From afbe70535ff1a8a7a32910cc15ebecc0ba92e7da Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Thu, 18 Jun 2015 18:47:19 +0200 Subject: [PATCH] main-loop: introduce qemu_mutex_iothread_locked 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! Cc: Frederic Konrad Message-Id: <1434646046-27150-3-git-send-email-pbonzini@redhat.com> Reviewed-by: Fam Zheng Signed-off-by: Paolo Bonzini --- 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 c09fbef5de..f547aebeaf 100644 --- a/cpus.c +++ b/cpus.c @@ -1146,6 +1146,13 @@ 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); @@ -1164,10 +1171,12 @@ 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 0f4a0fd4b2..bc18ca30e4 100644 --- a/include/qemu/main-loop.h +++ b/include/qemu/main-loop.h @@ -222,6 +222,16 @@ void qemu_set_fd_handler(int fd, 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. * diff --git a/stubs/iothread-lock.c b/stubs/iothread-lock.c index 5d8aca1b37..dda6f6b58d 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) { }