Unity eats cpu after switching to the greeter/back in the session

[Sebastien Bacher]

That started in xenial, often Unity is eating cpu after going to unity-greeter/switching users and coming back to the session

gdb indicates that some code is "stucked" in pam (while the session is unlocked)

"Thread 2 (Thread 0xa881cb40 (LWP 26675)):
#0 0xb77c1c38 in __kernel_vsyscall ()
#1 0xb744d5d7 in syscall () at ../sysdeps/unix/sysv/linux/i386/syscall.S:29
#2 0xb75dd1b6 in std::__atomic_futex_unsigned_base::_M_futex_wait_until(unsigned int*, unsigned int, bool, std::chrono::duration<long long, std::ratio<1ll, 1ll> >, std::chrono::duration<long long, std::ratio<1ll, 1000000000ll> >) () from /usr/lib/i386-linux-gnu/libstdc++.so.6
#3 0xafa5af4f in std::__atomic_futex_unsigned<2147483648u>::_M_load_and_test_until (__ns=..., __s=..., __has_timeout=<optimized out>, __mo=<optimized out>,
    __equal=<optimized out>, __operand=<optimized out>, __assumed=<optimized out>, this=<optimized out>) at /usr/include/c++/5/bits/atomic_futex.h:104
#4 std::__atomic_futex_unsigned<2147483648u>::_M_load_and_test (__mo=<optimized out>, __equal=<optimized out>, __operand=<optimized out>, __assumed=<optimized out>,
    this=<optimized out>) at /usr/include/c++/5/bits/atomic_futex.h:122
#5 std::__atomic_futex_unsigned<2147483648u>::_M_load_when_equal (__mo=std::memory_order_acquire, __val=1, this=0xa0bf74ec) at /usr/include/c++/5/bits/atomic_futex.h:162
#6 std::__future_base::_State_baseV2::wait (this=0xa0bf74e4) at /usr/include/c++/5/future:322
#7 std::__basic_future<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::_M_get_result (this=<synthetic pointer>)
    at /usr/include/c++/5/future:681
#8 std::future<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::get (this=<synthetic pointer>) at /usr/include/c++/5/future:760
#9 unity::lockscreen::UserAuthenticatorPam::ConversationFunction (num_msg=1, msg=0xa881c098, resp=0xa881c094, appdata_ptr=0xb06370c)
    at /build/unity-QwPAJG/unity-7.4.0+16.04.20151211/lockscreen/UserAuthenticatorPam.cpp:151
#10 0xae955243 in pam_vprompt (pamh=0xb343d38, style=1, response=0xa881c144, fmt=0xaab31b7f "%s", args=0xa881c110 "\374F\034\254\005") at pam_vprompt.c:83
#11 0xae95540c in pam_prompt (pamh=0xb343d38, style=1, response=0xa881c144, fmt=0xaab31b7f "%s") at pam_vprompt.c:111
#12 0xaab2ca7e in _unix_read_password (pamh=0xb343d38, ctrl=276824064, comment=0x0, prompt1=0xac1c46fc "Mot de passe : ", prompt2=0x0, data_name=0xaab31ab9 "-UN*X-PASS",
    pass=0xa881c1a8) at support.c:957
#13 0xaab2a093 in pam_sm_authenticate (pamh=0xb343d38, flags=0, argc=1, argv=0xa9e2480) at pam_unix_auth.c:160
#14 0xae950450 in _pam_dispatch_aux (use_cached_chain=0, resumed=<optimized out>, h=<optimized out>, flags=0, pamh=0xb343d38) at pam_dispatch.c:110
#15 _pam_dispatch (pamh=0xb343d38, flags=0, choice=1) at pam_dispatch.c:395
#16 0xae94fc7a in pam_authenticate (pamh=0xb343d38, flags=0) at pam_auth.c:34
#17 0xafa59892 in unity::lockscreen::UserAuthenticatorPam::<lambda(GTask*, gpointer, gpointer, GCancellable*)>::operator() (__closure=0x0, task=0xa371198, data=0xb06370c)
    at /build/unity-QwPAJG/unity-7.4.0+16.04.20151211/lockscreen/UserAuthenticatorPam.cpp:55
#18 unity::lockscreen::UserAuthenticatorPam::<lambda(GTask*, gpointer, gpointer, GCancellable*)>::_FUN(GTask *, gpointer, gpointer, GCancellable *) ()
    at /build/unity-QwPAJG/unity-7.4.0+16.04.20151211/lockscreen/UserAuthenticatorPam.cpp:62
#19 0xb45f365d in g_task_thread_pool_thread (thread_data=0xa371198, pool_data=0x0) at /build/glib2.0-4hziuO/glib2.0-2.47.3/./gio/gtask.c:1288
#20 0xb707f9f4 in g_thread_pool_thread_proxy (data=0xb3b0b8a8) at /build/glib2.0-4hziuO/glib2.0-2.47.3/./glib/gthreadpool.c:307
#21 0xb707ef9a in g_thread_proxy (data=0x9d28c00) at /build/glib2.0-4hziuO/glib2.0-2.47.3/./glib/gthread.c:780
#22 0xb6fe818a in start_thread (arg=0xa881cb40) at pthread_create.c:333
#23 0xb745200e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:122"

[Sebastien Bacher (seb128)]

it stopped doing it for me and there is no other report, let's close it