Interrupts

Comments

• marc Posts: 647

  July 2011

  ravi.chakram wrote:

  while understanding the interrupts, I saw the below line in hardirq.h.
  
  #define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT)
  
  Can anyone please tell me what is 1UL means in the above line.
  
  
  Thanks in advance
  
  Regards
  Kumar

  
  1 Unsigned Long = an "unsigned long" full of ones
  
  Regards
  
  PS: I'm no expert, I might be wrong!

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• asedt Posts: 96

  July 2011

  Correct:
  
  unsigned long int
  
  http://www.cs.umbc.edu/portal/help/C++/summary.shtml#Expressions

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• rulingminds Posts: 2

  July 2011

  UL means long unsigned int. 1 is left shifted by SOFTIRQ_SHIFT(8) times, that means value will be
  
  0x00000100.
  
  This macro is used to increment the softirq count in the thread_info->preempt_count at 0x00000100 offset.
  
  For linux kernel articles, visit www.rulingminds.com

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• Goineasy9 Posts: 1,116

  July 2011

  That RulingMinds site is very interesting, especially for kernel newbies like me. The videos could have been edited better, well, they could have been edited, but, all in all, I'd still say it's an interesting site.

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• chekkizhar Posts: 182

  August 2011

  rulingminds wrote:

  
  This macro is used to increment the softirq count in the thread_info->preempt_count at 0x00000100 offset.

  
  Great . But, is it incrementing the count at 0x00000100 or assigning/updating the soft-interrupts value as 0x00000100
  
  My opinion is, it is doing the second one.

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• rulingminds Posts: 2

  August 2011

  No, This is incrementing or decrementing the softirq count. The preempt_count is divided as follows.
  
  For softirq count there are 8 bits(from 8 - 15) in preempt_count variable. So, the variable can be incremented upto 255.
  This count says number of levels the softirqs are disabled(number of tymes local_bh_disable() is called) . decremented using local_bh_enable(). See the following code in softirq.c and preempt.h
  
  # define add_preempt_count(val) do { preempt_count() += (val); } while (0)
  
  static inline void __local_bh_disable(unsigned long ip, unsigned int cnt)
  {
  add_preempt_count(cnt);
  barrier();
  }
  
  Here, count is 0x100.
  
  
  -Rulingminds

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