[libvirt] [PATCH] qemu: Fix deadlock across fork() in QEMU driver

[Michal Privoznik]

On 02/21/2017 01:11 PM, Marc Hartmayer wrote:
> The functions in virCommand() after fork() must be careful with regard
> to accessing any mutexes that may have been locked by other threads in
> the parent process. It is possible that another thread in the parent
> process holds the lock for the virQEMUDriver while fork() is called.
> This leads to a deadlock in the child process when
> 'virQEMUDriverGetConfig(driver)' is called and therefore the handshake
> never completes between the child and the parent process. Ultimately
> the virDomainObjectPtr will never be unlocked.
> 
> It gets much worse if the other thread of the parent process, that
> holds the lock for the virQEMUDriver, tries to lock the already locked
> virDomainObject. This leads to a completely unresponsive libvirtd.
> 
> It's possible to reproduce this case with calling 'virsh start XXX'
> and 'virsh managedsave XXX' in a tight loop for multiple domains.
> 
> This commit fixes the deadlock in the same way as it is described in
> commit 61b52d2e3813cc8c9ff3ab67f232bd0c65f7318d.
> 
> Signed-off-by: Marc Hartmayer <mhartmay at linux.vnet.ibm.com>
> Reviewed-by: Boris Fiuczynski <fiuczy at linux.vnet.ibm.com>
> ---
>  src/qemu/qemu_domain.c  | 73 +++++++++++++++++++++++--------------------------
>  src/qemu/qemu_domain.h  |  3 +-
>  src/qemu/qemu_process.c |  2 +-
>  3 files changed, 37 insertions(+), 41 deletions(-)

ACKed and pushed. Nice catch.

Michal