* Gonglei (Arei) (arei.gong...@huawei.com) wrote: > Resizing the xbzrle cache during migration causes qemu-crash, > because the main-thread and migration-thread modify the xbzrle > cache size concurrently without lock-protection. > > Signed-off-by: ChenLiang <chenlian...@huawei.com> > Signed-off-by: Gonglei <arei.gong...@huawei.com> > --- > Changes against the previous version: > *Remove function cache_max_num_items and cache_page_size > *Protect migration_end and ram_save_setup by XBZRLE.lock > > arch_init.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---- > 1 files changed, 56 insertions(+), 5 deletions(-) > > diff --git a/arch_init.c b/arch_init.c > index 80574a0..d295237 100644 > --- a/arch_init.c > +++ b/arch_init.c > @@ -164,26 +164,69 @@ static struct { > uint8_t *encoded_buf; > /* buffer for storing page content */ > uint8_t *current_buf; > - /* Cache for XBZRLE */ > + /* Cache for XBZRLE, Protected by lock. */ > PageCache *cache; > + QemuMutex lock; > } XBZRLE = { > .encoded_buf = NULL, > .current_buf = NULL, > .cache = NULL, > + /* use the lock carefully */ > + .lock = {PTHREAD_MUTEX_INITIALIZER}, > }; > /* buffer used for XBZRLE decoding */ > static uint8_t *xbzrle_decoded_buf; > > +static void XBZRLE_cache_lock(void) > +{ > + qemu_mutex_lock(&XBZRLE.lock); > +} > + > +static void XBZRLE_cache_unlock(void) > +{ > + qemu_mutex_unlock(&XBZRLE.lock); > +} > +
You might want to make these only bother with the lock if xbzrle is enabled - however actually, I think it's probably just best to keep them as is, and simple. > int64_t xbzrle_cache_resize(int64_t new_size) > { > + PageCache *new_cache, *old_cache, *cache; > + > if (new_size < TARGET_PAGE_SIZE) { > return -1; > } > > - if (XBZRLE.cache != NULL) { > - return cache_resize(XBZRLE.cache, new_size / TARGET_PAGE_SIZE) * > - TARGET_PAGE_SIZE; > + XBZRLE_cache_lock(); > + /* check XBZRLE.cache again later */ > + cache = XBZRLE.cache; > + XBZRLE_cache_unlock(); That's a bit of an odd way of atomically reading a pointer; I think an smp_rmb(); cache = atomic_read(&XBZRLE.cache); would have done the trick. Personally I would have just kept the lock at the top and not released it until the end of the following section. > + if (cache != NULL) { > + if (pow2floor(new_size) == migrate_xbzrle_cache_size()) { > + goto ret; Hmm goto not really needed there. > + } > + new_cache = cache_init(new_size / TARGET_PAGE_SIZE, > + TARGET_PAGE_SIZE); > + if (!new_cache) { > + DPRINTF("Error creating cache\n"); > + return -1; > + } > + > + XBZRLE_cache_lock(); > + /* the XBZRLE.cache may have be destroyed, check it again */ > + if (XBZRLE.cache != NULL) { > + old_cache = XBZRLE.cache; > + XBZRLE.cache = new_cache; > + new_cache = NULL; > + } > + XBZRLE_cache_unlock(); > + > + if (NULL == new_cache) { > + cache_fini(old_cache); > + } else { > + cache_fini(new_cache); > + } Yes, it took me a few readings to understand that; i.e. if new_cache=NULL then it's actually now in use in XBZRLE.cache so only the old_cache needs freeing. > } > +ret: > return pow2floor(new_size); > } > > @@ -522,6 +565,8 @@ static int ram_save_block(QEMUFile *f, bool last_stage) > ret = ram_control_save_page(f, block->offset, > offset, TARGET_PAGE_SIZE, &bytes_sent); > > + XBZRLE_cache_lock(); > + > if (ret != RAM_SAVE_CONTROL_NOT_SUPP) { > if (ret != RAM_SAVE_CONTROL_DELAYED) { > if (bytes_sent > 0) { > @@ -553,6 +598,8 @@ static int ram_save_block(QEMUFile *f, bool last_stage) > acct_info.norm_pages++; > } > > + XBZRLE_cache_unlock(); > + > /* if page is unmodified, continue to the next */ > if (bytes_sent > 0) { > last_sent_block = block; Note that's not actually safe until my other xbzrle patch lands, because the cache data is currently passed into put_buffer_async that will read it later (possibly after you've reallocated) - but that's ok, that should get fixed in the mean time. > @@ -620,6 +667,7 @@ static void migration_end(void) > migration_bitmap = NULL; > } > > + XBZRLE_cache_lock(); > if (XBZRLE.cache) { > cache_fini(XBZRLE.cache); > g_free(XBZRLE.cache); > @@ -629,6 +677,7 @@ static void migration_end(void) > XBZRLE.encoded_buf = NULL; > XBZRLE.current_buf = NULL; > } > + XBZRLE_cache_unlock(); > } > > static void ram_migration_cancel(void *opaque) > @@ -659,13 +708,16 @@ static int ram_save_setup(QEMUFile *f, void *opaque) > dirty_rate_high_cnt = 0; > > if (migrate_use_xbzrle()) { > + XBZRLE_cache_lock(); > XBZRLE.cache = cache_init(migrate_xbzrle_cache_size() / > TARGET_PAGE_SIZE, > TARGET_PAGE_SIZE); > if (!XBZRLE.cache) { > + XBZRLE_cache_unlock(); > DPRINTF("Error creating cache\n"); > return -1; > } > + XBZRLE_cache_unlock(); > > /* We prefer not to abort if there is no memory */ > XBZRLE.encoded_buf = g_try_malloc0(TARGET_PAGE_SIZE); > @@ -681,7 +733,6 @@ static int ram_save_setup(QEMUFile *f, void *opaque) > XBZRLE.encoded_buf = NULL; > return -1; > } > - > acct_clear(); > } > > -- > 1.6.0.2 > > Best regards, > -Gonglei Thanks, Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com> -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK