Re: [Qemu-devel] [PATCH 1/1] Stop reinit of XBZRLE.lock

[陈梁]

nice catch

> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> Markus Armbruster spotted that the XBZRLE.lock might get initalised
> multiple times in the case of a second attempted migration, and
> that's undefined behaviour for pthread_mutex_init.
> 
> This patch adds a flag to stop re-initialisation - finding somewhere
> to cleanly destroy it where we can guarantee isn't happening
> at the same place we might want to take the lock is tricky.
> 
> It also adds comments to explain the lock use more.
> 
> (I considered rewriting this lockless but can't justify it given
> the simplicity of the fix).
> 
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
> arch_init.c | 30 ++++++++++++++++++++++++++----
> 1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 60c975d..16474b5 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -167,10 +167,13 @@ static struct {
>     /* Cache for XBZRLE, Protected by lock. */
>     PageCache *cache;
>     QemuMutex lock;

QemuMutex *lock;

> +    bool lock_init; /* True once we've init'd lock */
> } XBZRLE = {
>     .encoded_buf = NULL,
>     .current_buf = NULL,
>     .cache = NULL,

.lock = NULL,

> +    /* .lock is initialised in ram_save_setup */
> +    .lock_init = false
> };

maybe it is better that we malloc the lock in ram_save_setup dynamic,
and free it in migration_end.

> /* buffer used for XBZRLE decoding */
> static uint8_t *xbzrle_decoded_buf;
> @@ -187,6 +190,11 @@ static void XBZRLE_cache_unlock(void)
>         qemu_mutex_unlock(&XBZRLE.lock);
> }
> 
> +/* called from qmp_migrate_set_cache_size in main thread, possibly while
> + * a migration is in progress.
> + * A running migration maybe using the cache and might finish during this
> + * call, hence changes to the cache are protected by XBZRLE.lock().
> + */
> int64_t xbzrle_cache_resize(int64_t new_size)
> {
>     PageCache *new_cache, *cache_to_free;
> @@ -195,9 +203,12 @@ int64_t xbzrle_cache_resize(int64_t new_size)
>         return -1;
>     }
> 
> -    /* no need to lock, the current thread holds qemu big lock */
> +    /* The current thread holds qemu big lock, and we hold it while creating
> +     * the cache in ram_save_setup, thus it's safe to test if the
> +     * cache exists yet without it's own lock (which might not have been
> +     * init'd yet)
> +     */
>     if (XBZRLE.cache != NULL) {
> -        /* check XBZRLE.cache again later */
>         if (pow2floor(new_size) == migrate_xbzrle_cache_size()) {
>             return pow2floor(new_size);
>         }
> @@ -209,7 +220,10 @@ int64_t xbzrle_cache_resize(int64_t new_size)
>         }
> 
>         XBZRLE_cache_lock();
> -        /* the XBZRLE.cache may have be destroyed, check it again */
> +        /* the migration might have finished between the check above and us
> +         * taking the lock,  causing XBZRLE.cache to be destroyed
> +         *   check it again
> +         */
>         if (XBZRLE.cache != NULL) {
>             cache_to_free = XBZRLE.cache;
>             XBZRLE.cache = new_cache;
> @@ -744,7 +758,15 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>             DPRINTF("Error creating cache\n");
>             return -1;
>         }
> -        qemu_mutex_init(&XBZRLE.lock);

we malloc the lock and init it. Then free it in migration_end. It is safe 
because
migration_end holds qemu big lock.

> +        /* mutex's can't be reinit'd without destroying them
> +         * and we've not got a good place to destroy it that
> +         * we can guarantee isn't being called when we might want
> +         * to hold the lock.
> +         */
> +        if (!XBZRLE.lock_init) {
> +            XBZRLE.lock_init = true;
> +            qemu_mutex_init(&XBZRLE.lock);
> +        }
>         qemu_mutex_unlock_iothread();
> 
>         /* We prefer not to abort if there is no memory */
> -- 
> 1.8.5.3
> 
>