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authorJim Meyering <meyering@redhat.com>2009-04-17 18:44:18 +0200
committerJim Meyering <meyering@redhat.com>2009-04-22 21:52:11 +0200
commitc74fbaefebe91ca60e3a8582613a3714c740df65 (patch)
tree45dcb7277ba56f6f879f1d64444643038791a49a /src
parent2ad7da759490a5680844e1e4b6b4ac1d13b95d3b (diff)
downloadcoreutils-c74fbaefebe91ca60e3a8582613a3714c740df65.tar.xz
cp: work around linux kernel bug: short-read != EOF on /proc
Remove the optimization that avoided up to 50% of cp's read syscalls. Do not assume that a short read on a regular file indicates EOF. When reading from a file in /proc on linux [at least 2.6.9 - 2.6.29] into a 4k-byte buffer or larger, a short read does not always indicate EOF. For example, "cp /proc/slabinfo /tmp" copies only 4068 of the total 7493 bytes. This optimization (25719a33154f0c62ea9881f0c79ae312dd4cec7a, Improve performance a bit by optimizing away; 2005-11-24) appears to have been worth less than a 2% speed-up (and usually much less), so the impact of removing it is negligible. * src/copy.c (copy_reg): Don't exit the loop early. * tests/cp/proc-short-read: New test, lightly based on a suggestion from Mike Frysinger, to exercise this fix. * tests/Makefile.am (TESTS): Add cp/proc-short-read. * NEWS (Improve robustness): Mention this change.
Diffstat (limited to 'src')
-rw-r--r--src/copy.c7
1 files changed, 4 insertions, 3 deletions
diff --git a/src/copy.c b/src/copy.c
index 9b0e139c6..c45224c8a 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -700,9 +700,10 @@ copy_reg (char const *src_name, char const *dst_name,
}
last_write_made_hole = false;
- /* A short read on a regular file means EOF. */
- if (n_read != buf_size && S_ISREG (src_open_sb.st_mode))
- break;
+ /* It is tempting to return early here upon a short read from a
+ regular file. That would save the final read syscall for each
+ file. Unfortunately that doesn't work for certain files in
+ /proc with linux kernels from at least 2.6.9 .. 2.6.29. */
}
}