Re: [Qemu-devel] [PATCH 4/4] blockjob: reimplement block_job_sleep_ns to allow cancellation

[Jeff Cody <jcody@redhat.com>]

On Wed, Nov 29, 2017 at 03:21:37PM +0100, Kevin Wolf wrote:
> Am 29.11.2017 um 14:56 hat Jeff Cody geschrieben:
> > On Wed, Nov 29, 2017 at 11:25:13AM +0100, Paolo Bonzini wrote:
> > > This reverts the effects of commit 4afeffc857 ("blockjob: do not allow
> > > coroutine double entry or entry-after-completion", 2017-11-21)
> > > 
> > > This fixed the symptom of a bug rather than the root cause. Canceling the
> > > wait on a sleeping blockjob coroutine is generally fine, we just need to
> > > make it work correctly across AioContexts.  To do so, use a QEMUTimer
> > > that calls block_job_enter.  Use a mutex to ensure that block_job_enter
> > > synchronizes correctly with block_job_sleep_ns.
> > > 
> > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> > My only concerns were regarding comments, and Kevin can fix them when
> > applying if he wants.
> 
> I'm squashing in the following changes.
> 
> Kevin
> 
> 
> diff --git a/include/block/blockjob.h b/include/block/blockjob.h
> index 956f0d6819..00403d9482 100644
> --- a/include/block/blockjob.h
> +++ b/include/block/blockjob.h
> @@ -135,7 +135,10 @@ typedef struct BlockJob {
>       */
>      int ret;
>  
> -    /** Timer that is used by @block_job_sleep_ns.  */
> +    /**
> +     * Timer that is used by @block_job_sleep_ns. Accessed under
> +     * block_job_mutex (in blockjob.c).
> +     */
>      QEMUTimer sleep_timer;
>  
>      /** Non-NULL if this job is part of a transaction */
> diff --git a/blockjob.c b/blockjob.c
> index 83f5c891be..dd8c685d8a 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -37,9 +37,9 @@
>  #include "qemu/timer.h"
>  #include "qapi-event.h"
>  
> -/* Right now, this mutex is only needed to synchronize accesses to job->busy,
> - * such as concurrent calls to block_job_do_yield and block_job_enter.
> - */
> +/* Right now, this mutex is only needed to synchronize accesses to job->busy
> + * and and job->sleep_timer, such as concurrent calls to block_job_do_yield and

s/and and/and

> + * block_job_enter. */
>  static QemuMutex block_job_mutex;
>  
>  static void block_job_lock(void)
> @@ -760,6 +760,12 @@ static bool block_job_should_pause(BlockJob *job)
>      return job->pause_count > 0;
>  }
>  
> +/* Yield, and schedule a timer to reenter the coroutine after @ns nanoseconds.
> + * Reentering the job coroutine with block_job_enter() before the timer has
> + * expired is allowed and cancels the timer.
> + *
> + * If @ns is (uint64_t) -1, no timer is scheduled and block_job_enter() must be
> + * called explicitly. */
>  static void block_job_do_yield(BlockJob *job, uint64_t ns)
>  {
>      block_job_lock();