Commit 758e1b2b authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

cpus-common: simplify locking for start_exclusive/end_exclusive 



It is not necessary to hold qemu_cpu_list_mutex throughout the
exclusive section, because no other exclusive section can run
while pending_cpus != 0.

exclusive_idle() is called in cpu_exec_start(), and that prevents
any CPUs created after start_exclusive() from entering cpu_exec()
during an exclusive section.

Reviewed-by: default avatarRichard Henderson <rth@twiddle.net>
Reviewed-by: default avatarAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent cf07da65

cpus-common.c

+8 −3
Original line number Diff line number Diff line
@@ -171,8 +171,7 @@ static inline void exclusive_idle(void) 
}



/* Start an exclusive operation.

   Must only be called from outside cpu_exec, takes

   qemu_cpu_list_lock.   */

   Must only be called from outside cpu_exec.  */

void start_exclusive(void)

{

    CPUState *other_cpu;
@@ -191,11 +190,17 @@ void start_exclusive(void) 
    while (pending_cpus > 1) {

        qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);

    }



    /* Can release mutex, no one will enter another exclusive

     * section until end_exclusive resets pending_cpus to 0.

     */

    qemu_mutex_unlock(&qemu_cpu_list_lock);

}



/* Finish an exclusive operation.  Releases qemu_cpu_list_lock.  */

/* Finish an exclusive operation.  */

void end_exclusive(void)

{

    qemu_mutex_lock(&qemu_cpu_list_lock);

    pending_cpus = 0;

    qemu_cond_broadcast(&exclusive_resume);

    qemu_mutex_unlock(&qemu_cpu_list_lock);

docs/tcg-exclusive.promela

+3 −1
Original line number Diff line number Diff line
@@ -98,9 +98,11 @@ byte has_waiter[N_CPUS]; 
    do                                                            \

      :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex);    \

      :: else             -> break;                               \

    od

    od;                                                           \

    MUTEX_UNLOCK(mutex);



#define end_exclusive()                                           \

    MUTEX_LOCK(mutex);                                            \

    pending_cpus = 0;                                             \

    COND_BROADCAST(exclusive_resume);                             \

    MUTEX_UNLOCK(mutex);

include/qom/cpu.h

+0 −4
Original line number Diff line number Diff line
@@ -846,9 +846,6 @@ void cpu_exec_end(CPUState *cpu); 
 * cpu_exec are exited immediately.  CPUs that call cpu_exec_start

 * during the exclusive section go to sleep until this CPU calls

 * end_exclusive.

 *

 * Returns with the CPU list lock taken (which nests outside all

 * other locks except the BQL).

 */

void start_exclusive(void);
@@ -856,7 +853,6 @@ void start_exclusive(void); 
 * end_exclusive:

 *

 * Concludes an exclusive execution section started by start_exclusive.

 * Releases the CPU list lock.

 */

void end_exclusive(void);