multifd: Document the locking of MultiFD{Send/Recv}Params

Reorder the structures so we can know if the fields are:
- Read only
- Their own locking (i.e. sems)
- Protected by 'mutex'
- Only for the multifd channel

Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20220531104318.7494-2-quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Typo fixes from Chen Zhang

migration/multifd.h

@@ -65,7 +65,9 @@ typedef struct {
 } MultiFDPages_t;
 
 typedef struct {
-    /* this fields are not changed once the thread is created */
+    /* Fields are only written at creating/deletion time */
+    /* No lock required for them, they are read only */
+
     /* channel number */
     uint8_t id;
     /* channel thread name */
@@ -74,39 +76,47 @@ typedef struct {
     QemuThread thread;
     /* communication channel */
     QIOChannel *c;
+    /* is the yank function registered */
+    bool registered_yank;
+    /* packet allocated len */
+    uint32_t packet_len;
+    /* multifd flags for sending ram */
+    int write_flags;
+
     /* sem where to wait for more work */
     QemuSemaphore sem;
+    /* syncs main thread and channels */
+    QemuSemaphore sem_sync;
+
     /* this mutex protects the following parameters */
     QemuMutex mutex;
     /* is this channel thread running */
     bool running;
     /* should this thread finish */
     bool quit;
-    /* is the yank function registered */
-    bool registered_yank;
-    /* thread has work to do */
-    int pending_job;
-    /* array of pages to sent */
-    MultiFDPages_t *pages;
-    /* packet allocated len */
-    uint32_t packet_len;
-    /* pointer to the packet */
-    MultiFDPacket_t *packet;
-    /* multifd flags for sending ram */
-    int write_flags;
     /* multifd flags for each packet */
     uint32_t flags;
-    /* size of the next packet that contains pages */
-    uint32_t next_packet_size;
     /* global number of generated multifd packets */
     uint64_t packet_num;
-    /* thread local variables */
+    /* thread has work to do */
+    int pending_job;
+    /* array of pages to sent.
+     * The owner of 'pages' depends of 'pending_job' value:
+     * pending_job == 0 -> migration_thread can use it.
+     * pending_job != 0 -> multifd_channel can use it.
+     */
+    MultiFDPages_t *pages;
+
+    /* thread local variables. No locking required */
+
+    /* pointer to the packet */
+    MultiFDPacket_t *packet;
+    /* size of the next packet that contains pages */
+    uint32_t next_packet_size;
     /* packets sent through this channel */
     uint64_t num_packets;
     /* non zero pages sent through this channel */
     uint64_t total_normal_pages;
-    /* syncs main thread and channels */
-    QemuSemaphore sem_sync;
     /* buffers to send */
     struct iovec *iov;
     /* number of iovs used */
@@ -120,7 +130,9 @@ typedef struct {
 }  MultiFDSendParams;
 
 typedef struct {
-    /* this fields are not changed once the thread is created */
+    /* Fields are only written at creating/deletion time */
+    /* No lock required for them, they are read only */
+
     /* channel number */
     uint8_t id;
     /* channel thread name */
@@ -129,31 +141,35 @@ typedef struct {
     QemuThread thread;
     /* communication channel */
     QIOChannel *c;
+    /* packet allocated len */
+    uint32_t packet_len;
+
+    /* syncs main thread and channels */
+    QemuSemaphore sem_sync;
+
     /* this mutex protects the following parameters */
     QemuMutex mutex;
     /* is this channel thread running */
     bool running;
     /* should this thread finish */
     bool quit;
-    /* ramblock host address */
-    uint8_t *host;
-    /* packet allocated len */
-    uint32_t packet_len;
-    /* pointer to the packet */
-    MultiFDPacket_t *packet;
     /* multifd flags for each packet */
     uint32_t flags;
     /* global number of generated multifd packets */
     uint64_t packet_num;
-    /* thread local variables */
+
+    /* thread local variables. No locking required */
+
+    /* pointer to the packet */
+    MultiFDPacket_t *packet;
     /* size of the next packet that contains pages */
     uint32_t next_packet_size;
     /* packets sent through this channel */
     uint64_t num_packets;
+    /* ramblock host address */
+    uint8_t *host;
     /* non zero pages recv through this channel */
     uint64_t total_normal_pages;
-    /* syncs main thread and channels */
-    QemuSemaphore sem_sync;
     /* buffers to recv */
     struct iovec *iov;
     /* Pages that are not zero */