On Tue, Jun 03, 2025 at 08:05:58AM +0200, Markus Armbruster wrote: > Stefan Hajnoczi <[email protected]> writes: > > > On Wed, May 28, 2025 at 05:12:14PM -0500, Eric Blake wrote: > >> On Wed, May 28, 2025 at 03:09:13PM -0400, Stefan Hajnoczi wrote: > >> > io_uring may not be available at runtime due to system policies (e.g. > >> > the io_uring_disabled sysctl) or creation could fail due to file > >> > descriptor resource limits. > >> > > >> > Handle failure scenarios as follows: > >> > > >> > If another AioContext already has io_uring, then fail AioContext > >> > creation so that the aio_add_sqe() API is available uniformly from all > >> > QEMU threads. Otherwise fall back to epoll(7) if io_uring is > >> > unavailable. > >> > > >> > Notes: > >> > - Update the comment about selecting the fastest fdmon implementation. > >> > At this point it's not about speed anymore, it's about aio_add_sqe() > >> > API availability. > >> > - Uppercase the error message when converting from error_report() to > >> > error_setg_errno() for consistency (but there are instances of > >> > lowercase in the codebase). > >> > - It's easier to move the #ifdefs from aio-posix.h to aio-posix.c. > >> > > >> > Signed-off-by: Stefan Hajnoczi <[email protected]> > >> > --- > >> > util/aio-posix.h | 12 ++---------- > >> > util/aio-posix.c | 29 ++++++++++++++++++++++++++--- > >> > util/fdmon-io_uring.c | 8 ++++---- > >> > 3 files changed, 32 insertions(+), 17 deletions(-) > >> > > >> > diff --git a/util/aio-posix.h b/util/aio-posix.h > >> > index f9994ed79e..6f9d97d866 100644 > >> > --- a/util/aio-posix.h > >> > +++ b/util/aio-posix.h > >> > @@ -18,6 +18,7 @@ > >> > #define AIO_POSIX_H > >> > > >> > #include "block/aio.h" > >> > +#include "qapi/error.h" > >> > > >> > struct AioHandler { > >> > GPollFD pfd; > >> > @@ -72,17 +73,8 @@ static inline void fdmon_epoll_disable(AioContext > >> > *ctx) > >> > #endif /* !CONFIG_EPOLL_CREATE1 */ > >> > > >> > #ifdef CONFIG_LINUX_IO_URING > >> > -bool fdmon_io_uring_setup(AioContext *ctx); > >> > +void fdmon_io_uring_setup(AioContext *ctx, Error **errp); > >> > >> Why change the return type to void? That forces you to have to check > >> errp. If you still return bool (true for errp unchanged, false when > >> errp set), callers might have a simpler interface. > > > > QEMU has both forms. I prefer void foo(Error **errp) because it > > eliminates these awkward states: > > 1. Return true with errp set. There is a risk of leaking errp here. > > 2. Return false with errp NULL. This results in no error message. > > > > Sometimes it is handy to have a return value but I think that void is a > > good default return type. > > I used to think this way, too. Experience changed my mind. > > The "awkward states" are bugs. > > The price to avoid them is more verbose error handling. Because such > bugs have been rare in practice, the vebosity has turned out to be too > much pain for too little gain. qapi/error.h's big comment: > > * = Rules = > [...] > * - Whenever practical, also return a value that indicates success / > * failure. This can make the error checking more concise, and can > * avoid useless error object creation and destruction. Note that > * we still have many functions returning void. We recommend > * • bool-valued functions return true on success / false on failure, > * • pointer-valued functions return non-null / null pointer, and > * • integer-valued functions return non-negative / negative. > > For what it's worth, this is exactly how GError wants to be used. We > deviated from it because we thought we were smarter, and came to regret > it. > > Further down, the big comment shows example code: > > * Call a function, receive an error from it, and pass it to the caller > * - when the function returns a value that indicates failure, say > * false: > * if (!foo(arg, errp)) { > * handle the error... > * } > * - when it does not, say because it is a void function: > * ERRP_GUARD(); > * foo(arg, errp); > * if (*errp) { > * handle the error... > * } > * More on ERRP_GUARD() below. > * > * Code predating ERRP_GUARD() still exists, and looks like this: > * Error *err = NULL; > * foo(arg, &err); > * if (err) { > * handle the error... > * error_propagate(errp, err); // deprecated > * } > * Avoid in new code. Do *not* "optimize" it to > * foo(arg, errp); > * if (*errp) { // WRONG! > * handle the error... > * } > * because errp may be NULL without the ERRP_GUARD() guard. > > In case you think the difference in readability isn't all that big: > error handling is *everywhere*, and any clutter adds up quickly, > obscuring the logic. Every line counts.
Thank you for the pointers! I missed that this was already documented.
I will switch to a bool return type.
>
> > I have CCed Markus in case he has suggestions.
>
> Thanks for that!
>
> >> > void fdmon_io_uring_destroy(AioContext *ctx);
> >> > -#else
> >> > -static inline bool fdmon_io_uring_setup(AioContext *ctx)
> >> > -{
> >> > - return false;
> >> > -}
> >> > -
> >> > -static inline void fdmon_io_uring_destroy(AioContext *ctx)
> >> > -{
> >> > -}
> >> > #endif /* !CONFIG_LINUX_IO_URING */
> >> >
> >> > #endif /* AIO_POSIX_H */
> >> > diff --git a/util/aio-posix.c b/util/aio-posix.c
> >> > index fa047fc7ad..44b3df61f9 100644
> >> > --- a/util/aio-posix.c
> >> > +++ b/util/aio-posix.c
> >> > @@ -16,6 +16,7 @@
> >> > #include "qemu/osdep.h"
> >> > #include "block/block.h"
> >> > #include "block/thread-pool.h"
> >> > +#include "qapi/error.h"
> >> > #include "qemu/main-loop.h"
> >> > #include "qemu/lockcnt.h"
> >> > #include "qemu/rcu.h"
> >> > @@ -717,17 +718,39 @@ void aio_context_setup(AioContext *ctx, Error
> >> > **errp)
> >> > ctx->epollfd = -1;
> >> > ctx->epollfd_tag = NULL;
> >> >
> >> > - /* Use the fastest fd monitoring implementation if available */
> >> > - if (fdmon_io_uring_setup(ctx)) {
> >> > - return;
> >> > +#ifdef CONFIG_LINUX_IO_URING
> >> > + {
> >> > + static bool need_io_uring;
> >> > + Error *local_err = NULL; /* ERRP_GUARD() doesn't handle
> >> > error_abort */
> >>
> >> Really? I thought that was part of why we added ERRP_GUARD, so that
> >> error_abort is pinned closer to the spot where the error is triggered
> >> rather than where it was chained. But your use of errp is a bit
> >> different than usual here, so you may be correct that it doesn't
> >> handle error_abort in the way that you want (allowing a graceful
> >> downgrade to epoll if it is the first failure, but aborting if it is
> >> the second AioContext that fails).
> >
> > The ERRP_GUARD() doc comment explains why it behaves this way:
> >
> > * Note: &error_abort is not rewritten, because that would move the
> > * abort from the place where the error is created to the place where
> > * it's propagated.
>
> Error propagation should be avoided when possible. It's not possible
> here; more on that below.
>
> Why avoid? Two reasons. One, it degrades stack backtraces, as Eric
> pointed out. Two, passing @errp directly is more obvious and less
> verbose, as we've seen above.
>
> >> > +
> >> > + /* io_uring takes precedence because it provides aio_add_sqe()
> >> > support */
> >> > + fdmon_io_uring_setup(ctx, &local_err);
> >> > + if (!local_err) {
> >> > + /*
> >> > + * If one AioContext gets io_uring, then all AioContexts
> >> > need io_uring
> >> > + * so that aio_add_sqe() support is available across all
> >> > threads.
> >> > + */
> >> > + need_io_uring = true;
> >> > + return;
> >> > + }
> >> > + if (need_io_uring) {
> >> > + error_propagate(errp, local_err);
> >> > + return;
> >> > + }
> >> > +
> >> > + warn_report_err_once(local_err); /* frees local_err */
> >> > + local_err = NULL;
>
> This is why we can't avoid error_propagate() here: when
> fdmon_io_uring_setup() fails, we either consume the error and succeed,
> or pass it to the caller and fail.
>
> Because of the former, passing @errp to fdmon_io_uring_setup() would be
> wrong; we need to pass a &local_err. Which we then need to propagate to
> do the latter.
>
> Similar code exists elsewhere. It's fairly rare, though.
>
> ERRP_GUARD() is not designed for this pattern. We have to propragate
> manually.
>
> I'd drop the /* ERRP_GUARD() doesn't handle error_abort */ comment. To
> make sense of it, I believe you need to understand a lot more. And if
> you do, you don't really need the comment.
Will fix.
>
> >> > }
> >> > +#endif /* CONFIG_LINUX_IO_URING */
> >> >
> >> > fdmon_epoll_setup(ctx);
> >> > }
> >> >
> >> > void aio_context_destroy(AioContext *ctx)
> >> > {
> >> > +#ifdef CONFIG_LINUX_IO_URING
> >> > fdmon_io_uring_destroy(ctx);
> >> > +#endif
> >> >
> >> > qemu_lockcnt_lock(&ctx->list_lock);
> >> > fdmon_epoll_disable(ctx);
>
> [...]
>
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