Re: KVM: x86: Prevent deadlock against tk_core.seq

[Peter Zijlstra]

On Thu, May 06, 2021 at 03:21:37PM +0200, Thomas Gleixner wrote:
> syzbot reported a possible deadlock in pvclock_gtod_notify():
> 
> CPU 0  		  	   	    	    CPU 1
> write_seqcount_begin(&tk_core.seq);
>   pvclock_gtod_notify()			    spin_lock(&pool->lock);
>     queue_work(..., &pvclock_gtod_work)	    ktime_get()
>      spin_lock(&pool->lock);		      do {
>      						seq = read_seqcount_begin(tk_core.seq)
> 						...
> 				              } while (read_seqcount_retry(&tk_core.seq, seq);
> 
> While this is unlikely to happen, it's possible.
> 
> Delegate queue_work() to irq_work() which postpones it until the
> tk_core.seq write held region is left and interrupts are reenabled.
> 
> Fixes: 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes")
> Reported-by: syzbot+6beae4000559d41d80f8@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>