Date: Wed, 21 Apr 1999 11:17:56 +0200 From: Graham Wheeler <gram@cdsec.com> To: hackers@freebsd.org Subject: Re: File locking problem Message-ID: <371D97C4.B7EFA0B1@cdsec.com> References: <199904201821.OAA16150@lor.watermarkgroup.com>
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Luoqi Chen wrote:
>
> > What exactly happens when a process obtains an exclusive lock on
> > a file and then forks? Does the lock get removed only after both
> > processes have closed the file, or after either one of them closes it?
> > And if the latter, is there a way I can get around this?
> >
> Try using the POSIX style fcntl(2) lock instead?
I need the open and lock to be atomic. I am currently trying to
write a small program which replicates the problem I am having so
I can post it. Currently everything works in the small program (as
it does when I run the real program), but a client sees the failures
I reported every now and then (about one failure per thousand files
processed).
I should mention I am not explicitly unlocking the file, but am
relying on it being unlocked upon the last close. This appears to
be the correct approach based on the flock, open and close manual
pages. I am also relying on the file being removed upon the last
close due to the call to unlink that happens before the last close.
However, the behaviour we are seeing seems to indicate that
on rare occasions, after the last close, the parent process is somehow
managing to open and lock the file again, even though it cannot
read anything (the first fgets() fails and feof() gets set).
If the problem was that my code was buggy, and unlink() wasn't
being called, then there would be no reason for the fgets() to
fail. So it seems that although my assorted processes are doing
things in the order:
...
child:unlink()
child:close() // and unlock
parent:open() // and lock - should not succeed due to unlink
parent:fgets() // should never occur as open() should fail
...
there is some kind of race happening between the file being removed
by the unlink and the parent:open() - the parent:open() is
succeeding when it shouldn't, and then the fgets() is failing (which
it shouldn't if the open was successful).
--
Dr Graham Wheeler E-mail: gram@cdsec.com
Citadel Data Security Phone: +27(21)423-6065/6/7
Firewalls/Virtual Private Networks Fax: +27(21)24-3656
Internet/Intranet Network Specialists
Data Security Products WWW: http://www.cdsec.com/
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