lock:
[ 499.259349] 000000009ec93a76 (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0xe0/0x160
[ 499.259597]
[ 499.259597] which lock already depends on the new lock.
[ 499.259597]
[ 499.259848]
[ 499.259848] the existing dependency chain (in reverse order) is:
[ 499.260086]
[ 499.260086] -> #4 (&mm->mmap_sem){++++}:
[ 499.260277] __lock_acquire+0x53c/0x910
[ 499.260442] lock_acquire+0xf4/0x2f0
[ 499.260595] down_write_killable+0x6c/0x150
[ 499.260764] copy_process.isra.34.part.35+0x1594/0x1be0
[ 499.260967] _do_fork+0xf8/0x910
[ 499.261090] ppc_clone+0x8/0xc
[ 499.261209]
[ 499.261209] -> #3 (&dup_mmap_sem){++++}:
[ 499.261378] __lock_acquire+0x53c/0x910
[ 499.261540] lock_acquire+0xf4/0x2f0
[ 499.261669] down_write+0x6c/0x110
[ 499.261793] percpu_down_write+0x48/0x140
[ 499.261954] register_for_each_vma+0x6c/0x2a0
[ 499.262116] uprobe_register+0x230/0x320
[ 499.262277] probe_event_enable+0x1cc/0x540
[ 499.262435] perf_trace_event_init+0x1e0/0x350
[ 499.262587] perf_trace_init+0xb0/0x110
[ 499.262750] perf_tp_event_init+0x38/0x90
[ 499.262910] perf_try_init_event+0x10c/0x150
[ 499.263075] perf_event_alloc+0xbb0/0xf10
[ 499.263235] sys_perf_event_open+0x2a8/0xdd0
[ 499.263396] system_call+0x58/0x6c
[ 499.263516]
[ 499.263516] -> #2 (&uprobe->register_rwsem){++++}:
[ 499.263723] __lock_acquire+0x53c/0x910
[ 499.263884] lock_acquire+0xf4/0x2f0
[ 499.264002] down_write+0x6c/0x110
[ 499.264118] uprobe_register+0x1ec/0x320
[ 499.264283] probe_event_enable+0x1cc/0x540
[ 499.264442] perf_trace_event_init+0x1e0/0x350
[ 499.264603] perf_trace_init+0xb0/0x110
[ 499.264766] perf_tp_event_init+0x38/0x90
[ 499.264930] perf_try_init_event+0x10c/0x150
[ 499.265092] perf_event_alloc+0xbb0/0xf10
[ 499.265261] sys_perf_event_open+0x2a8/0xdd0
[ 499.265424] system_call+0x58/0x6c
[ 499.265542]
[ 499.265542] -> #1 (event_mutex){+.+.}:
[ 499.265738] __lock_acquire+0x53c/0x910
[ 499.265896] lock_acquire+0xf4/0x2f0
[ 499.266019] __mutex_lock+0xa0/0xab0
[ 499.266142] trace_add_event_call+0x44/0x100
[ 499.266310] create_trace_uprobe+0x4a0/0x8b0
[ 499.266474] trace_run_command+0xa4/0xc0
[ 499.266631] trace_parse_run_command+0xe4/0x200
[ 499.266799] probes_write+0x20/0x40
[ 499.266922] __vfs_write+0x6c/0x240
[ 499.267041] vfs_write+0xd0/0x240
[ 499.267166] ksys_write+0x6c/0x110
[ 499.267295] system_call+0x58/0x6c
[ 499.267413]
[ 499.267413] -> #0 (uprobe_lock){+.+.}:
[ 499.267591] validate_chain.isra.34+0xbd0/0x1000
[ 499.267747] __lock_acquire+0x53c/0x910
[ 499.267917] lock_acquire+0xf4/0x2f0
[ 499.268048] __mutex_lock+0xa0/0xab0
[ 499.268170] trace_uprobe_mmap+0x78/0x130
[ 499.268335] uprobe_mmap+0x80/0x3b0
[ 499.268464] mmap_region+0x290/0x660
[ 499.268590] do_mmap+0x40c/0x500
[ 499.268718] vm_mmap_pgoff+0x114/0x160
[ 499.268870] ksys_mmap_pgoff+0xe8/0x2e0
[ 499.269034] sys_mmap+0x84/0xf0
[ 499.269161] system_call+0x58/0x6c
[ 499.269279]
[ 499.269279] other info that might help us debug this:
[ 499.269279]
[ 499.269524] Chain exists of:
[ 499.269524] uprobe_lock --> &dup_mmap_sem --> &mm->mmap_sem
[ 499.269524]
[ 499.269856] Possible unsafe locking scenario:
[ 499.269856]
[ 499.270058] CPU0 CPU1
[ 499.270223] ---- ----
[ 499.270384] lock(&mm->mmap_sem);
[ 499.270514] lock(&dup_mmap_sem);
[ 499.270711] lock(&mm->mmap_sem);
[ 499.270923] lock(uprobe_lock);
[ 499.271046]
[ 499.271046] *** DEADLOCK ***
[ 499.271046]
[ 499.271256] 1 lock held by perf/6744:
[ 499.271377] #0: 000000009ec93a76 (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0xe0/0x160
[ 499.271628]
[ 499.271628] stack backtrace:
[ 499.271797] CPU: 25 PID: 6744 Comm: perf Not tainted 4.17.0-rc3+ #76
[ 499.272003] Call Trace:
[ 499.272094] [c0000000e32d74a0] [c000000000b00174] dump_stack+0xe8/0x164 (unreliable)
[ 499.272349] [c0000000e32d74f0] [c0000000001a905c] print_circular_bug.isra.30+0x354/0x388
[ 499.272590] [c0000000e32d7590] [c0000000001a3050] check_prev_add.constprop.38+0x8f0/0x910
[ 499.272828] [c0000000e32d7690] [c0000000001a3c40] validate_chain.isra.34+0xbd0/0x1000
[ 499.273070] [c0000000e32d7780] [c0000000001a57cc] __lock_acquire+0x53c/0x910
[ 499.273311] [c0000000e32d7860] [c0000000001a65b4] lock_acquire+0xf4/0x2f0
[ 499.273510] [c0000000e32d7930] [c000000000b1d1f0] __mutex_lock+0xa0/0xab0
[ 499.273717] [c0000000e32d7a40] [c0000000002b01b8] trace_uprobe_mmap+0x78/0x130
[ 499.273952] [c0000000e32d7a90] [c0000000002d7070] uprobe_mmap+0x80/0x3b0
[ 499.274153] [c0000000e32d7b20] [c0000000003550a0] mmap_region+0x290/0x660
[ 499.274353] [c0000000e32d7c00] [c00000000035587c] do_mmap+0x40c/0x500
[ 499.274560] [c0000000e32d7c80] [c00000000031ebc4] vm_mmap_pgoff+0x114/0x160
[ 499.274763] [c0000000e32d7d60] [c000000000352818] ksys_mmap_pgoff+0xe8/0x2e0
[ 499.275013] [c0000000e32d7de0] [c000000000016864] sys_mmap+0x84/0xf0
[ 499.275207] [c0000000e32d7e30] [c00000000000b404] system_call+0x58/0x6c


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