[Bug 90230] New: intel driver hangs in waitpid() for backlight helper process on output hotplug

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90230

            Bug ID: 90230
           Summary: intel driver hangs in waitpid() for backlight helper
                    process on output hotplug
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: fourdan at xfce.org
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Using Fedora 22, when undocking/docking my laptop with an external monitor
attached to the docking station, Xorg will hang on waitpid() for the backlight
helper process.

Backtrace shows that the PID is -1:

#0  0x00007feccda90f3b in __GI___waitpid (pid=-1, stat_loc=stat_loc at entry=0x0,
options=options at entry=0) at ../sysdeps/unix/sysv/linux/waitpid.c:40
#1  0x00007fecc8601e58 in backlight_close (b=b at entry=0x97c8c8) at
backlight.c:522
#2  0x00007fecc864b85a in sna_output_destroy (output=output at entry=0x97c960) at
sna_display.c:3202
#3  0x00007fecc864f1e4 in sna_output_add (sna=sna at entry=0x7feccfeed000,
id=<optimized out>, serial=serial at entry=5) at sna_display.c:3944
#4  0x00007fecc8651f21 in sna_mode_discover (sna=sna at entry=0x7feccfeed000) at
sna_display.c:4211
#5  0x00007fecc8658198 in sna_handle_uevents (fd=<optimized out>,
closure=0x7feccfeed000) at sna_driver.c:771
#6  0x00000000004799e7 in xf86Wakeup (blockData=<optimized out>, err=<optimized
out>, pReadmask=0x836360 <LastSelectMask>) at xf86Events.c:284
#7  0x000000000043ee0d in WakeupHandler (result=result at entry=2,
pReadmask=pReadmask at entry=0x836360 <LastSelectMask>) at dixutils.c:423
#8  0x0000000000592dc7 in WaitForSomething
(pClientsReady=pClientsReady at entry=0x14189c0) at WaitFor.c:229
#9  0x0000000000439fd1 in Dispatch () at dispatch.c:361
#10 0x000000000043e2fb in dix_main (argc=15, argv=0x7ffd512ec9a8,
envp=<optimized out>) at main.c:298
#11 0x00007feccd9e9790 in __libc_start_main (main=0x4286f0 <main>, argc=15,
argv=0x7ffd512ec9a8, init=<optimized out>, fini=<optimized out>,
rtld_fini=<optimized out>, 
    stack_end=0x7ffd512ec998) at libc-start.c:289
#12 0x0000000000428729 in _start ()

It could be a Fedora 22 issue with SElinux (I dont know) but I think it would
be safer to check for (b->pid > 0) instead of just ->pid) in backlight_close so
that if the PID is not initialized somehow (default value is -1), it won hang.

Will send a patch to the ML in a minute for further review/discussion...

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