race in cron

Thread View: net.bugs.v7

2 messages

[T] Back to Threads [V] Tree View [G] Group Articles

2 total messages Started by Henry Spencer Thu, 18 Oct 1984 20:58

#143

Author: Henry Spencer
Date: Thu, 18 Oct 1984 20:58

24 lines
1663 bytes

We saw an odd phenomenon with cron(8) the other day.  A while after we'd
taken some entries out of crontab, cron seemed to have forgotten about
some others.  The entries removed were at the end of crontab, and the
forgotten ones just preceded them.  We're not entirely certain why this
occurred, but one thin possibility occurred to us:  cron checks the
modification time on the file to decide whether re-reading is in order.
The mod time's resolution is one second.  Much can happen in one second.
Consider:

	1. Editor commences rewriting crontab.
	2. Kernel clock ticks one second.
	3. Cron stats crontab, decides re-read is in order.
	4. Cron finishes re-read, getting an incompletely-rewritten version.
	5. Editor finishes rewriting crontab.
	6. Crontab inode updated and time-stamped.
	7. Kernel clock ticks another second.

Cron has gotten a truncated crontab and has no way to realize this.  Does
anyone have any other suggestions as to possible causes?  The fix for this
one is to have cron check the size of the file to see whether it matches
the number of bytes in the last re-read.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry

Re: race in cron

#144

Author: matt
Date: Wed, 24 Oct 1984 01:28

18 lines
1237 bytes

#endif lineeater

One could `touch' the new copy of crontab, to force cron to re-read the
crontab.  Or one could edit a scratch copy of crontab (in the same 
filesystem), then `mv scratch /usr/lib/crontab', which will prevent 
this race from occuring (since only links and unlinks are involved in 
the `mv').

A different fix than the one suggested would be to sleep for a few seconds 
after detecting the changed crontab.  This would give the editor time to 
write the file (except on floppy-based systems :-) ), and is even simpler
than a stat.

					- Matt
					  (Dept. of Old Crons)
-------
UUCP:	{ucbvax,ihnp4,randvax,trwrb!trwspp,ism780}!ucla-cs!matt
ARPA:	matt@ucla-locus

Thread Navigation

This is a paginated view of messages in the thread with full content displayed inline.

Messages are displayed in chronological order, with the original post highlighted in green.

Use pagination controls to navigate through all messages in large threads.

Back to All Threads