Re: [PATCH] xen: fix xen_qlock_wait()

[Boris Ostrovsky]

On 11/9/18 7:04 AM, Juergen Gross wrote:
> Commit a856531951dc80 ("xen: make xen_qlock_wait() nestable")
> introduced a regression for Xen guests running fully virtualized
> (HVM or PVH mode). The Xen hypervisor wouldn't return from the poll
> hypercall with interrupts disabled in case of an interrupt (for PV
> guests it does).
>
> So instead of disabling interrupts in xen_qlock_wait() use a nesting
> counter to avoid calling xen_clear_irq_pending() in case
> xen_qlock_wait() is nested.
>
> Fixes: a856531951dc80 ("xen: make xen_qlock_wait() nestable")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

This needs Sander's Reported-by.

> ---
>  arch/x86/xen/spinlock.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 441c88262169..1c8a8816a402 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -9,6 +9,7 @@
>  #include <linux/log2.h>
>  #include <linux/gfp.h>
>  #include <linux/slab.h>
> +#include <linux/atomic.h>
>  
>  #include <asm/paravirt.h>
>  #include <asm/qspinlock.h>
> @@ -21,6 +22,7 @@
>  
>  static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
>  static DEFINE_PER_CPU(char *, irq_name);
> +static DEFINE_PER_CPU(atomic_t, xen_qlock_wait_nest);

I'd move this to xen_qlock_wait().

Either way,

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

>  static bool xen_pvspin = true;
>  
>  static void xen_qlock_kick(int cpu)
> @@ -39,25 +41,25 @@ static void xen_qlock_kick(int cpu)
>   */
>  static void xen_qlock_wait(u8 *byte, u8 val)
>  {
> -	unsigned long flags;
>  	int irq = __this_cpu_read(lock_kicker_irq);
> +	atomic_t *nest_cnt = this_cpu_ptr(&xen_qlock_wait_nest);
>  
>  	/* If kicker interrupts not initialized yet, just spin */
>  	if (irq == -1 || in_nmi())
>  		return;
>  
> -	/* Guard against reentry. */
> -	local_irq_save(flags);
> +	/* Detect reentry. */
> +	atomic_inc(nest_cnt);
>  
> -	/* If irq pending already clear it. */
> -	if (xen_test_irq_pending(irq)) {
> +	/* If irq pending already and no nested call clear it. */
> +	if (atomic_read(nest_cnt) == 1 && xen_test_irq_pending(irq)) {
>  		xen_clear_irq_pending(irq);
>  	} else if (READ_ONCE(*byte) == val) {
>  		/* Block until irq becomes pending (or a spurious wakeup) */
>  		xen_poll_irq(irq);
>  	}
>  
> -	local_irq_restore(flags);
> +	atomic_dec(nest_cnt);
>  }
>  
>  static irqreturn_t dummy_handler(int irq, void *dev_id)