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aio-posix: support userspace polling of fd monitoring

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Unlike ppoll(2) and epoll(7), Linux io_uring completions can be polled
from userspace.  Previously userspace polling was only allowed when all
AioHandler's had an ->io_poll() callback.  This prevented starvation of
fds by userspace pollable handlers.

Add the FDMonOps->need_wait() callback that enables userspace polling
even when some AioHandlers lack ->io_poll().

For example, it's now possible to do userspace polling when a TCP/IP
socket is monitored thanks to Linux io_uring.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-7-stefanha@redhat.com
Message-Id: <20200305170806.1313245-7-stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-03-05 17:08:05 +00:00
parent 73fd282e7b
commit aa38e19f05
5 changed files with 35 additions and 3 deletions
Download patch file Download diff file Expand all files Collapse all files

19
include/block/aio.h
View file

@ -55,6 +55,9 @@ struct ThreadPool;
struct LinuxAioState;
struct LuringState;
/* Is polling disabled? */
bool aio_poll_disabled(AioContext *ctx);
/* Callbacks for file descriptor monitoring implementations */
typedef struct {
/*
@ -84,6 +87,22 @@ typedef struct {
* Returns: number of ready file descriptors.
*/
int (*wait)(AioContext *ctx, AioHandlerList *ready_list, int64_t timeout);
/*
* need_wait:
* @ctx: the AioContext
*
* Tell aio_poll() when to stop userspace polling early because ->wait()
* has fds ready.
*
* File descriptor monitoring implementations that cannot poll fd readiness
* from userspace should use aio_poll_disabled() here. This ensures that
* file descriptors are not starved by handlers that frequently make
* progress via userspace polling.
*
* Returns: true if ->wait() should be called, false otherwise.
*/
bool (*need_wait)(AioContext *ctx);
} FDMonOps;
/*