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

[Juergen Gross <jgross@suse.com>]

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@vger.kernel.org
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> Although you don't seem too interested, you can stick on a:
> Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
> if you like.

I am interested.

OTOH I wanted to post the patch officially to give others the chance to
send remarks.


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)
>>
> 
>