linux-kernel - Re: Strange problem with FUTEX_WAIT

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Date:	Fri, 06 Mar 2009 04:58:53 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	g3vbv@...eyonder.co.uk
CC:	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: Strange problem with FUTEX_WAIT_PRIVATE

Sid Boyce a écrit :
> Darren Hart wrote:
>> Sid Boyce wrote:
>>> I don't know if it's a kernel problem, I have already posted to openSUSE
>>> Factory list without any response. It's either an openSUSE problem or
>>> one encountered since about 2.6.29-rc4 I'd guess and through to
>>> 2.6.29-rc7-git1.
>>> If I execute certain applications as root I never get the GUI output, as
>>> user everything works as expected.
>>> Below is part of the strace output from "qjackctl". VirtualBox  and
>>> others do the same.
>>> <<<Previous lines deleted>>
>>> stat("/etc/kde4/share/config/oxygenrc", {st_mode=S_IFREG|0644,
>>> st_size=55, ...}) = 0
>>> stat("/etc/kde4/share/config/oxygenrc", {st_mode=S_IFREG|0644,
>>> st_size=55, ...}) = 0
>>> fstat(8, {st_mode=S_IFREG|0644, st_size=55, ...}) = 0
>>> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
>>> = 0x7fba0d1a9000
>>> read(8, "[Windeco]\nBlendTitlebarColors=false\nShowStripes=false\n\n",
>>> 4096) = 55
>>> read(8, "", 4096)                       = 0
>>> close(8)                                = 0
>>> munmap(0x7fba0d1a9000, 4096)            = 0
>>> socket(PF_FILE, SOCK_STREAM, 0)         = 8
>>> connect(8, {sa_family=AF_FILE, path=@"/tmp/dbus-8XYkB058j7"}, 23) = 0
>>> fcntl(8, F_GETFL)                       = 0x2 (flags O_RDWR)
>>> fcntl(8, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
>>> fcntl(8, F_GETFD)                       = 0
>>> fcntl(8, F_SETFD, FD_CLOEXEC)           = 0
>>> geteuid()                               = 0
>>> rt_sigaction(SIGPIPE, {0x1, [PIPE], SA_RESTORER|SA_RESTART,
>>> 0x7fba0869d6e0}, {SIG_DFL, [], 0}, 8) = 0
>>> poll([{fd=8, events=POLLOUT}], 1, 0)    = 1 ([{fd=8, revents=POLLOUT}])
>>> write(8, "\0", 1)                       = 1
>>> write(8, "AUTH EXTERNAL 30\r\n", 18)    = 18
>>> poll([{fd=8, events=POLLIN}], 1, -1)    = 1 ([{fd=8, revents=POLLIN}])
>>> read(8, "OK 7c5d389345212834a5f2258649afe483\r\n", 2048) = 37
>>> poll([{fd=8, events=POLLOUT}], 1, -1)   = 1 ([{fd=8, revents=POLLOUT}])
>>> write(8, "BEGIN\r\n", 7)                = 7
>>> poll([{fd=8, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=8,
>>> revents=POLLIN|POLLOUT|POLLHUP}])
>>> read(8, "", 2048)                       = 0
>>> close(8)                                = 0
>>> sched_yield()                           = 0
>>> sched_yield()                           = 0
>>> sched_yield()                           = 0
>>> sched_yield()                           = 0
>>> <<< many lines of the same truncated>>>
>>> sched_yield()                           = 0
>>> sched_yield()                           = 0
>> tsk tsk tsk
>>
>>> futex(0x7722ac, FUTEX_WAIT_PRIVATE, 1, NULL
>>>
>>> It never moves beyond that point and I have to CTRL-C back to the prompt.
>>> Regards
>>> Sid.
>> So without an accompanying FUTEX_WAKE this thread will never return
>> (since the timeout is NULL).  I suggest 'strace -f' to follow all
>> threads - do you see the FUTEX_WAKE?  Do you see it when not running as
>> root?
>>
> It was run with "strace -s 256 -f" and FUTEX_WAKE occurs earlier in the
> strace, it just hangs on the last one. Attaching the full strace output.
> Regards
> Sid.
> 
grep futex QJ.out

futex(0x7fff861119dc, FUTEX_WAKE_PRIVATE, 1) = 0
futex(0x7ff17b172b60, FUTEX_WAKE_PRIVATE, 2147483647) = 0
futex(0x7ff1798acf40, FUTEX_WAKE_PRIVATE, 2147483647) = 0
futex(0x7ff17a2310ec, FUTEX_WAKE_PRIVATE, 2147483647) = 0
futex(0x7ff17a019b88, FUTEX_WAKE_PRIVATE, 2147483647) = 0
futex(0x76deac, FUTEX_WAIT_PRIVATE, 1, NULL

So nothing calls futex(0x76deac, FUTEX_WAKE_PRIVATE, 1)

According to your strace, this program is monothreaded (no additional 
thread was created), so I would say program has a bug.

Maybe not 64bit clean, since 0x76deac seems 32bit truncated...


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