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When ->poll() succeeds the AioHandler is placed on the ready list with
revents set to the magic value 0. This magic value causes
aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
for G_IO_IN or ->io_write() for G_IO_OUT.
This magic value 0 hack works for the IOThread where AioHandlers are
placed on ->ready_list and processed by aio_dispatch_ready_handlers().
It does not work for the main loop where all AioHandlers are processed
by aio_dispatch_handlers(), even those that are not ready and have a
revents value of 0.
As a result the main loop invokes ->poll_ready() on AioHandlers that are
not ready. These spurious ->poll_ready() calls waste CPU cycles and
could lead to crashes if the code assumes ->poll() must have succeeded
before ->poll_ready() is called (a reasonable asumption but I haven't
seen it in practice).
Stop using revents to track whether ->poll_ready() will be called on an
AioHandler. Introduce a separate AioHandler->poll_ready field instead.
This eliminates spurious ->poll_ready() calls in the main loop.
Fixes: 826cc32423 ("aio-posix: split poll check from ready handler")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Jason Wang <jasowang@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Message-id: 20220223155703.136833-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
84 lines
2.1 KiB
C
84 lines
2.1 KiB
C
/*
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* AioContext POSIX event loop implementation internal APIs
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*
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* Copyright IBM, Corp. 2008
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* Copyright Red Hat, Inc. 2020
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*
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* Authors:
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* Anthony Liguori <aliguori@us.ibm.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2. See
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* the COPYING file in the top-level directory.
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*
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* Contributions after 2012-01-13 are licensed under the terms of the
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* GNU GPL, version 2 or (at your option) any later version.
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*/
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#ifndef AIO_POSIX_H
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#define AIO_POSIX_H
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#include "block/aio.h"
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struct AioHandler {
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GPollFD pfd;
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IOHandler *io_read;
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IOHandler *io_write;
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AioPollFn *io_poll;
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IOHandler *io_poll_ready;
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IOHandler *io_poll_begin;
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IOHandler *io_poll_end;
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void *opaque;
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QLIST_ENTRY(AioHandler) node;
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QLIST_ENTRY(AioHandler) node_ready; /* only used during aio_poll() */
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QLIST_ENTRY(AioHandler) node_deleted;
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QLIST_ENTRY(AioHandler) node_poll;
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#ifdef CONFIG_LINUX_IO_URING
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QSLIST_ENTRY(AioHandler) node_submitted;
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unsigned flags; /* see fdmon-io_uring.c */
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#endif
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int64_t poll_idle_timeout; /* when to stop userspace polling */
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bool poll_ready; /* has polling detected an event? */
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bool is_external;
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};
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/* Add a handler to a ready list */
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void aio_add_ready_handler(AioHandlerList *ready_list, AioHandler *node,
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int revents);
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extern const FDMonOps fdmon_poll_ops;
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#ifdef CONFIG_EPOLL_CREATE1
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bool fdmon_epoll_try_upgrade(AioContext *ctx, unsigned npfd);
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void fdmon_epoll_setup(AioContext *ctx);
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void fdmon_epoll_disable(AioContext *ctx);
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#else
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static inline bool fdmon_epoll_try_upgrade(AioContext *ctx, unsigned npfd)
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{
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return false;
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}
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static inline void fdmon_epoll_setup(AioContext *ctx)
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{
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}
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static inline void fdmon_epoll_disable(AioContext *ctx)
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{
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}
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#endif /* !CONFIG_EPOLL_CREATE1 */
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#ifdef CONFIG_LINUX_IO_URING
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bool fdmon_io_uring_setup(AioContext *ctx);
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void fdmon_io_uring_destroy(AioContext *ctx);
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#else
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static inline bool fdmon_io_uring_setup(AioContext *ctx)
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{
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return false;
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}
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static inline void fdmon_io_uring_destroy(AioContext *ctx)
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{
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}
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#endif /* !CONFIG_LINUX_IO_URING */
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#endif /* AIO_POSIX_H */
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