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

[Daniel P . Berrangé]

On Wed, Jul 31, 2024 at 04:24:45PM +0100, Peter Maydell wrote:
> 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).

Ah ok, so RETRY_ON_EINTR conceivably makes sense in the linux-user
/ bsd-user code in certain scenarios......but it seems almost every
single use today is in system emulator code !

With regards,
Daniel
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