Dne 10.12.2015 v 12:04 IceBsi napsal(a):
> Dear ALSA developers:
>
> I found a root cause of the following issue by using strace:
> unable to create IPC shm instance
>
> Firstly, please see the test code below:
> 1 #include <stdio.h>
> 2 #include <unistd.h>
> 3 #include <sys/ipc.h>
> 4 #include <sys/shm.h>
> 5 #include <errno.h>
> 6 #include <string.h>
> 7
> 8 int main(int argc, char *argv[])
> 9 {
> 10 int r = 0;
> 11 struct shmid_ds buf;
> 12 key_t ipc_key = (key_t)325559;
> 13 size_t shmsize = 488;
> 14 int shmid = -1;
> 15 mode_t ipc_perm = 0666;
> 16 void *shmptr = NULL;
> 17 pid_t pid;
> 18
> 19 printf("___%d:%d call shmget(%d,%u,%d)\n", __LINE__, getpid(), (int)(ipc_key), shmsize, IPC_CREAT | ipc_perm);
> 20 shmid = shmget(ipc_key, shmsize, IPC_CREAT | ipc_perm);
> 21 printf("___%d:%d ret shmget=%d\n", __LINE__, getpid(), shmid);
> 22
> 23 printf("___%d:%d call shmat(%d,0,0)\n", __LINE__, getpid(), shmid);
> 24 shmptr = shmat(shmid, 0, 0);
> 25 printf("___%d:%d ret shmat=%p\n", __LINE__, getpid(), shmptr);
> 26
> 27 printf("+++%d:%d call fork\n", __LINE__, getpid());
> 28 pid = fork();
> 29 printf("+++%d:%d ret fork=%d\n", __LINE__, getpid(), (int)pid);
> 30
> 31 printf("___%d:%d call shmctl(%d,IPC_STAT,%p)\n", __LINE__, getpid(), shmid, &buf);
> 32 r = shmctl(shmid, IPC_STAT, &buf);
> 33 printf("___%d:%d ret shmctl=%d\n", __LINE__, getpid(), r);
> 34 printf("___%d:%d nattch=%lu\n", __LINE__, getpid(), buf.shm_nattch);
> 35
> 36 printf("___%d:%d call shmdt(%p)\n", __LINE__, getpid(), shmptr);
> 37 r = shmdt(shmptr);
> 38 printf("___%d:%d ret shmdt=%d\n", __LINE__, getpid(), r);
> 39
> 40 printf("___%d:%d call shmctl(%d,IPC_RMID,NULL)\n", __LINE__, getpid(), shmid);
> 41 r = shmctl(shmid, IPC_RMID, NULL);
> 42 printf("___%d:%d ret shmctl=%d, %s\n", __LINE__, getpid(), r, strerror(errno));
> 43
> 44 if ((pid_t)0 != pid) {
> 45 printf("+++%d:%d call wait(NULL)\n", __LINE__, getpid());
> 46 pid = wait(NULL);
> 47 printf("+++%d:%d ret wait=%d\n", __LINE__, getpid(), (int)pid);
> 48 }
> 49
> 50 return 0;
> 51 }
> The output of the test code is:
> ___19:11782 call shmget(325559,488,950)
> ___21:11782 ret shmget=2850820
> ___23:11782 call shmat(2850820,0,0)
> ___25:11782 ret shmat=0x7f452c223000
> +++27:11782 call fork
> +++29:11782 ret fork=11783
> ___31:11782 call shmctl(2850820,IPC_STAT,0x7fffdaf0a750)
> ___33:11782 ret shmctl=0
> ___34:11782 nattch=2
> ___36:11782 call shmdt(0x7f452c223000)
> +++29:11783 ret fork=0
> ___38:11782 ret shmdt=0
> ___40:11782 call shmctl(2850820,IPC_RMID,NULL)
> ___31:11783 call shmctl(2850820,IPC_STAT,0x7fffdaf0a750)
> ___42:11782 ret shmctl=0, Success
> +++45:11782 call wait(NULL)
> ___33:11783 ret shmctl=0
> ___34:11783 nattch=1
> ___36:11783 call shmdt(0x7f452c223000)
> ___38:11783 ret shmdt=0
> ___40:11783 call shmctl(2850820,IPC_RMID,NULL)
> ___42:11783 ret shmctl=-1, Invalid argument
> +++47:11782 ret wait=11783
> And sometimes output is:
> ___19:11798 call shmget(325559,488,950)
> ___21:11798 ret shmget=3112964
> ___23:11798 call shmat(3112964,0,0)
> ___25:11798 ret shmat=0x7f6b39378000
> +++27:11798 call fork
> +++29:11798 ret fork=11799
> ___31:11798 call shmctl(3112964,IPC_STAT,0x7fff3a6472a0)
> ___33:11798 ret shmctl=0
> +++29:11799 ret fork=0
> ___31:11799 call shmctl(3112964,IPC_STAT,0x7fff3a6472a0)
> ___34:11798 nattch=2
> ___36:11798 call shmdt(0x7f6b39378000)
> ___33:11799 ret shmctl=0
> ___38:11798 ret shmdt=0
> ___34:11799 nattch=2
> ___40:11798 call shmctl(3112964,IPC_RMID,NULL)
> ___36:11799 call shmdt(0x7f6b39378000)
> ___42:11798 ret shmctl=0, Success
> +++45:11798 call wait(NULL)
> ___38:11799 ret shmdt=0
> ___40:11799 call shmctl(3112964,IPC_RMID,NULL)
> ___42:11799 ret shmctl=-1, Invalid argument
> +++47:11798 ret wait=11799
>>From the outputs the parent process always get nattch=2, the child process
> get nattch=1 or sometimes get nattch=2.
>
> Then please see the following lines of src/pcm/pcm_direct.c
> (from alsa-lib-1.1.0.tar.bz2):
> 110 dmix->shmptr = shmat(dmix->shmid, 0, 0);
> 111 if (dmix->shmptr == (void *) -1) {
> 112 err = -errno;
> 113 snd_pcm_direct_shm_discard(dmix);
> 114 return err;
> 115 }
> 116 mlock(dmix->shmptr, sizeof(snd_pcm_direct_share_t));
> 117 if (shmctl(dmix->shmid, IPC_STAT, &buf) < 0) {
> 118 err = -errno;
> 119 snd_pcm_direct_shm_discard(dmix);
> 120 return err;
> 121 }
> 122 if (buf.shm_nattch == 1) { /* we're the first user, clear the segment */
> 123 memset(dmix->shmptr, 0, sizeof(snd_pcm_direct_share_t));
> 124 if (dmix->ipc_gid>= 0) {
> 125 buf.shm_perm.gid = dmix->ipc_gid;
> 126 shmctl(dmix->shmid, IPC_SET, &buf);
> 127 }
> 128 dmix->shmptr->magic = SND_PCM_DIRECT_MAGIC;
> 129 return 1;
> 130 } else {
> 131 if (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC) {
> 132 snd_pcm_direct_shm_discard(dmix);
> 133 return -EINVAL;
> 134 }
> 135 }
> If an applicatioin that uses alsa-lib invokes fork() and the fork() is invoked
> just after shmat(line 110) and just before shmctl(line 117), buf.shm_nattch
> will be 2, so "buf.shm_nattch == 1" will be false. But while buf.shm_nattch
> is 2, we're still the first user this time, and we should executes line 123 to
> line 129 of the codes above.
>
> There is the following statement in `man shmget`:
> When a new shared memory segment is created, its contents are initialized to zero values
> According to the statement above, dmix->shmptr->magic is 0 on shm first created.
> So, to fix this issue, we should change the following line:
> 122 if (buf.shm_nattch == 1) { /* we're the first user, clear the segment */
> to:
> 122 if (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC) { /* we're the first user, clear the segment */
You're talking about an application using threads ? Then a mutex should
be used here. Thanks for the analysis.
Jaroslav
--
Jaroslav Kysela <perex@xxxxxxxx>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.
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