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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: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2016-09-02 23:33:38 +02:00
parent cf07da65f3
commit 758e1b2b62
3 changed files with 11 additions and 8 deletions
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11
cpus-common.c
View File

@ -171,8 +171,7 @@ static inline void exclusive_idle(void)
} }
/* Start an exclusive operation. /* Start an exclusive operation.
Must only be called from outside cpu_exec, takes Must only be called from outside cpu_exec. */
qemu_cpu_list_lock. */
void start_exclusive(void) void start_exclusive(void)
{ {
CPUState *other_cpu; CPUState *other_cpu;
@ -191,11 +190,17 @@ void start_exclusive(void)
while (pending_cpus > 1) { while (pending_cpus > 1) {
qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock); 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) void end_exclusive(void)
{ {
qemu_mutex_lock(&qemu_cpu_list_lock);
pending_cpus = 0; pending_cpus = 0;
qemu_cond_broadcast(&exclusive_resume); qemu_cond_broadcast(&exclusive_resume);
qemu_mutex_unlock(&qemu_cpu_list_lock); qemu_mutex_unlock(&qemu_cpu_list_lock);

4
docs/tcg-exclusive.promela
View File

@ -98,9 +98,11 @@ byte has_waiter[N_CPUS];
do \ do \
:: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex); \ :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex); \
:: else -> break; \ :: else -> break; \
od od; \
MUTEX_UNLOCK(mutex);
#define end_exclusive() \ #define end_exclusive() \
MUTEX_LOCK(mutex); \
pending_cpus = 0; \ pending_cpus = 0; \
COND_BROADCAST(exclusive_resume); \ COND_BROADCAST(exclusive_resume); \
MUTEX_UNLOCK(mutex); MUTEX_UNLOCK(mutex);

4
include/qom/cpu.h
View File

@ -846,9 +846,6 @@ void cpu_exec_end(CPUState *cpu);
* cpu_exec are exited immediately. CPUs that call cpu_exec_start * cpu_exec are exited immediately. CPUs that call cpu_exec_start
* during the exclusive section go to sleep until this CPU calls * during the exclusive section go to sleep until this CPU calls
* end_exclusive. * end_exclusive.
*
* Returns with the CPU list lock taken (which nests outside all
* other locks except the BQL).
*/ */
void start_exclusive(void); void start_exclusive(void);
@ -856,7 +853,6 @@ void start_exclusive(void);
* end_exclusive: * end_exclusive:
* *
* Concludes an exclusive execution section started by start_exclusive. * Concludes an exclusive execution section started by start_exclusive.
* Releases the CPU list lock.
*/ */
void end_exclusive(void); void end_exclusive(void);