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aio-wait: delegate polling of main AioContext if BQL not held

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Any thread that is not a iothread returns NULL for qemu_get_current_aio_context().
As a result, it would also return true for
in_aio_context_home_thread(qemu_get_aio_context()), causing
AIO_WAIT_WHILE to invoke aio_poll() directly.  This is incorrect
if the BQL is not held, because aio_poll() does not expect to
run concurrently from multiple threads, and it can actually
happen when savevm writes to the vmstate file from the
migration thread.

Therefore, restrict in_aio_context_home_thread to return true
for the main AioContext only if the BQL is held.

The function is moved to aio-wait.h because it is mostly used
there and to avoid a circular reference between main-loop.h
and block/aio.h.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200407140746.8041-5-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2020-04-07 10:07:45 -04:00 committed by Stefan Hajnoczi
parent 636b836d5f
commit 3c18a92dc4
2 changed files with 32 additions and 19 deletions
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22
include/block/aio-wait.h
View File

@ -26,6 +26,7 @@
#define QEMU_AIO_WAIT_H
#include "block/aio.h"
#include "qemu/main-loop.h"
/**
* AioWait:
@ -124,4 +125,25 @@ void aio_wait_kick(void);
*/
void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
/**
* in_aio_context_home_thread:
* @ctx: the aio context
*
* Return whether we are running in the thread that normally runs @ctx. Note
* that acquiring/releasing ctx does not affect the outcome, each AioContext
* still only has one home thread that is responsible for running it.
*/
static inline bool in_aio_context_home_thread(AioContext *ctx)
{
if (ctx == qemu_get_current_aio_context()) {
return true;
}
if (ctx == qemu_get_aio_context()) {
return qemu_mutex_iothread_locked();
} else {
return false;
}
}
#endif /* QEMU_AIO_WAIT_H */

29
include/block/aio.h
View File

@ -133,12 +133,16 @@ struct AioContext {
AioHandlerList deleted_aio_handlers;
/* Used to avoid unnecessary event_notifier_set calls in aio_notify;
* accessed with atomic primitives. If this field is 0, everything
* (file descriptors, bottom halves, timers) will be re-evaluated
* before the next blocking poll(), thus the event_notifier_set call
* can be skipped. If it is non-zero, you may need to wake up a
* concurrent aio_poll or the glib main event loop, making
* event_notifier_set necessary.
* only written from the AioContext home thread, or under the BQL in
* the case of the main AioContext. However, it is read from any
* thread so it is still accessed with atomic primitives.
*
* If this field is 0, everything (file descriptors, bottom halves,
* timers) will be re-evaluated before the next blocking poll() or
* io_uring wait; therefore, the event_notifier_set call can be
* skipped. If it is non-zero, you may need to wake up a concurrent
* aio_poll or the glib main event loop, making event_notifier_set
* necessary.
*
* Bit 0 is reserved for GSource usage of the AioContext, and is 1
* between a call to aio_ctx_prepare and the next call to aio_ctx_check.
@ -681,19 +685,6 @@ void aio_co_enter(AioContext *ctx, struct Coroutine *co);
*/
AioContext *qemu_get_current_aio_context(void);
/**
* in_aio_context_home_thread:
* @ctx: the aio context
*
* Return whether we are running in the thread that normally runs @ctx. Note
* that acquiring/releasing ctx does not affect the outcome, each AioContext
* still only has one home thread that is responsible for running it.
*/
static inline bool in_aio_context_home_thread(AioContext *ctx)
{
return ctx == qemu_get_current_aio_context();
}
/**
* aio_context_setup:
* @ctx: the aio context