Commit 7376eda7 authored by Stefan Hajnoczi's avatar Stefan Hajnoczi
Browse files

block: make BDRV_POLL_WHILE() re-entrancy safe 



Nested BDRV_POLL_WHILE() calls can occur.  Currently
assert(!wait_->wakeup) fails in AIO_WAIT_WHILE() when this happens.

This patch converts the bool wait_->need_kick flag to an unsigned
wait_->num_waiters counter.

Nesting works correctly because outer AIO_WAIT_WHILE() callers evaluate
the condition again after the inner caller completes (invoking the inner
caller counts as aio_poll() progress).

Reported-by: default avatar"fuweiwei (C)" <fuweiwei2@huawei.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180307124619.6218-1-stefanha@redhat.com
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent e4ae62b8

include/block/aio-wait.h

+30 −31
Original line number Diff line number Diff line
@@ -50,8 +50,8 @@ 
 *   }

 */

typedef struct {

    /* Is the main loop waiting for a kick?  Accessed with atomic ops. */

    bool need_kick;

    /* Number of waiting AIO_WAIT_WHILE() callers. Accessed with atomic ops. */

    unsigned num_waiters;

} AioWait;



/**
@@ -84,9 +84,8 @@ typedef struct { 
    } else {                                                       \

        assert(qemu_get_current_aio_context() ==                   \

               qemu_get_aio_context());                            \

        assert(!wait_->need_kick);                          \

        /* Set wait_->need_kick before evaluating cond.  */ \

        atomic_mb_set(&wait_->need_kick, true);             \

        /* Increment wait_->num_waiters before evaluating cond. */ \

        atomic_inc(&wait_->num_waiters);                           \

        while (busy_) {                                            \

            if ((cond)) {                                          \

                waited_ = busy_ = true;                            \
@@ -98,7 +97,7 @@ typedef struct { 
                waited_ |= busy_;                                  \

            }                                                      \

        }                                                          \

        atomic_set(&wait_->need_kick, false);               \

        atomic_dec(&wait_->num_waiters);                           \

    }                                                              \

    waited_; })

util/aio-wait.c

+1 −1
Original line number Diff line number Diff line
@@ -34,7 +34,7 @@ static void dummy_bh_cb(void *opaque) 
void aio_wait_kick(AioWait *wait)

{

    /* The barrier (or an atomic op) is in the caller.  */

    if (atomic_read(&wait->need_kick)) {

    if (atomic_read(&wait->num_waiters)) {

        aio_bh_schedule_oneshot(qemu_get_aio_context(), dummy_bh_cb, NULL);

    }

}