ratelimit: protect with a mutex

Right now, rate limiting is protected by the AioContext mutex, which is
taken for example both by the block jobs and by qmp_block_job_set_speed
(via find_block_job).

We would like to remove the dependency of block layer code on the
AioContext mutex, since most drivers and the core I/O code are already
not relying on it.  However, there is no existing lock that can easily
be taken by both ratelimit_set_speed and ratelimit_calculate_delay,
especially because the latter might run in coroutine context (and
therefore under a CoMutex) but the former will not.

Since concurrent calls to ratelimit_calculate_delay are not possible,
one idea could be to use a seqlock to get a snapshot of slice_ns and
slice_quota.  But for now keep it simple, and just add a mutex to the
RateLimit struct; block jobs are generally not performance critical to
the point of optimizing the clock cycles spent in synchronization.

This also requires the introduction of init/destroy functions, so
add them to the two users of ratelimit.h.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/qemu/ratelimit.h b/include/qemu/ratelimit.h
index 01da8d63f1..003ea6d5a3 100644
--- a/include/qemu/ratelimit.h
+++ b/include/qemu/ratelimit.h
@@ -14,9 +14,11 @@
 #ifndef QEMU_RATELIMIT_H
 #define QEMU_RATELIMIT_H
 
+#include "qemu/lockable.h"
 #include "qemu/timer.h"
 
 typedef struct {
+    QemuMutex lock;
     int64_t slice_start_time;
     int64_t slice_end_time;
     uint64_t slice_quota;
@@ -40,6 +42,7 @@ static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
     int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     double delay_slices;
 
+    QEMU_LOCK_GUARD(&limit->lock);
     assert(limit->slice_quota && limit->slice_ns);
 
     if (limit->slice_end_time < now) {
@@ -65,9 +68,20 @@ static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
     return limit->slice_end_time - now;
 }
 
+static inline void ratelimit_init(RateLimit *limit)
+{
+    qemu_mutex_init(&limit->lock);
+}
+
+static inline void ratelimit_destroy(RateLimit *limit)
+{
+    qemu_mutex_destroy(&limit->lock);
+}
+
 static inline void ratelimit_set_speed(RateLimit *limit, uint64_t speed,
                                        uint64_t slice_ns)
 {
+    QEMU_LOCK_GUARD(&limit->lock);
     limit->slice_ns = slice_ns;
     limit->slice_quota = MAX(((double)speed * slice_ns) / 1000000000ULL, 1);
 }