Re: [PATCH] virtio_ring: Fix the stale index in available ring

[Michael S. Tsirkin]

On Fri, Mar 15, 2024 at 09:24:36PM +1000, Gavin Shan wrote:
> 
> On 3/15/24 21:05, Michael S. Tsirkin wrote:
> > On Fri, Mar 15, 2024 at 08:45:10PM +1000, Gavin Shan wrote:
> > > > > Yes, I guess smp_wmb() ('dmb') is buggy on NVidia's grace-hopper platform. I tried
> > > to reproduce it with my own driver where one thread writes to the shared buffer
> > > and another thread reads from the buffer. I don't hit the out-of-order issue so
> > > far.
> > 
> > Make sure the 2 areas you are accessing are in different cache lines.
> > 
> 
> Yes, I already put those 2 areas to separate cache lines.
> 
> > 
> > > My driver may be not correct somewhere and I will update if I can reproduce
> > > the issue with my driver in the future.
> > 
> > Then maybe your change is just making virtio slower and masks the bug
> > that is actually elsewhere?
> > 
> > You don't really need a driver. Here's a simple test: without barriers
> > assertion will fail. With barriers it will not.
> > (Warning: didn't bother testing too much, could be buggy.
> > 
> > ---
> > 
> > #include <pthread.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <assert.h>
> > 
> > #define FIRST values[0]
> > #define SECOND values[64]
> > 
> > volatile int values[100] = {};
> > 
> > void* writer_thread(void* arg) {
> > 	while (1) {
> > 	FIRST++;
> > 	// NEED smp_wmb here
>         __asm__ volatile("dmb ishst" : : : "memory");
> > 	SECOND++;
> > 	}
> > }
> > 
> > void* reader_thread(void* arg) {
> >      while (1) {
> > 	int first = FIRST;
> > 	// NEED smp_rmb here
>         __asm__ volatile("dmb ishld" : : : "memory");
> > 	int second = SECOND;
> > 	assert(first - second == 1 || first - second == 0);
> >      }
> > }
> > 
> > int main() {
> >      pthread_t writer, reader;
> > 
> >      pthread_create(&writer, NULL, writer_thread, NULL);
> >      pthread_create(&reader, NULL, reader_thread, NULL);
> > 
> >      pthread_join(writer, NULL);
> >      pthread_join(reader, NULL);
> > 
> >      return 0;
> > }
> > 
> 
> Had a quick test on NVidia's grace-hopper and Ampere's CPUs. I hit
> the assert on both of them. After replacing 'dmb' with 'dsb', I can
> hit assert on both of them too. I need to look at the code closely.
> 
> [root@virt-mtcollins-02 test]# ./a
> a: a.c:26: reader_thread: Assertion `first - second == 1 || first - second == 0' failed.
> Aborted (core dumped)
> 
> [root@nvidia-grace-hopper-05 test]# ./a
> a: a.c:26: reader_thread: Assertion `first - second == 1 || first - second == 0' failed.
> Aborted (core dumped)
> 
> Thanks,
> Gavin


Actually this test is broken. No need for ordering it's a simple race.
The following works on x86 though (x86 does not need barriers
though).


#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#if 0
#define x86_rmb()  asm volatile("lfence":::"memory")
#define x86_mb()  asm volatile("mfence":::"memory")
#define x86_smb()  asm volatile("sfence":::"memory")
#else
#define x86_rmb()  asm volatile("":::"memory")
#define x86_mb()  asm volatile("":::"memory")
#define x86_smb()  asm volatile("":::"memory")
#endif

#define FIRST values[0]
#define SECOND values[640]
#define FLAG values[1280]

volatile unsigned values[2000] = {};

void* writer_thread(void* arg) {
	while (1) {
	/* Now synchronize with reader */
	while(FLAG);
	FIRST++;
	x86_smb();
	SECOND++;
	x86_smb();
	FLAG = 1;
	}
}

void* reader_thread(void* arg) {
    while (1) {
	/* Now synchronize with writer */
	while(!FLAG);
	x86_rmb();
	unsigned first = FIRST;
	x86_rmb();
	unsigned second = SECOND;
	assert(first - second == 1 || first - second == 0);
	FLAG = 0;

	if (!(first %1000000))
		printf("%d\n", first);
   }
}

int main() {
    pthread_t writer, reader;

    pthread_create(&writer, NULL, writer_thread, NULL);
    pthread_create(&reader, NULL, reader_thread, NULL);

    pthread_join(writer, NULL);
    pthread_join(reader, NULL);

    return 0;
}