Re: [Xen-devel] [PATCH] xen: fix xen_qlock_wait()

[Sander Eikelenboom]

On 09/11/18 16:20, Juergen Gross wrote:
> On 09/11/2018 16:02, Sander Eikelenboom wrote:
>> On 09/11/18 13:04, 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>
>>
>> Although you don't seem too interested, you can stick on a:
>> Tested-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
>> if you like.
> 
> I am interested.
> 
> OTOH I wanted to post the patch officially to give others the chance to
> send remarks.

OK, would be nice to at least be CC'ed on patches going upstream when
you have been reporting stuff.

--
Sander


> 
> Juergen
> 
>>
>> --
>> Sander
>>
>>> ---
>>>  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);
>>>  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)
>>>
>>
>>
>