linux-kernel - gzip sleeps until 'strace'ing the process

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Date:	Wed, 16 Sep 2009 16:21:37 +0200
From:	Henning Fehrmann <henning.fehrmann@....mpg.de>
To:	linux-kernel@...r.kernel.org
Subject: gzip sleeps until 'strace'ing the process

Hello,

If you have some hints, please put me in the CC.

We launched a job which forks a 
gzip -cd 
which is basically zcat.
This process is accessing a file within a NFS mounted directory: 

# ps aux|grep gzip
user   32078  0.0  0.0   4140   552 pts/0    S+   15:06   0:00 gzip -cd -f file_in_NFS_mounted_directory

The file is properly ziped.

The process is in the sleep state - also the file can be accessed.

If I 'strace' it the process finishes successfully. 

# strace -p 32078
Process 32078 attached - interrupt to quit
write(1, "e-010 5.5565\n121.951418712 2.2249"..., 24578) = 24578
write(1, "3 -0.3036801 -3.1699e-011 5.30998"..., 32768) = 32768
write(1, "2015e-010 5.5565\n121.9529017461 2"..., 32768) = 32768
write(1, "\n121.9543512274 0.166872 -0.13840"..., 32768) = 32768
write(1, "-011 5.5565\n121.9558745249 5.2519"..., 32768) = 32768
write(1, "08331213 -4.2015e-010 5.80302\n121"..., 32768) = 32768
write(1, "1.9587130924 4.603975 -0.2485881 "..., 32768) = 32768
read(8, "\361\234a\325\305\3161\373x\307\324\312\240+\307\354\211Y\357cH\234\360\362\315\235NV\310O\\\212\236"..., 32768) = 14630
read(8, ""..., 18138)                   = 0
write(1, "600686259 5.408658 -0.5240481 -1."..., 32768) = 32768
write(1, "5.707316 -0.3587721 -4.2015e-010 "..., 32768) = 32768
write(1, "1.9627260083 5.766155 -0.3587721 "..., 30733) = 30733
close(8)                                = 0
exit_group(0)                           = ?
Process 32078 detached

I found it puzzling since strace should not have an influence on the state of a process.

In the /proc directory I found the follwing:

/proc/32078# cat status
Name:   gzip
State:  S (sleeping)
Tgid:   32078
Pid:    32078
PPid:   31592
TracerPid:      0
Uid:    5059    5059    5059    5059
Gid:    5000    5000    5000    5000
FDSize: 256
Groups: 5000 5001 5002
VmPeak:     4140 kB
VmSize:     4140 kB
VmLck:         0 kB
VmHWM:       552 kB
VmRSS:       552 kB
VmData:      492 kB
VmStk:        84 kB
VmExe:        52 kB
VmLib:      1432 kB
VmPTE:        32 kB
Threads:        1
SigQ:   0/68608
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000001300001
SigCgt: 0000000000805002
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: fffffffffffffeff
voluntary_ctxt_switches:        55
nonvoluntary_ctxt_switches:     1

Do you have any idea?

Cheers,
Henning Fehrmann
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