Created attachment 126163 [details]
my modmap

Issuing 'xmodmap mymodmap' causes the x system to freeze for minutes on a high CPU load, or rather, become unresponsive to keyboard or mouse input.

This issue has been reported elsewhere for years, for instance in 

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/998310
https://bugs.launchpad.net/ubuntu/+source/openbox/+bug/1391679
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=550533
http://www.archivum.info/ubuntu-bugs@lists.ubuntu.com/2014-11/10065/(Bug-1391679)-Re-xmodmap-freezes-openbox-in-lubuntu-14.10.html

I use openbox Openbox 3.6.1 in Arch Linux. Right now I use xserver 1.18.4-1, but this has been happening to me for years ( and the people on the links above ).

Steps to reproduce

- xmodmap mymodmap

Find mymodmap attached to this bug report.

Some observations:

- The bigger mymodmap file is ( the more lines ) the longer it takes for the system to come back to responsiveness.

- I anaylised the Xorg process using perf, and it seems to me that the culprits are ResourceClientBits, GrabMatchesSecond,DeletePassiveGrabFromList and so on, as you can see on this output:

14,63%  Xorg     Xorg                   [.] ResourceClientBits        
11,83%  Xorg     Xorg                   [.] GrabMatchesSecond         
5,17%   Xorg     Xorg                   [.] DeletePassiveGrabFromList 
5,08%   Xorg     Xorg                   [.] AddPassiveGrabToList      
2,44%   Xorg     Xorg                   [.] 0x000000000004fb70       
1,80%   Xorg     [kernel.vmlinux]       [k] do_select               
1,49%   Xorg     [kernel.vmlinux]       [k] unix_poll              
1,33%   Xorg     Xorg                   [.] 0x000000000004fb85    
1,31%   Xorg     Xorg                   [.] 0x000000000004fbd1   
1,21%   Xorg     Xorg                   [.] 0x000000000004fbde  
1,14%   Xorg     Xorg                   [.] 0x000000000004fbc0

- My system tops at around 23% CPU on all 6 CPUs while irresponsive.