linux-kernel - Re: Strange problem with FUTEX_WAIT

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Message-ID: <49B06BDE.4060103@us.ibm.com>
Date:	Thu, 05 Mar 2009 16:18:38 -0800
From:	Darren Hart <dvhltc@...ibm.com>
To:	g3vbv@...eyonder.co.uk
CC:	linux-kernel@...r.kernel.org
Subject: Re: Strange problem with FUTEX_WAIT_PRIVATE

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?

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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