APPLIED: [B][PATCH 0/2] Fix for LP#1821259 (pending patches for) Fix for deadlock in cpu_stopper

[Khaled Elmously]

On 2019-03-21 20:48:34 , Mauricio Faria de Oliveira wrote:
> BugLink: https://bugs.launchpad.net/bugs/1821259
> 
> Bionic only needs 2 of the 4 patches submitted for Xenial.
> All patches are applied / not needed on Cosmic and later.
> 
> [Impact] 
> 
>  * This problem hard locks up 2 CPUs in a deadlock, and this
>    soft locks up other CPUs as an effect; the system becomes
>    unusable.
> 
>  * This is relatively rare / difficult to hit because it's a
>    corner case in scheduling/load balancing that needs timing
>    with CPU stopper code. And it needs SMP plus _NUMA_ system.
>    (but it can be hit with synthetic test case attached in LP.)
> 
>  * Since SMP plus NUMA usually equals _servers_ it looks like
>    a good idea to prevent this bug / hard lockups / rebooting.
> 
>  * The fix resolves the potential deadlock by removing one of
>    the calls required to deadlock from under the locked code.
> 
> [Test Case]
> 
>  * There's a synthetic test case to reproduce this problem
>    (although without the stack traces - just a system hang)
>    attached to this LP bug.
> 
>  * It uses kprobes/mdelay/cpu stopper calls to force the code
>    to execute and force the timing/locking condition to occur.
> 
>  * $ sudo insmod kmod-stopper.ko 
> 
>    Some dmesg logging occurs, and systems either hangs or not.
>    See examples in comments.
>    
> [Regression Potential] 
> 
>  * These are patches to the cpu stop_machine.c code, and they
>    change a bit how it works;  however, there are no upstream
>    fixes for these patches anymore and they are still the top
>    of the 'git log --oneline -- kernel/stop_machine.c' output.
> 
>  * These patches have been verified with the synthetic test case
>    and 'stress-ng --class scheduler --sequential 0' (no regressions)
>    on guest with 2 CPUs and one physical system with 24 CPUs.
> 
> [Other Info]
>  
>  * The patches are required on Xenial and later.
>  * There are 4 patches for Xenial, and 2 patches pending for Bionic.
>  * All patches are applied from Cosmic onwards.
> 
> Isaac J. Manjarres (1):
>   stop_machine: Disable preemption after queueing stopper threads
> 
> Prasad Sodagudi (1):
>   stop_machine: Atomically queue and wake stopper threads
> 
>  kernel/stop_machine.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> -- 
> 2.17.1
> 
> 
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