On Thu, 06/18 18:47, Paolo Bonzini wrote:
> This function will be used to avoid recursive locking of the iothread lock
> whenever address_space_rw/ld*/st* are called with the BQL held, which is
> almost always the case.
>
> Tracking whether the iothread is owned is very cheap (just use a TLS
> variable) but requires some care because now the lock must always be
> taken with qemu_mutex_lock_iothread(). Previously this wasn't the case.
> Outside TCG mode this is not a problem. In TCG mode, we need to be
> careful and avoid the "prod out of compiled code" step if already
> in a VCPU thread. This is easily done with a check on current_cpu,
> i.e. qemu_in_vcpu_thread().
>
> Hopefully, multithreaded TCG will get rid of the whole logic to kick
> VCPUs whenever an I/O event occurs!
>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> cpus.c | 9 +++++++++
> include/qemu/main-loop.h | 10 ++++++++++
> stubs/iothread-lock.c | 5 +++++
> 3 files changed, 24 insertions(+)
>
> diff --git a/cpus.c b/cpus.c
> index 2e807f9..9531d03 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1116,6 +1116,13 @@ bool qemu_in_vcpu_thread(void)
> return current_cpu && qemu_cpu_is_self(current_cpu);
> }
>
> +static __thread bool iothread_locked = false;
> +
> +bool qemu_mutex_iothread_locked(void)
> +{
> + return iothread_locked;
> +}
> +
> void qemu_mutex_lock_iothread(void)
> {
> atomic_inc(&iothread_requesting_mutex);
> @@ -1133,10 +1140,12 @@ void qemu_mutex_lock_iothread(void)
> atomic_dec(&iothread_requesting_mutex);
> qemu_cond_broadcast(&qemu_io_proceeded_cond);
> }
> + iothread_locked = true;
> }
>
> void qemu_mutex_unlock_iothread(void)
> {
> + iothread_locked = false;
> qemu_mutex_unlock(&qemu_global_mutex);
> }
>
> diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
> index 62c68c0..6b74eb9 100644
> --- a/include/qemu/main-loop.h
> +++ b/include/qemu/main-loop.h
> @@ -270,6 +270,16 @@ int qemu_add_child_watch(pid_t pid);
> #endif
>
> /**
> + * qemu_mutex_iothread_locked: Return lock status of the main loop mutex.
> + *
> + * The main loop mutex is the coarsest lock in QEMU, and as such it
> + * must always be taken outside other locks. This function helps
> + * functions take different paths depending on whether the current
> + * thread is running within the main loop mutex.
> + */
> +bool qemu_mutex_iothread_locked(void);
> +
> +/**
> * qemu_mutex_lock_iothread: Lock the main loop mutex.
> *
> * This function locks the main loop mutex. The mutex is taken by
> diff --git a/stubs/iothread-lock.c b/stubs/iothread-lock.c
> index 5d8aca1..dda6f6b 100644
> --- a/stubs/iothread-lock.c
> +++ b/stubs/iothread-lock.c
> @@ -1,6 +1,11 @@
> #include "qemu-common.h"
> #include "qemu/main-loop.h"
>
> +bool qemu_mutex_iothread_locked(void)
> +{
> + return true;
> +}
> +
> void qemu_mutex_lock_iothread(void)
> {
> }
> --
> 1.8.3.1
>
>
>
Reviewed-by: Fam Zheng <f...@redhat.com>
