Openmoko Bug #2279: toggling backlight very fast blocks reads of /proc/apm

[Openmoko Public Trac]

#2279: toggling backlight very fast blocks reads of /proc/apm
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 Reporter:  lindi            |          Owner:  openmoko-kernel     
     Type:  defect           |         Status:  new                 
 Priority:  normal           |      Milestone:                      
Component:  System Software  |        Version:  unspecified         
 Severity:  minor            |       Keywords:  kernel backlight DoS
 Haspatch:  0                |      Blockedby:                      
Estimated:                   |    Patchreview:                      
 Blocking:                   |   Reproducible:  always              
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 Steps to reproduce:
 1) Start "while cat /proc/apm; do sleep 1; done" in one terminal
 2) Start
 while true; do
     echo 255 > /sys/class/backlight/gta02-bl/brightness
     echo 15 > /sys/class/backlight/gta02-bl/brightness
 done
 in another terminal

 Expected results:
 2) backlight brightness changes very fast but cat is still sometimes
 scheduled and can read from /proc/apm

 Actual results:
 2) backlight brightness changes very fast but the read() to /proc/apm
 never returns.

 More info:
 1) I was just trying to make freerunner work as a strobe light when I
 noticed that I could not stop the program. This probably happened because
 Xorg was stuck reading from some file and would never succeed.
 2) I am using andy-tracking 9ecc089861ab238e

-- 
Ticket URL: <https://docs.openmoko.org/trac/ticket/2279>
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