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cpus: take seqlock across qemu_icount updates

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Even though writes of qemu_icount can safely race with reads in
qemu_icount_raw, qemu_icount is also read by icount_adjust, which
runs in the I/O thread.  Therefore, writes do needs protection of
the vm_clock_lock; for simplicity the patch protects it with both
seqlock+spinlock, which we already do for hosts that lack 64-bit atomics.

The bug actually predated the introduction of vm_clock_lock;
cpu_update_icount would have needed the BQL before the spinlock was
introduced.

Reported-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2018-09-11 13:15:32 +02:00
parent 39fe576c82
commit 9b4e6f4966
1 changed files with 14 additions and 8 deletions
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22
cpus.c
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@ -245,21 +245,27 @@ static int64_t cpu_get_icount_executed(CPUState *cpu)
* account executed instructions. This is done by the TCG vCPU
* thread so the main-loop can see time has moved forward.
*/
void cpu_update_icount(CPUState *cpu)
static void cpu_update_icount_locked(CPUState *cpu)
{
int64_t executed = cpu_get_icount_executed(cpu);
cpu->icount_budget -= executed;
#ifndef CONFIG_ATOMIC64
seqlock_write_lock(&timers_state.vm_clock_seqlock,
&timers_state.vm_clock_lock);
#endif
atomic_set__nocheck(&timers_state.qemu_icount,
timers_state.qemu_icount + executed);
#ifndef CONFIG_ATOMIC64
}
/*
* Update the global shared timer_state.qemu_icount to take into
* account executed instructions. This is done by the TCG vCPU
* thread so the main-loop can see time has moved forward.
*/
void cpu_update_icount(CPUState *cpu)
{
seqlock_write_lock(&timers_state.vm_clock_seqlock,
&timers_state.vm_clock_lock);
cpu_update_icount_locked(cpu);
seqlock_write_unlock(&timers_state.vm_clock_seqlock,
&timers_state.vm_clock_lock);
#endif
}
static int64_t cpu_get_icount_raw_locked(void)
@ -272,7 +278,7 @@ static int64_t cpu_get_icount_raw_locked(void)
exit(1);
}
/* Take into account what has run */
cpu_update_icount(cpu);
cpu_update_icount_locked(cpu);
}
/* The read is protected by the seqlock, so __nocheck is okay. */
return atomic_read__nocheck(&timers_state.qemu_icount);