Re: [PATCH] util: retry open() when it gets interrupted by a signal

[Peter Maydell]

On Wed, 31 Jul 2024 at 16:21, Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Wed, Jul 31, 2024 at 03:32:52PM +0100, Peter Maydell wrote:
> > This is why we have the RETRY_ON_EINTR() macro, right?
> >
> > Currently we have some places that call qemu_open_old() inside
> > RETRY_ON_EINTR -- we should decide whether we want to
> > handle EINTR inside the qemu_open family of functions,
> > or make the caller deal with it, and put the macro uses
> > in the right place consistently.
>
> It is incredibly arbitrary where we use RETRY_ON_EINTR, which I think
> points towards it being a sub-optimal solution to the general problem.

Agreed (and agreed that SA_RESTART is the usual approach to
avoid this mess). Partly I just vaguely recall discussions
about this back when we added/improved the RETRY_ON_EINTR
macro in the first place: maybe there's a reason we have it
still...

> > I agree that it would be nicer if we could use SA_RESTART,
> > but presumably there's a reason why we don't. (At any
> > rate code that's shared with the user-mode emulation
> > has to be EINTR-resistant, because we can't force the
> > user-mode guest code to avoid registering signal handlers
> > that aren't SA_RESTART.)
>
> For user mode emulation isn't it valid to just propagage the
> EINTR back up to the application, since EINTR is a valid errno
> they have to be willing to handle unless the app has itself
> use SA_RESTART.

Yes, that's what we must do for cases where we are doing some
syscall on behalf of the guest. But for cases where we're
doing a syscall because of something QEMU itself needs to do,
we may need to retry, because we might not be in a position
to be able to back out of what we're doing (or we might not
even be inside the "handle a guest syscall" codepath at all).

-- PMM