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Date: Wed, 09 Oct 2013 13:36:05 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] rfifolock: add recursive FIFO lock
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, xiawenc@linux.vnet.ibm.com, qemu-devel@nongnu.org, Michael Roth <mdroth@linux.vnet.ibm.com>

Il 09/10/2013 13:26, Stefan Hajnoczi ha scritto:
> On Wed, Oct 09, 2013 at 12:05:43PM +0200, Paolo Bonzini wrote:
>> Il 09/10/2013 11:55, Stefan Hajnoczi ha scritto:
>>> +    /* Take a ticket */
>>> +    unsigned int ticket = r->tail++;
>>> +
>>> +    if (r->nesting > 0) {
>>> +        if (qemu_thread_is_self(&r->owner_thread)) {
>>> +            r->tail--; /* put ticket back, we're nesting */
>>> +        } else {
>>
>> ticket is dead in the "nested" path, why not move it (and the increment)
>> directly after the else?
> 
> The increment cannot be moved because there are 3 cases:
> 
> 1. Uncontended lock: r->tail++
> 2. Recursive lock: do not change tail
> 3. Wait for contended lock: r->tail++
> 
> Case #1 needs r->tail++ too.
> 
> I find this approach clearest:
> 
> if (r->nesting == 0) {
>     /* Uncontended, just take a ticket */
>     r->tail++;
> } else if (qemu_thread_is_self(&r->owner_thread)) {
>     /* Recursive lock, no need to take a ticket */
> } else {
>     /* Contended case, take a ticket and wait for it */
>     unsigned int ticket = r->tail++;
> 
>     while (ticket != r->head) {
>         ...
>     }
> }

I like this one too.

Paolo