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SubjectRe: fcntl and flock wakeups not FIFO?
Matthew Wilcox wrote:
> [cc'ing the person or list mentioned in MAINTAINERS would get you
> a better response :-P]

Hmm...that might be a good idea. :)

>>I've been doing some experimenting with locking on 2.4.18 and have
>>noticed that if I have a number of writers waiting on a lock, they are
>>not woken up in the order in which they requested the lock.
>>
>>Is this expected? If so, what was the reasoning for this and are there
>>any patches to give FIFO wakeups?
>
>
> That certainly isn't what's supposed to happen. They should get woken
> up in-order. The code in 2.4.18 seems to be doing that. Are you
> doing anything clever with scheduling?

Well maybe a little bit on the production box, but I don't think its the
cause since the same thing happens on my home machine with a stock
Mandrake 9 kernel (2.4.19-16mdk).

Here's the test app:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/file.h>

int main(int argc, char **argv)
{
int fd = open("/dev/null", O_RDWR);
if (fd < 0)
{
perror("open");
exit(-1);
}

printf("aquiring exclusive lock\n");
int rc = flock(fd, LOCK_EX);
if (rc < 0)
{
perror("flock");
exit(-1);
}

printf("got lock\n");

while(1)
pause();

return 0;
}

I start up four different instances of it in different windows, then
kill them (ctrl-c) in the order that I started them.

It doesn't happen every time, but they don't always get the lock in the
same order that I started them.

Chris



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