Shell quiz: implement a mutex Post: 302318325

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Top Forums Shell Programming and Scripting Shell quiz: implement a mutex Post 302318325 by colemar on Thursday 21st of May 2009 10:04:16 AM

05-21-2009

Registered User

Quote:

Originally Posted by methyl

After creating the file append your shell's process id. Read back the file and check the process id to be 100% sure that your instance holds the interlock before processing, otherwise back off.

Depends on what you mean for "back off".

Suppose that while you write the pid another process is doing the same. This results in the other pid appearing inside the file before or after your pid, the position does not matter here.
Then you check and see another pid:

You wait, leaving in place your pid. Deadlock: both processes waiting for the other pid to disappear.
You delete your pid (not a trivial task) and wait for some interval, then you write your pid again and check. The other process is doing the same, therefore it is likely that both processes again see another pid and have to wait again. A pseudo-random wait interval would take care of the problem... it reminds me of Ethernet collision detection. It would perhaps work, but it seems just too involved.

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Ok, now I got it: the serialization protocol dictates that the pid appearing in line 1 of the interlock file is the one whose process is allowed to go on, while the other processes are inside a wait-and-check loop (well, perhaps it is smarter to just wait $pid1 where pid1 is read from line 1); when the process ends, it removes its pid; then another pid is in line 1... and so on, until the pid queue is empty.

Not bad. Still a bit involved though.

You have to be absolutely sure that the script cannot end without having removed its pid from the queue. An exit handler implemented with trap would be enough, but a SIGKILL would bypass the trap.

Last edited by colemar; 05-21-2009 at 11:36 AM.. Reason: Solved

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LEARN ABOUT BSD

gcore

GCORE(1)						      General Commands Manual							  GCORE(1)

NAME

       gcore - get core image of running process

SYNOPSIS

       gcore [-s][-c core] pid

DESCRIPTION

       gcore  creates  a  core	image  of  each specified process, suitable for use with adb(1).  By default the core image is written to the file
       <pid>.core.

       The options are:

       -c     Write the core file to the specified file instead of <pid>.core.

       -s     Stop the process while creating the core image and resume it when done.  This makes sure that the core dump will be in a	consistent
	      state.   The  process is resumed even if it was already stopped.	Of course, you can obtain the same result by manually stopping the
	      process with kill(1).

       The core image name was changed from core.<pid> to <pid>.core to prevent matching names like core.h and core.c when using programs such	as
       find(1).

FILES

       <process-id>.core   The core image.

BUGS

       If gcore encounters an error while creating the core image and the -s option was used the process will remain stopped.

       Swapped out processes and system processes (the swapper) may not be gcore'd.

4.2 Berkeley Distribution					  April 15, 1994							  GCORE(1)