qemu/include/block/aio.h
Stefan Hajnoczi ca96ac44dc AioContext: force event loop iteration using BH
The notify_me optimization introduced in commit eabc977973
("AioContext: fix broken ctx->dispatching optimization") skips
event_notifier_set() calls when the event loop thread is not blocked in
ppoll(2).

This optimization causes a deadlock if two aio_context_acquire() calls
race.  notify_me = 0 during the race so the winning thread can enter
ppoll(2) unaware that the other thread is waiting its turn to acquire
the AioContext.

This patch forces ppoll(2) to return by scheduling a BH instead of
calling aio_notify().

The following deadlock with virtio-blk dataplane is fixed:

  qemu ... -object iothread,id=iothread0 \
           -drive if=none,id=drive0,file=test.img,... \
           -device virtio-blk-pci,iothread=iothread0,drive=drive0

This command-line results in a hang early on without this patch.

Thanks to Paolo Bonzini <pbonzini@redhat.com> for investigating this bug
with me.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1438101249-25166-4-git-send-email-pbonzini@redhat.com
Message-Id: <1438014819-18125-3-git-send-email-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-29 10:02:06 +01:00

377 lines
13 KiB
C

/*
* QEMU aio implementation
*
* Copyright IBM, Corp. 2008
*
* Authors:
* Anthony Liguori <aliguori@us.ibm.com>
*
* This work is licensed under the terms of the GNU GPL, version 2. See
* the COPYING file in the top-level directory.
*
*/
#ifndef QEMU_AIO_H
#define QEMU_AIO_H
#include "qemu/typedefs.h"
#include "qemu-common.h"
#include "qemu/queue.h"
#include "qemu/event_notifier.h"
#include "qemu/thread.h"
#include "qemu/rfifolock.h"
#include "qemu/timer.h"
typedef struct BlockAIOCB BlockAIOCB;
typedef void BlockCompletionFunc(void *opaque, int ret);
typedef struct AIOCBInfo {
void (*cancel_async)(BlockAIOCB *acb);
AioContext *(*get_aio_context)(BlockAIOCB *acb);
size_t aiocb_size;
} AIOCBInfo;
struct BlockAIOCB {
const AIOCBInfo *aiocb_info;
BlockDriverState *bs;
BlockCompletionFunc *cb;
void *opaque;
int refcnt;
};
void *qemu_aio_get(const AIOCBInfo *aiocb_info, BlockDriverState *bs,
BlockCompletionFunc *cb, void *opaque);
void qemu_aio_unref(void *p);
void qemu_aio_ref(void *p);
typedef struct AioHandler AioHandler;
typedef void QEMUBHFunc(void *opaque);
typedef void IOHandler(void *opaque);
struct AioContext {
GSource source;
/* Protects all fields from multi-threaded access */
RFifoLock lock;
/* The list of registered AIO handlers */
QLIST_HEAD(, AioHandler) aio_handlers;
/* This is a simple lock used to protect the aio_handlers list.
* Specifically, it's used to ensure that no callbacks are removed while
* we're walking and dispatching callbacks.
*/
int walking_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.
*
* Bit 0 is reserved for GSource usage of the AioContext, and is 1
* between a call to aio_ctx_check and the next call to aio_ctx_dispatch.
* Bits 1-31 simply count the number of active calls to aio_poll
* that are in the prepare or poll phase.
*
* The GSource and aio_poll must use a different mechanism because
* there is no certainty that a call to GSource's prepare callback
* (via g_main_context_prepare) is indeed followed by check and
* dispatch. It's not clear whether this would be a bug, but let's
* play safe and allow it---it will just cause extra calls to
* event_notifier_set until the next call to dispatch.
*
* Instead, the aio_poll calls include both the prepare and the
* dispatch phase, hence a simple counter is enough for them.
*/
uint32_t notify_me;
/* lock to protect between bh's adders and deleter */
QemuMutex bh_lock;
/* Anchor of the list of Bottom Halves belonging to the context */
struct QEMUBH *first_bh;
/* A simple lock used to protect the first_bh list, and ensure that
* no callbacks are removed while we're walking and dispatching callbacks.
*/
int walking_bh;
/* Used by aio_notify.
*
* "notified" is used to avoid expensive event_notifier_test_and_clear
* calls. When it is clear, the EventNotifier is clear, or one thread
* is going to clear "notified" before processing more events. False
* positives are possible, i.e. "notified" could be set even though the
* EventNotifier is clear.
*
* Note that event_notifier_set *cannot* be optimized the same way. For
* more information on the problem that would result, see "#ifdef BUG2"
* in the docs/aio_notify_accept.promela formal model.
*/
bool notified;
EventNotifier notifier;
/* Scheduling this BH forces the event loop it iterate */
QEMUBH *notify_dummy_bh;
/* Thread pool for performing work and receiving completion callbacks */
struct ThreadPool *thread_pool;
/* TimerLists for calling timers - one per clock type */
QEMUTimerListGroup tlg;
};
/**
* aio_context_new: Allocate a new AioContext.
*
* AioContext provide a mini event-loop that can be waited on synchronously.
* They also provide bottom halves, a service to execute a piece of code
* as soon as possible.
*/
AioContext *aio_context_new(Error **errp);
/**
* aio_context_ref:
* @ctx: The AioContext to operate on.
*
* Add a reference to an AioContext.
*/
void aio_context_ref(AioContext *ctx);
/**
* aio_context_unref:
* @ctx: The AioContext to operate on.
*
* Drop a reference to an AioContext.
*/
void aio_context_unref(AioContext *ctx);
/* Take ownership of the AioContext. If the AioContext will be shared between
* threads, and a thread does not want to be interrupted, it will have to
* take ownership around calls to aio_poll(). Otherwise, aio_poll()
* automatically takes care of calling aio_context_acquire and
* aio_context_release.
*
* Access to timers and BHs from a thread that has not acquired AioContext
* is possible. Access to callbacks for now must be done while the AioContext
* is owned by the thread (FIXME).
*/
void aio_context_acquire(AioContext *ctx);
/* Relinquish ownership of the AioContext. */
void aio_context_release(AioContext *ctx);
/**
* aio_bh_new: Allocate a new bottom half structure.
*
* Bottom halves are lightweight callbacks whose invocation is guaranteed
* to be wait-free, thread-safe and signal-safe. The #QEMUBH structure
* is opaque and must be allocated prior to its use.
*/
QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
/**
* aio_notify: Force processing of pending events.
*
* Similar to signaling a condition variable, aio_notify forces
* aio_wait to exit, so that the next call will re-examine pending events.
* The caller of aio_notify will usually call aio_wait again very soon,
* or go through another iteration of the GLib main loop. Hence, aio_notify
* also has the side effect of recalculating the sets of file descriptors
* that the main loop waits for.
*
* Calling aio_notify is rarely necessary, because for example scheduling
* a bottom half calls it already.
*/
void aio_notify(AioContext *ctx);
/**
* aio_notify_accept: Acknowledge receiving an aio_notify.
*
* aio_notify() uses an EventNotifier in order to wake up a sleeping
* aio_poll() or g_main_context_iteration(). Calls to aio_notify() are
* usually rare, but the AioContext has to clear the EventNotifier on
* every aio_poll() or g_main_context_iteration() in order to avoid
* busy waiting. This event_notifier_test_and_clear() cannot be done
* using the usual aio_context_set_event_notifier(), because it must
* be done before processing all events (file descriptors, bottom halves,
* timers).
*
* aio_notify_accept() is an optimized event_notifier_test_and_clear()
* that is specific to an AioContext's notifier; it is used internally
* to clear the EventNotifier only if aio_notify() had been called.
*/
void aio_notify_accept(AioContext *ctx);
/**
* aio_bh_poll: Poll bottom halves for an AioContext.
*
* These are internal functions used by the QEMU main loop.
* And notice that multiple occurrences of aio_bh_poll cannot
* be called concurrently
*/
int aio_bh_poll(AioContext *ctx);
/**
* qemu_bh_schedule: Schedule a bottom half.
*
* Scheduling a bottom half interrupts the main loop and causes the
* execution of the callback that was passed to qemu_bh_new.
*
* Bottom halves that are scheduled from a bottom half handler are instantly
* invoked. This can create an infinite loop if a bottom half handler
* schedules itself.
*
* @bh: The bottom half to be scheduled.
*/
void qemu_bh_schedule(QEMUBH *bh);
/**
* qemu_bh_cancel: Cancel execution of a bottom half.
*
* Canceling execution of a bottom half undoes the effect of calls to
* qemu_bh_schedule without freeing its resources yet. While cancellation
* itself is also wait-free and thread-safe, it can of course race with the
* loop that executes bottom halves unless you are holding the iothread
* mutex. This makes it mostly useless if you are not holding the mutex.
*
* @bh: The bottom half to be canceled.
*/
void qemu_bh_cancel(QEMUBH *bh);
/**
*qemu_bh_delete: Cancel execution of a bottom half and free its resources.
*
* Deleting a bottom half frees the memory that was allocated for it by
* qemu_bh_new. It also implies canceling the bottom half if it was
* scheduled.
* This func is async. The bottom half will do the delete action at the finial
* end.
*
* @bh: The bottom half to be deleted.
*/
void qemu_bh_delete(QEMUBH *bh);
/* Return whether there are any pending callbacks from the GSource
* attached to the AioContext, before g_poll is invoked.
*
* This is used internally in the implementation of the GSource.
*/
bool aio_prepare(AioContext *ctx);
/* Return whether there are any pending callbacks from the GSource
* attached to the AioContext, after g_poll is invoked.
*
* This is used internally in the implementation of the GSource.
*/
bool aio_pending(AioContext *ctx);
/* Dispatch any pending callbacks from the GSource attached to the AioContext.
*
* This is used internally in the implementation of the GSource.
*/
bool aio_dispatch(AioContext *ctx);
/* Progress in completing AIO work to occur. This can issue new pending
* aio as a result of executing I/O completion or bh callbacks.
*
* Return whether any progress was made by executing AIO or bottom half
* handlers. If @blocking == true, this should always be true except
* if someone called aio_notify.
*
* If there are no pending bottom halves, but there are pending AIO
* operations, it may not be possible to make any progress without
* blocking. If @blocking is true, this function will wait until one
* or more AIO events have completed, to ensure something has moved
* before returning.
*/
bool aio_poll(AioContext *ctx, bool blocking);
/* Register a file descriptor and associated callbacks. Behaves very similarly
* to qemu_set_fd_handler. Unlike qemu_set_fd_handler, these callbacks will
* be invoked when using aio_poll().
*
* Code that invokes AIO completion functions should rely on this function
* instead of qemu_set_fd_handler[2].
*/
void aio_set_fd_handler(AioContext *ctx,
int fd,
IOHandler *io_read,
IOHandler *io_write,
void *opaque);
/* Register an event notifier and associated callbacks. Behaves very similarly
* to event_notifier_set_handler. Unlike event_notifier_set_handler, these callbacks
* will be invoked when using aio_poll().
*
* Code that invokes AIO completion functions should rely on this function
* instead of event_notifier_set_handler.
*/
void aio_set_event_notifier(AioContext *ctx,
EventNotifier *notifier,
EventNotifierHandler *io_read);
/* Return a GSource that lets the main loop poll the file descriptors attached
* to this AioContext.
*/
GSource *aio_get_g_source(AioContext *ctx);
/* Return the ThreadPool bound to this AioContext */
struct ThreadPool *aio_get_thread_pool(AioContext *ctx);
/**
* aio_timer_new:
* @ctx: the aio context
* @type: the clock type
* @scale: the scale
* @cb: the callback to call on timer expiry
* @opaque: the opaque pointer to pass to the callback
*
* Allocate a new timer attached to the context @ctx.
* The function is responsible for memory allocation.
*
* The preferred interface is aio_timer_init. Use that
* unless you really need dynamic memory allocation.
*
* Returns: a pointer to the new timer
*/
static inline QEMUTimer *aio_timer_new(AioContext *ctx, QEMUClockType type,
int scale,
QEMUTimerCB *cb, void *opaque)
{
return timer_new_tl(ctx->tlg.tl[type], scale, cb, opaque);
}
/**
* aio_timer_init:
* @ctx: the aio context
* @ts: the timer
* @type: the clock type
* @scale: the scale
* @cb: the callback to call on timer expiry
* @opaque: the opaque pointer to pass to the callback
*
* Initialise a new timer attached to the context @ctx.
* The caller is responsible for memory allocation.
*/
static inline void aio_timer_init(AioContext *ctx,
QEMUTimer *ts, QEMUClockType type,
int scale,
QEMUTimerCB *cb, void *opaque)
{
timer_init_tl(ts, ctx->tlg.tl[type], scale, cb, opaque);
}
/**
* aio_compute_timeout:
* @ctx: the aio context
*
* Compute the timeout that a blocking aio_poll should use.
*/
int64_t aio_compute_timeout(AioContext *ctx);
#endif