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Re: [RESEND PATCH] hw/dma: fix crash caused by race condition

From: Tong Zhang
Subject: Re: [RESEND PATCH] hw/dma: fix crash caused by race condition
Date: Wed, 1 Jun 2022 18:04:35 -0700

Hi Stefan,

On Wed, Jun 1, 2022 at 6:56 AM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > This patch makes sense to me. Can you rephrase your concern?
> >
> > The locking is around dbs->io_func().
> >
> > aio_context_acquire(dbs->ctx);
> > dbs->acb = dbs->io_func()
> > aio_context_release(dbs->ctx);
> >
> >
> > So where exactly would the lock that's now still held stop someone from
> > modifying dbs->acb = NULL at the beginning of the function, which seems
> > to be not protected by that lock?
> >
> > Maybe I'm missing some locking magic due to the lock being a recursive lock.
> Tong Zhang: Can you share a backtrace of all threads when the
> assertion failure occurs?
Sorry I couldn't get the trace now -- but I can tell that we have some
internal code uses
this dma related code and will grab dbs->ctx lock in another thread
and could overwrite dbs->acb.

>From my understanding, one of the reasons that the lock is required
here is to protect dbs->acb,
we could not reliably test io_func()'s return value after releasing
the lock here.

Since this code affects our internal code base and I did not reproduce
on master branch,
feel free to ignore it.

- Tong

> Stefan

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