fcntl couldn't lock /dev/null and hang
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1
A minimal working example, but only for my machine:
int main(int argc, char* argv) {
int fd = open ("/dev/null", O_RDWR|O_CREAT);
if (fd < 0) {
printf("Failed to open filen");
}
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
int res = fcntl(fd, F_SETLKW,&lock); // this hangs
if (res < 0) {
printf("Failed to lockn");
}
close (fd);
return (0);
}
The program above hangs only on my machine, and completed instantly on 7 other machines. Is there anything that I can look into to investigate this problem?
strace shows that fcntl was getting stuck when the program (./t) was killed by ^C.
5249 execve("./t", ["./t"], [/* 23 vars */]) = 0
5249 brk(NULL) = 0x1cf6000
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
5249 open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
5249 fstat(3, {st_mode=S_IFREG|0644, st_size=98358, ...}) = 0
5249 mmap(NULL, 98358, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f31a1118000
5249 close(3) = 0
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
5249 read(3, "177ELF21133>1Pt2"..., 832) = 832
5249 fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1117000
5249 mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f31a0b42000
5249 mprotect(0x7f31a0d02000, 2097152, PROT_NONE) = 0
5249 mmap(0x7f31a0f02000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f31a0f02000
5249 mmap(0x7f31a0f08000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f31a0f08000
5249 close(3) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1116000
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1115000
5249 arch_prctl(ARCH_SET_FS, 0x7f31a1116700) = 0
5249 mprotect(0x7f31a0f02000, 16384, PROT_READ) = 0
5249 mprotect(0x600000, 4096, PROT_READ) = 0
5249 mprotect(0x7f31a1131000, 4096, PROT_READ) = 0
5249 munmap(0x7f31a1118000, 98358) = 0
5249 open("/dev/null", O_RDWR|O_CREAT, 03777762203636510) = 3
5249 fcntl(3, F_SETLKW, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
5249 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
5249 +++ killed by SIGINT +++
files
asked yesterday
qspqsp
93110
add a comment |
1
A minimal working example, but only for my machine:
int main(int argc, char* argv) {
int fd = open ("/dev/null", O_RDWR|O_CREAT);
if (fd < 0) {
printf("Failed to open filen");
}
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
int res = fcntl(fd, F_SETLKW,&lock); // this hangs
if (res < 0) {
printf("Failed to lockn");
}
close (fd);
return (0);
}
The program above hangs only on my machine, and completed instantly on 7 other machines. Is there anything that I can look into to investigate this problem?
strace shows that fcntl was getting stuck when the program (./t) was killed by ^C.
5249 execve("./t", ["./t"], [/* 23 vars */]) = 0
5249 brk(NULL) = 0x1cf6000
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
5249 open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
5249 fstat(3, {st_mode=S_IFREG|0644, st_size=98358, ...}) = 0
5249 mmap(NULL, 98358, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f31a1118000
5249 close(3) = 0
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
5249 read(3, "177ELF21133>1Pt2"..., 832) = 832
5249 fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1117000
5249 mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f31a0b42000
5249 mprotect(0x7f31a0d02000, 2097152, PROT_NONE) = 0
5249 mmap(0x7f31a0f02000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f31a0f02000
5249 mmap(0x7f31a0f08000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f31a0f08000
5249 close(3) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1116000
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1115000
5249 arch_prctl(ARCH_SET_FS, 0x7f31a1116700) = 0
5249 mprotect(0x7f31a0f02000, 16384, PROT_READ) = 0
5249 mprotect(0x600000, 4096, PROT_READ) = 0
5249 mprotect(0x7f31a1131000, 4096, PROT_READ) = 0
5249 munmap(0x7f31a1118000, 98358) = 0
5249 open("/dev/null", O_RDWR|O_CREAT, 03777762203636510) = 3
5249 fcntl(3, F_SETLKW, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
5249 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
5249 +++ killed by SIGINT +++
files
asked yesterday
qspqsp
93110
First thing to look into would bestrace.
– Jos
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
1
Why do you want a lock on/dev/null?
– vidarlo
yesterday
You are askingfnctlto obtain an exclusive (write) lockl_type=F_WRLCK, where (according toman fnctl)F_SETLKWmeans "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock usinglslocks- see How to list processes locking file?. If you don't wantfnctlto wait in the case of conflict, then tryF_SETLKin place ofF_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking/dev/null.
– steeldriver
13 hours ago
add a comment |
1
1
1
A minimal working example, but only for my machine:
int main(int argc, char* argv) {
int fd = open ("/dev/null", O_RDWR|O_CREAT);
if (fd < 0) {
printf("Failed to open filen");
}
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
int res = fcntl(fd, F_SETLKW,&lock); // this hangs
if (res < 0) {
printf("Failed to lockn");
}
close (fd);
return (0);
}
The program above hangs only on my machine, and completed instantly on 7 other machines. Is there anything that I can look into to investigate this problem?
strace shows that fcntl was getting stuck when the program (./t) was killed by ^C.
5249 execve("./t", ["./t"], [/* 23 vars */]) = 0
5249 brk(NULL) = 0x1cf6000
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
5249 open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
5249 fstat(3, {st_mode=S_IFREG|0644, st_size=98358, ...}) = 0
5249 mmap(NULL, 98358, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f31a1118000
5249 close(3) = 0
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
5249 read(3, "177ELF21133>1Pt2"..., 832) = 832
5249 fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1117000
5249 mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f31a0b42000
5249 mprotect(0x7f31a0d02000, 2097152, PROT_NONE) = 0
5249 mmap(0x7f31a0f02000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f31a0f02000
5249 mmap(0x7f31a0f08000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f31a0f08000
5249 close(3) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1116000
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1115000
5249 arch_prctl(ARCH_SET_FS, 0x7f31a1116700) = 0
5249 mprotect(0x7f31a0f02000, 16384, PROT_READ) = 0
5249 mprotect(0x600000, 4096, PROT_READ) = 0
5249 mprotect(0x7f31a1131000, 4096, PROT_READ) = 0
5249 munmap(0x7f31a1118000, 98358) = 0
5249 open("/dev/null", O_RDWR|O_CREAT, 03777762203636510) = 3
5249 fcntl(3, F_SETLKW, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
5249 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
5249 +++ killed by SIGINT +++
files
asked yesterday
qspqsp
93110
A minimal working example, but only for my machine:
int main(int argc, char* argv) {
int fd = open ("/dev/null", O_RDWR|O_CREAT);
if (fd < 0) {
printf("Failed to open filen");
}
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
int res = fcntl(fd, F_SETLKW,&lock); // this hangs
if (res < 0) {
printf("Failed to lockn");
}
close (fd);
return (0);
}
The program above hangs only on my machine, and completed instantly on 7 other machines. Is there anything that I can look into to investigate this problem?
strace shows that fcntl was getting stuck when the program (./t) was killed by ^C.
5249 execve("./t", ["./t"], [/* 23 vars */]) = 0
5249 brk(NULL) = 0x1cf6000
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
5249 open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
5249 fstat(3, {st_mode=S_IFREG|0644, st_size=98358, ...}) = 0
5249 mmap(NULL, 98358, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f31a1118000
5249 close(3) = 0
5249 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
5249 open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
5249 read(3, "177ELF21133>1Pt2"..., 832) = 832
5249 fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1117000
5249 mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f31a0b42000
5249 mprotect(0x7f31a0d02000, 2097152, PROT_NONE) = 0
5249 mmap(0x7f31a0f02000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f31a0f02000
5249 mmap(0x7f31a0f08000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f31a0f08000
5249 close(3) = 0
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1116000
5249 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f31a1115000
5249 arch_prctl(ARCH_SET_FS, 0x7f31a1116700) = 0
5249 mprotect(0x7f31a0f02000, 16384, PROT_READ) = 0
5249 mprotect(0x600000, 4096, PROT_READ) = 0
5249 mprotect(0x7f31a1131000, 4096, PROT_READ) = 0
5249 munmap(0x7f31a1118000, 98358) = 0
5249 open("/dev/null", O_RDWR|O_CREAT, 03777762203636510) = 3
5249 fcntl(3, F_SETLKW, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
5249 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
5249 +++ killed by SIGINT +++
files
files
asked yesterday
qspqsp
93110
asked yesterday
qspqsp
93110
edited yesterday
qsp
asked yesterday
qspqsp
93110
asked yesterday
qspqsp
93110
asked yesterday
qspqsp
93110
93110
First thing to look into would bestrace.
– Jos
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
1
Why do you want a lock on/dev/null?
– vidarlo
yesterday
You are askingfnctlto obtain an exclusive (write) lockl_type=F_WRLCK, where (according toman fnctl)F_SETLKWmeans "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock usinglslocks- see How to list processes locking file?. If you don't wantfnctlto wait in the case of conflict, then tryF_SETLKin place ofF_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking/dev/null.
– steeldriver
13 hours ago
add a comment |
First thing to look into would bestrace.
– Jos
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
1
Why do you want a lock on/dev/null?
– vidarlo
yesterday
You are askingfnctlto obtain an exclusive (write) lockl_type=F_WRLCK, where (according toman fnctl)F_SETLKWmeans "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock usinglslocks- see How to list processes locking file?. If you don't wantfnctlto wait in the case of conflict, then tryF_SETLKin place ofF_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking/dev/null.
– steeldriver
13 hours ago
First thing to look into would be
strace.– Jos
yesterday
First thing to look into would be
strace.– Jos
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
1
1
Why do you want a lock on
/dev/null?– vidarlo
yesterday
Why do you want a lock on
/dev/null?– vidarlo
yesterday
You are asking
fnctl to obtain an exclusive (write) lock l_type=F_WRLCK, where (according to man fnctl) F_SETLKW means "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock using lslocks - see How to list processes locking file?. If you don't want fnctl to wait in the case of conflict, then try F_SETLK in place of F_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking /dev/null.– steeldriver
13 hours ago
You are asking
fnctl to obtain an exclusive (write) lock l_type=F_WRLCK, where (according to man fnctl) F_SETLKW means "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock using lslocks - see How to list processes locking file?. If you don't want fnctl to wait in the case of conflict, then try F_SETLK in place of F_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking /dev/null.– steeldriver
13 hours ago
add a comment |
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First thing to look into would be
strace.– Jos
yesterday
@Jos Thanks, I added strace output, what else I can look into?
– qsp
yesterday
1
Why do you want a lock on
/dev/null?– vidarlo
yesterday
You are asking
fnctlto obtain an exclusive (write) lockl_type=F_WRLCK, where (according toman fnctl)F_SETLKWmeans "... but if a conflicting lock is held on the file, then wait for that lock to be released". You may be able to see why there is a conflicting lock usinglslocks- see How to list processes locking file?. If you don't wantfnctlto wait in the case of conflict, then tryF_SETLKin place ofF_SETLKW. Regardless, @vidarlo 's point applies - it doesn't seem like a good idea to be write-locking/dev/null.– steeldriver
13 hours ago