Re: [Qemu-devel] QEMU leaves pidfile behind on exit

[Daniel P . Berrangé]

On Tue, Feb 13, 2018 at 08:35:23PM +0100, Laszlo Ersek wrote:
> On 02/13/18 17:28, Daniel P. Berrangé wrote:
> > On Fri, Feb 09, 2018 at 07:12:59PM +0000, Shaun Reitan wrote:
> >> QEMU leaves the pidfile behind on a clean exit when using the option
> >> -pidfile /var/run/qemu.pid.
> >>
> >> Should QEMU leave it behind or should it clean up after itself?
> >>
> >> I'm willing to take a crack at a patch to fix the issue, but before I do, I
> >> want to make sure that leaving the pidfile behind was not intentional?
> > 
> > If QEMU deletes the pidfile on exit then, with the current pidfile
> > acquisition logic, there's a race condition possible:
> > 
> > To acquire we do
> > 
> >  1. fd = open()
> >  2. lockf(fd)
> > 
> > If the first QEMU that currently owns the pidfile unlinks in, while
> > a second qemu is in betweeen steps 1 & 2, the second QEMU will
> > acquire the pidfile successfully (which is fine) but the pidfile
> > is now unlinked. This is not fine, because a 3rd qemu can now come
> > and try to acquire the pidfile (by creating a new one) and succeed,
> > despite the second qemu still owning the (now unlinked) pidfile.
> > 
> > It is possible to deal with this race by making qemu_create_pidfile
> > more intelligent [1]. It would have todo
> > 
> >   1. fd = open(filename)
> >   2. fstat(fd)
> >   3. lockf(fd)
> >   4. stat(filename)
> > 
> > It must then compare the results of 2 + 4 to ensure the pidfile it
> > acquired is the same as the one on disk. With this change, it would
> > be safe for QEMU to delete the pidfile on exit.
> 
> Why don't we just open the pidfile with (O_CREAT | O_EXCL)? O_EXCL is
> supposed to be atomic.

O_EXCL isn't a good idea because if QEMU crashes without cleaning up
you have a stale pidfile and O_EXCL will turn that into a failure to
acquire pidfile. The key point of using lockf() is to ensure we can
cope reliably with stale pidfiles

> 
> ... The open(2) manual on Linux says,
> 
>               On  NFS,  O_EXCL  is  supported only when using NFSv3 or
>               later on kernel 2.6 or later.  In NFS environments where
>               O_EXCL support is not provided, programs that rely on it
>               for performing locking tasks will contain a race  condi-
>               tion.   [...]
> 
> Sigh.
> 
> > [1] See the equiv libvirt logic for pidfile acquisition in
> >      
> > https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virpidfile.c;h=58ab29f77f2cfb8583447112dae77a07446bc627;hb=HEAD#l384
> > 
> 
> To my knowledge, "same file" should be checked with:
> 
>   a.st_dev == b.st_dev && a.st_ino == b.st_ino
> 
> Example:
> - "filename" is "/var/run/qemu.pid"
> - "/var/run" is originally a symbolic link to "/mnt/fs1/"
> - between steps #1 and #4, "/var/run" is re-created as a symbolic link
>   to "/mnt/fs2/" -- a different filesystem from fs1
> - "/mnt/fs2/qemu.pid" happens to have the same inode number as
>   "/mnt/fs1/qemu.pid"

I don't really think we need to worry about the admin changing symlinks
like this while QEMU is in middle of acquiring the PID.

Regards,
Daniel
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