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Re: [PATCH 7/7] aio-posix: remove idle poll handlers to improve scalabil

From: Stefan Hajnoczi
Subject: Re: [PATCH 7/7] aio-posix: remove idle poll handlers to improve scalability
Date: Fri, 6 Mar 2020 13:50:36 +0000

On Thu, Mar 05, 2020 at 06:28:29PM +0100, Paolo Bonzini wrote:
> On 05/03/20 18:08, Stefan Hajnoczi wrote:
> > +    /*
> > +     * List of handlers participating in userspace polling.  Accessed 
> > almost
> > +     * exclusively from aio_poll() and therefore not an RCU list.  
> > Protected by
> > +     * ctx->list_lock.
> > +     */
> > +    AioHandlerList poll_aio_handlers;
> > +
> Not sure I understand the "almost" part.  If it's accessed only from
> aio_poll() it is protected via either AIO_WAIT_WHILE or the BQL, not by
> ctx->list_lock; if it's protected by ctx->list_lock (using
> qemu_lockcnt_inc in readers), it is an RCU list.

aio_remove_fd_handler() removes nodes from the list during
aio_set_fd_handler(), but only while holding ctx->list_lock and the
count is zero (no readers).

All other access is done from with ctx->list_lock incremented.  This
code needs to be reentrant in case of nested aio_poll() but nothing else
will access the list at the same time.

Therefore RCU is not needed.  ctx->list_lock acts more like a rwlock.


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