Handle CPU interrupts by inline checking of a flag

Fix some of the nasty TCG race conditions and crashes by implementing
cpu_exit() as setting a flag which is checked at the start of each TB.
This avoids crashes if a thread or signal handler calls cpu_exit()
while the execution thread is itself modifying the TB graph (which
may happen in system emulation mode as well as in linux-user mode
with a multithreaded guest binary).

This fixes the crashes seen in LP:668799; however there are another
class of crashes described in LP:1098729 which stem from the fact
that in linux-user with a multithreaded guest all threads will
use and modify the same global TCG date structures (including the
generated code buffer) without any kind of locking. This means that
multithreaded guest binaries are still in the "unsupported"
category.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

diff --git a/cpu-exec.c b/cpu-exec.c
index f9ea080e31df57d291d4b06d78b92092fb3aaf2d..9092145d0b4e6493dd329d1e598bb18d76297f59 100644 (file)
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -64,6 +64,12 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, uint8_t *tb_ptr)
         TranslationBlock *tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
         cpu_pc_from_tb(env, tb);
     }
+    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_REQUESTED) {
+        /* We were asked to stop executing TBs (probably a pending
+         * interrupt. We've now stopped, so clear the flag.
+         */
+        cpu->tcg_exit_req = 0;
+    }
     return next_tb;
 }
 
@@ -608,7 +614,20 @@ int cpu_exec(CPUArchState *env)
                     tc_ptr = tb->tc_ptr;
                     /* execute the generated code */
                     next_tb = cpu_tb_exec(cpu, tc_ptr);
-                    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_ICOUNT_EXPIRED) {
+                    switch (next_tb & TB_EXIT_MASK) {
+                    case TB_EXIT_REQUESTED:
+                        /* Something asked us to stop executing
+                         * chained TBs; just continue round the main
+                         * loop. Whatever requested the exit will also
+                         * have set something else (eg exit_request or
+                         * interrupt_request) which we will handle
+                         * next time around the loop.
+                         */
+                        tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
+                        next_tb = 0;
+                        break;
+                    case TB_EXIT_ICOUNT_EXPIRED:
+                    {
                         /* Instruction counter expired.  */
                         int insns_left;
                         tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
@@ -632,6 +651,10 @@ int cpu_exec(CPUArchState *env)
                             next_tb = 0;
                             cpu_loop_exit(env);
                         }
+                        break;
+                    }
+                    default:
+                        break;
                     }
                 }
                 cpu->current_tb = NULL;

diff --git a/exec.c b/exec.c
index a41bcb86943821496e21158dd11bfc93c25b92e5..46a283071a2e908bd9f3aa77af7d2c2694909246 100644 (file)
--- a/exec.c
+++ b/exec.c
@@ -495,7 +495,7 @@ void cpu_exit(CPUArchState *env)
     CPUState *cpu = ENV_GET_CPU(env);
 
     cpu->exit_request = 1;
-    cpu_unlink_tb(cpu);
+    cpu->tcg_exit_req = 1;
 }
 
 void cpu_abort(CPUArchState *env, const char *fmt, ...)

diff --git a/include/exec/gen-icount.h b/include/exec/gen-icount.h
index c858a738c0b885157cf80ca548f56d1a85670bda..384153b4c4f0999dd1b6781afc28d991b33f0a11 100644 (file)
--- a/include/exec/gen-icount.h
+++ b/include/exec/gen-icount.h
@@ -7,10 +7,19 @@
 
 static TCGArg *icount_arg;
 static int icount_label;
+static int exitreq_label;
 
 static inline void gen_icount_start(void)
 {
     TCGv_i32 count;
+    TCGv_i32 flag;
+
+    exitreq_label = gen_new_label();
+    flag = tcg_temp_local_new_i32();
+    tcg_gen_ld_i32(flag, cpu_env,
+                   offsetof(CPUState, tcg_exit_req) - ENV_OFFSET);
+    tcg_gen_brcondi_i32(TCG_COND_NE, flag, 0, exitreq_label);
+    tcg_temp_free_i32(flag);
 
     if (!use_icount)
         return;
@@ -29,6 +38,9 @@ static inline void gen_icount_start(void)
 
 static void gen_icount_end(TranslationBlock *tb, int num_insns)
 {
+    gen_set_label(exitreq_label);
+    tcg_gen_exit_tb((tcg_target_long)tb + TB_EXIT_REQUESTED);
+
     if (use_icount) {
         *icount_arg = num_insns;
         gen_set_label(icount_label);

diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index ee1a7c878a04a1b3683eabfc2f0376f40427672f..ab2657c558397330aee1097b327e70755a221f9c 100644 (file)
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -71,6 +71,8 @@ struct kvm_run;
  * @created: Indicates whether the CPU thread has been successfully created.
  * @stop: Indicates a pending stop request.
  * @stopped: Indicates the CPU has been artificially stopped.
+ * @tcg_exit_req: Set to force TCG to stop executing linked TBs for this
+ *           CPU and return to its top level loop.
  * @env_ptr: Pointer to subclass-specific CPUArchState field.
  * @current_tb: Currently executing TB.
  * @kvm_fd: vCPU file descriptor for KVM.
@@ -100,6 +102,7 @@ struct CPUState {
     bool stop;
     bool stopped;
     volatile sig_atomic_t exit_request;
+    volatile sig_atomic_t tcg_exit_req;
 
     void *env_ptr; /* CPUArchState */
     struct TranslationBlock *current_tb;

diff --git a/tcg/tcg.h b/tcg/tcg.h
index f5d0aed495d8f5a607ca7bfc8f79a1bec77dc415..df375cf31e4c246df11695bf5b7bced94b4b2e8d 100644 (file)
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -703,6 +703,10 @@ TCGv_i64 tcg_const_local_i64(int64_t val);
  *        would hit zero midway through it. In this case the next-TB pointer
  *        returned is the TB we were about to execute, and the caller must
  *        arrange to execute the remaining count of instructions.
+ *  3:    we stopped because the CPU's exit_request flag was set
+ *        (usually meaning that there is an interrupt that needs to be
+ *        handled). The next-TB pointer returned is the TB we were
+ *        about to execute when we noticed the pending exit request.
  *
  * If the bottom two bits indicate an exit-via-index then the CPU
  * state is correctly synchronised and ready for execution of the next
@@ -719,6 +723,7 @@ TCGv_i64 tcg_const_local_i64(int64_t val);
 #define TB_EXIT_IDX0 0
 #define TB_EXIT_IDX1 1
 #define TB_EXIT_ICOUNT_EXPIRED 2
+#define TB_EXIT_REQUESTED 3
 
 #if !defined(tcg_qemu_tb_exec)
 # define tcg_qemu_tb_exec(env, tb_ptr) \

diff --git a/translate-all.c b/translate-all.c
index b50fb89528df707e3a0d323648e1507745684f57..9741d9639545ec6fe41d34dafb9c3e80a0ff2836 100644 (file)
--- a/translate-all.c
+++ b/translate-all.c
@@ -1475,7 +1475,7 @@ static void tcg_handle_interrupt(CPUArchState *env, int mask)
             cpu_abort(env, "Raised interrupt while not in I/O function");
         }
     } else {
-        cpu_unlink_tb(cpu);
+        cpu->tcg_exit_req = 1;
     }
 }
 
@@ -1626,7 +1626,7 @@ void cpu_interrupt(CPUArchState *env, int mask)
     CPUState *cpu = ENV_GET_CPU(env);
 
     env->interrupt_request |= mask;
-    cpu_unlink_tb(cpu);
+    cpu->tcg_exit_req = 1;
 }
 
 /*