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