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Re: [Qemu-devel] [PATCH 1/2] rfifolock: add recursive FIFO lock
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] rfifolock: add recursive FIFO lock |
Date: |
Wed, 9 Oct 2013 13:26:09 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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) {
...
}
}
There's also a shorter approach which collapses everything into a single
if statement, but I find the if expression a harder to understand:
if (r->nesting == 0 || !qemu_thread_is_self(&r->owner_thread)) {
unsigned int ticket = r->tail++;
while (ticket != r->head) {
...
}
}
Any preferences?
Stefan
[Qemu-devel] [PATCH 2/2] aio: add aio_context_acquire() and aio_context_release(), Stefan Hajnoczi, 2013/10/09
Re: [Qemu-devel] [PATCH 0/2] aio: add aio_context_acquire() and aio_context_release(), Paolo Bonzini, 2013/10/09