[Qemu-devel] [RFC v2 04/11] tcg: comment on which functions have to be called with tb_lock held

[Alex Bennée]

From: Paolo Bonzini <address@hidden>

softmmu requires more functions to be thread-safe, because translation
blocks can be invalidated from e.g. notdirty callbacks.  Probably the
same holds for user-mode emulation, it's just that no one has ever
tried to produce a coherent locking there.

This patch will guide the introduction of more tb_lock and tb_unlock
calls for system emulation.

Note that after this patch some (most) of the mentioned functions are
still called outside tb_lock/tb_unlock.  The next one will rectify this.

Signed-off-by: Paolo Bonzini <address@hidden>
Signed-off-by: Alex Bennée <address@hidden>

---
v1(ajb):
  - just s-o-b
v2
  - clarify write lock on tb_jump_cache
---
 exec.c                  |  1 +
 include/exec/exec-all.h |  1 +
 include/qom/cpu.h       |  3 +++
 tcg/tcg.h               |  2 ++
 translate-all.c         | 32 ++++++++++++++++++++++++++------
 5 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/exec.c b/exec.c
index f46e596..17f390e 100644
--- a/exec.c
+++ b/exec.c
@@ -826,6 +826,7 @@ int cpu_breakpoint_insert(CPUState *cpu, vaddr pc, int 
flags,
 {
     CPUBreakpoint *bp;
 
+    /* TODO: locking (RCU?) */
     bp = g_malloc(sizeof(*bp));
 
     bp->pc = pc;
diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index bbd9807..60716ae 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -386,6 +386,7 @@ static inline void tb_set_jmp_target(TranslationBlock *tb,
 
 #endif
 
+/* Called with tb_lock held.  */
 static inline void tb_add_jump(TranslationBlock *tb, int n,
                                TranslationBlock *tb_next)
 {
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 6931db9..13eeaae 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -306,7 +306,10 @@ struct CPUState {
 
     void *env_ptr; /* CPUArchState */
     struct TranslationBlock *current_tb;
+
+    /* Writes protected by tb_lock, reads not thread-safe  */
     struct TranslationBlock *tb_jmp_cache[TB_JMP_CACHE_SIZE];
+
     struct GDBRegisterState *gdb_regs;
     int gdb_num_regs;
     int gdb_num_g_regs;
diff --git a/tcg/tcg.h b/tcg/tcg.h
index ea4ff02..a46d17c 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -609,6 +609,7 @@ static inline bool tcg_op_buf_full(void)
 
 /* pool based memory allocation */
 
+/* tb_lock must be held for tcg_malloc_internal. */
 void *tcg_malloc_internal(TCGContext *s, int size);
 void tcg_pool_reset(TCGContext *s);
 void tcg_pool_delete(TCGContext *s);
@@ -617,6 +618,7 @@ void tb_lock(void);
 void tb_unlock(void);
 void tb_lock_reset(void);
 
+/* Called with tb_lock held.  */
 static inline void *tcg_malloc(int size)
 {
     TCGContext *s = &tcg_ctx;
diff --git a/translate-all.c b/translate-all.c
index d923008..a7ff5e7 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -248,7 +248,9 @@ static int encode_search(TranslationBlock *tb, uint8_t 
*block)
     return p - block;
 }
 
-/* The cpu state corresponding to 'searched_pc' is restored.  */
+/* The cpu state corresponding to 'searched_pc' is restored.
+ * Called with tb_lock held.
+ */
 static int cpu_restore_state_from_tb(CPUState *cpu, TranslationBlock *tb,
                                      uintptr_t searched_pc)
 {
@@ -306,8 +308,10 @@ bool cpu_restore_state(CPUState *cpu, uintptr_t retaddr)
         if (tb->cflags & CF_NOCACHE) {
             /* one-shot translation, invalidate it immediately */
             cpu->current_tb = NULL;
+            tb_lock();
             tb_phys_invalidate(tb, -1);
             tb_free(tb);
+            tb_unlock();
         }
         return true;
     }
@@ -399,6 +403,7 @@ static void page_init(void)
 }
 
 /* If alloc=1:
+ * Called with tb_lock held for system emulation.
  * Called with mmap_lock held for user-mode emulation.
  */
 static PageDesc *page_find_alloc(tb_page_addr_t index, int alloc)
@@ -754,8 +759,12 @@ bool tcg_enabled(void)
     return tcg_ctx.code_gen_buffer != NULL;
 }
 
-/* Allocate a new translation block. Flush the translation buffer if
-   too many translation blocks or too much generated code. */
+/*
+ * Allocate a new translation block. Flush the translation buffer if
+ * too many translation blocks or too much generated code.
+ *
+ * Called with tb_lock held.
+ */
 static TranslationBlock *tb_alloc(target_ulong pc)
 {
     TranslationBlock *tb;
@@ -769,6 +778,7 @@ static TranslationBlock *tb_alloc(target_ulong pc)
     return tb;
 }
 
+/* Called with tb_lock held.  */
 void tb_free(TranslationBlock *tb)
 {
     /* In practice this is mostly used for single use temporary TB
@@ -874,7 +884,10 @@ static void tb_invalidate_check(target_ulong address)
     }
 }
 
-/* verify that all the pages have correct rights for code */
+/* verify that all the pages have correct rights for code
+ *
+ * Called with tb_lock held.
+ */
 static void tb_page_check(void)
 {
     TranslationBlock *tb;
@@ -985,7 +998,10 @@ static inline void tb_jmp_unlink(TranslationBlock *tb)
     }
 }
 
-/* invalidate one TB */
+/* invalidate one TB
+ *
+ * Called with tb_lock held.
+ */
 void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
 {
     CPUState *cpu;
@@ -1062,6 +1078,7 @@ static void build_page_bitmap(PageDesc *p)
 
 /* add the tb in the target page and protect it if necessary
  *
+ * Called with tb_lock held.
  * Called with mmap_lock held for user-mode emulation.
  */
 static inline void tb_alloc_page(TranslationBlock *tb,
@@ -1429,7 +1446,9 @@ void tb_invalidate_phys_page_fast(tb_page_addr_t start, 
int len)
     }
     if (!p->code_bitmap &&
         ++p->code_write_count >= SMC_BITMAP_USE_THRESHOLD) {
-        /* build code bitmap */
+        /* build code bitmap.  FIXME: writes should be protected by
+         * tb_lock, reads by tb_lock or RCU.
+         */
         build_page_bitmap(p);
     }
     if (p->code_bitmap) {
@@ -1571,6 +1590,7 @@ void tb_invalidate_phys_addr(AddressSpace *as, hwaddr 
addr)
 }
 #endif /* !defined(CONFIG_USER_ONLY) */
 
+/* Called with tb_lock held.  */
 void tb_check_watchpoint(CPUState *cpu)
 {
     TranslationBlock *tb;
-- 
2.7.4